The Development of Beneficial Utilization of Coal Combustion Products - Part 3

We Energies Bottom Ash as Fine Aggregate in “Eco-Pad” (In-Situ Mixed Concrete Pavement) (67)

The scope of this research was to identify the mixture proportions and develop a high recycled content in-situ mixed concrete for a 3.5 acre outdoor storage pad for bottom ash and synthetic gypsum produced at We Energies’ Pleasant Prairie Power Plant (PPPP) as shown in Figure 6-12. The storage pad was constructed in the fall of 2004 at PPPP, located in Kenosha County, Wisconsin. Development of the “eco-pad” allowed the usage of an alternative paving material and also development of an economic and environmentally friendly construction process.

The “eco-pad” is a concrete mixture that includes recycled concrete for coarse aggregate, bottom ash for fine aggregate, cementitious materials (Class C fly ash and either Portland cement or slag cement yielding a 93% or 100% recycled content, respectively) and water for obtaining the optimum moisture density of materials. The class C fly ash and bottom ash used in this project were produced at We Energies PPPP and is a by-product of Powder River Basin, Wyoming sub-bituminous coal combustion. The 1½-inch topsize recycled concrete was supplied from a crushed and screened stockpile managed by an asphaltic concrete producer in Racine County, Wisconsin. The Portland (Type I/II) cement was used in conjunction with Class C fly ash on the western half of the site and slag cement in conjunction with Class C fly ash was used on the eastern half. During construction in the late Fall season, the temperature was progressively getting colder and a shortage of Portland cement led to substituting with slag cement. The chemical composition of the cementitious material is shown in Tables 6-12. The end result was an 8-inch thick concrete pavement on a 12-inch bottom ash base.

Figure 6-12: Eco-Pad construction at Pleasant Prairie power plant.

Table 6-12: Chemical Composition of the Cementitious Materials Used in the “Eco-pad” Pavement

Laboratory Testing

Prior to the placement of the in-situ mixed concrete pavement on the field site, a laboratory mix analysis was performed to determine the mixture proportions. The cementitious material (Class C fly ash and Portland cement mixture) and aggregate materials (recycled concrete and bottom ash) were evaluated for moisture content using ASTM D-2216, grainsize analysis (ASTM D-422), moisture density relationship by Modified Proctor method (ATM D-1557, except that the 5 lift requirement was replaced with 3 lifts), and to simulate the conditions of roller compacted concrete, the compressive strength analysis used was per ASTM D-1633 (Compressive Strength of Molded Soil-Cement Cylinders) and a 4-inch diameter split mold was used to facilitate the removal of each specimen with minimal disturbance to the samples. Upon completion, the specimens were sealed in plastic bags and curing was accelerated for seven days at 100ºF (per ASTM C-593) to approximate conditions of a 28-day cure period. After curing the samples were capped with a gypsum cap and the compressive strength was determined using a constant drive calibrated load frame. The tested specimen had a height to diameter ratio of 1.5:1 rather than the conventional ratio of 2:1 for a relative measure of the compressive strength.

The preliminary mixture proportion testing was performed in two phases. Initially, samples of the proposed recycled concrete (coarse aggregate) and the bottom ash (fine aggregate) were tested to determine their optimum blend for grainsize distribution and density. The second phase consisted of mixing the selected aggregate blend with varying amounts of the blended cementitious binder material for determination of the mixture’s optimum density and strength characteristics.

Laboratory Results

The grainsize analysis of the proposed recycled aggregate indicated that the coarse aggregate is described as a poorly to well-graded crushed concrete with about 48-67% gravel, 31-45% sand, and 2.6-6.6% silt/clay sized particles. The fine aggregate is described as a bottom ash with about 4-6% gravel, 77-85% sand, and 13-17% silt/clay sized particles. The dry loose unit weight of the coarse and fine aggregates resulted in 99 pcf to 105 pcf and 65 pcf, respectively. The grainsize analysis test results are shown in Table 6-13.

Generally, the results of the coarse/fine aggregate blends 50/50, 60/40, 70/30, and 80/20 indicated a poorly graded aggregate with about 35-54% gravel, 41-57% sand, and 5-8% silt/clay sized particles. As shown in Table 6-13, the compacted unit weights of the blends ranged from 102.8 pcf to 109.1 pcf with the 60/40 and 70/30 blends producing the higher densities. Based on the blended aggregate testing the 60/40 blend was selected for moisture density and moisture strength relationship testing with 12%, 15% and 18% (50% Portland cement/50% Class C Fly Ash) cementitious content. A blend of 50% Portland cement and 50% Class C fly ash (denoted as 50PC/50FA), by mass, was selected due to excellent experience on numerous construction projects and to reduce the number of variables on this project. The test results shown in Table 6-14 indicate that using an 18% 50PC/50FA cementitious content with a 60/40 aggregate material had the highest compressive strength. However, since the difference between 15% and the 18% mixtures was less than 5 psi, the 15% 50PC/50FA cementitious content of 60/40 aggregate material was selected for economic and environmental benefits.

Additional testing was performed with varying aggregate blends when a second sample of recycled concrete showed a denser gradation. The aggregate blends of 50/50, 60/40 and 70/30 recycled concrete/bottom ash were mixed with a constant 15% content of 50PC/50FA to determine their moisture-density and moisture-strength relationship, as shown in Table 6-14. The 70/30 aggregate blend with 15% blended cementitious content produced the higher strength and density. This was due to the material’s denser graded nature which allowed for a more compact arrangement of particles yielding a higher density and potentially higher strength.

Table 6-13: Summary of Aggregate Trial Blending Tests

Table 6-14: Summary of Moisture Density/Strength Tests of RC/BA Aggregate Blend with Percent Cement Content

Eco-Pad Construction Overview

A 12-inch thick compacted bottom ash base grade was established for the Eco-pad pavement of which 3-inches will later be incorporated into the concrete by in-situ mixing and 9-inches remains as the base. The in-situ mixing phase consisted of placing 5-inches of crushed recycled concrete across the proposed pavement area with dump trucks and using a road grader to create a uniform layer. The recycled concrete and bottom ash were then pre-mixed with a Wirtgen WR2500 asphalt reclaimer/ pulverizer set an 8 inch depth.

Lafarge pre-blended 50% Portland cement and 50% Class C fly ash was supplied from their bulk terminal in Milwaukee, Wisconsin. The 50PC/50FA blend of cementitious material was delivered to the jobsite via bulk pneumatic tanker trucks. The cementitious material was pneumatically conveyed to the vane spreader. The dry cementitious materials were placed with a vane spreader over the previously mixed aggregates. The 50PC/50FA blend was spread at a rate of 110 pounds per square yard. This rate was based on a 15% dry unit weight basis of the maximum dry density of the laboratory blended mixture.

Moisture conditioning was not required on this project due to relatively wet site conditions due to a rainy period prior to mixing. After mixing from the second pass of the pulverizer, the aggregate and cementitious materials mixture was compacted with a large vibratory sheepsfoot compactor, graded, and final rolling was accomplished with a smooth drum roller in the static mode. A target mixture moisture content of 10.5% for optimum strength was recommended along with directions to minimize the delay period from mixing of the cementitious materials to compaction. Compaction of the in-situ mixture was specified at 95% of the maximum dry density as determined by the Modified Proctor method. Saw cuts on a 20 foot grid followed the next day. A curing compound was applied following installation of the saw cuts and finally the elastomeric joint filler was applied to seal the saw cut joints.

Weather conditions during the in-situ mixing were challenging during construction. Due to a regional cement shortage, construction was delayed into late October and early November when temperatures were cold in Wisconsin. This was complicated by a rainy period that made obtaining optimum moisture content for compaction a challenge at the beginning of the project. A bottle neck in construction operation was the rate at which the cementitious material could be blended at the terminal and delivered to the project. An operating issue at the cement terminal also threatened to delay the project because cement could not be unloaded. However, ground granulated slag cement was available at the terminal and was substituted for Portland cement, thus adding another interesting dimension to this project. A call to the Slag Cement Association indicated that they were not aware of a prior use of a 50/50 fly ash/slag cement blend without Portland cement on a large construction project. The slag cement and Class C fly ash binder pavement combined with the recycled aggregates provided a 100% recycled material content in approximately two thirds of the pavement area.

Field Testing

The construction of the Eco-Pad test pavement was performed in three stages. The initial stage consisted of performing a grainsize analysis on samples of the field blended aggregates. A laboratory mixture analysis of the field aggregate blend with 15% of the blended cementitious material was also performed to establish laboratory moisture-density and moisture-strength relationships.

The second stage of the testing was performed during the field mixing of the blended aggregate and cementitious materials. Using the nuclear gauge method (ASTM D -2922), a field density test was performed during the compaction phase to assess the in-situ moisture content and percent compaction. Additionally, samples of the in-situ mixed concrete were obtained and compacted in the field by the Modified Proctor method. The field molded specimens were delivered to the laboratory and cured for a period of 7 to 365 days to assess the compressive strength development of the mixture.

The final phase of the testing included obtaining in-situ core specimens after approximately one and two years to assess the in-place strength of the pavement. The cores were obtained with a rotary type drill with a diamond impregnated core barrel in general accordance with ASTM D-42. Samples were subsequently air dried for 7 days, capped with a gypsum capping compound and compressive strengths were determined in accordance with ASTM C-39.

Field Test Results

Evaluation of the in-situ recycled concrete mixture constructed in the Eco-Pad pavement was based on the 5-inch thick recycled concrete and a 3-inch thick bottom ash aggregate blend and 15% blended cementitious materials (50PC/50FA) at the western side of the Eco-pad or 15% blended 50% slag cement and 50% fly ash, denoted as 50SC/50FA, at the eastern side of the Eco-Pad.

The grainsize analysis of the individual bottom ash and recycled concrete samples used on-site indicated gradations similar to the results obtained in the laboratory testing phase and this also held true for the field blended aggregate samples. The 5-inch recycled concrete and 3-inch thick bottom ash volumetric field blend has shown similarity to the 70/30 blend, by mass, prepared for the laboratory mixture analysis.

Results of the moisture density relationship testing indicated a higher maximum dry density at about the same optimum moisture content as in the preliminary laboratory mix proportioning phase. This is likely due to a well-graded sample resulting in a more densely compacted mixture. The higher result in the compressive strength may also be due to the higher density characteristics and lower optimum moisture contents. Subsequently, two additional samples of the previously sampled and combined field blended aggregate were mixed in the lab, one with 15% PC/FA and the other with 15% SC/FA cementitious blend to further assess the moisture-density and moisture-strength relationships. Results of the tests on the PC/FA blend showed similar results to those of the PC/FA blend of the first aggregate field blend mixture. Results of the SC/FA cementitious blend also provided results that were similar to those of the first aggregate field blend mixture. Results of the SC/FA cementitious blend resulted with similar moisture density relationships but with lower strengths, 1600 psi vs. 2225 psi and 2700 psi. This is likely due to the fact that the slag cement contained less CaO and also generally develops its strength at a slower rate than Portland cement. Results are shown in Table 6-15.

The second phase of the field testing included performing field moisture and density testing during the placement and compaction phase of the construction. In summary, the field blended aggregate had moisture contents initially of 14 to 19 percent, which was above the recommended optimum target of 10.5 percent. However, during the mixing process the moisture contents were generally found to range from 10 to 16 percent based on the in-place field density testing. The field density testing also indicated an in-place compaction ranging from 92 to 99 percent with an average compaction of 96.5 percent of the Modified Proctor density.

Table 6-15: Laboratory Summary of Moisture Density/Strength Tests on Field Blended Aggregate Samples

Results of the field molded compressive strength specimens are summarized in Table 6-16. In summary, the field molded samples of the PC/FA cementitious blend indicated compressive strengths (2440 psi at 28 days and 2525 psi at 56 days) are similar to those of the laboratory mixtures with the field blend aggregates (2225 psi and 2700 psi) and somewhat higher than the mixtures with the laboratory blended aggregates (1880 psi and 1920 psi). The field molded samples with the SC/FA cementitious blend indicated compressive strengths on the order of 195 psi and 175 psi at 28 days which turned out to be much less than the laboratory mixture which yielded a strength of 1600 psi using the accelerated core method. This is probably due to the much lower curing temperatures of the field samples and the fact that slag cement generally develops strength at a slower rate at lower temperatures. The 365- day test results indicated compressive strengths on the order of 4325 psi and 2565 psi for the PC/FA and SC/FA mixtures, respectively.

The final phase of the field testing included obtaining field core samples from the eco- pad pavement section after one and two years of field curing. Results of the core strength tests indicated an average compressive strength of 3150 psi and 1852 psi after one year and 2960 psi and 2266 psi after two years for the PC/FA and SC/FA mixtures. In comparing these results to the molded field samples, it must be recognized that the molded specimens have a height to diameter ratio of 2. Therefore, the molded samples will yield a somewhat higher strength value. Correcting the shorter molded samples with a correction factor of 0.91 as suggested in ASTM C-42, the molded samples would indicate strengths of 3930 psi and 2334 psi, respectively. The test results for the field molded and cored samples are summarized in Table 6-16.

Table 6-16: Summary of Field Molded and Core Specimens Compressive Strength Test Results

Summary

Based on the data recorded in this project, the following general conclusion is drawn:

• When 70% crushed recycled concrete and 30% bottom ash are blended and mixed with a 15% blended (50PC/50FA) cementitious material by mass, in-situ mixed with an asphalt reclaimer/pulverizer, moisture conditioned and compacted, a compressive strength on the order of 3100 psi in one year was attainable.
• When the aggregate blend is mixed using slag cement in lieu of Portland cement in the cementitious material blend, a compressive strength on the order of 1700 to 2000 psi in one year and 2000 to 2500 psi in 2 years was attainable.
• After 2 years of service, the concrete is not showing any significant distress due to freezing and thawing, except for some scaling near the storm water outlet that had excessive moisture contents during construction. There are no indications of structural failure despite high compressive loads from trucks, loaders and cranes that have used the pad. Typically, the Eco-Pad was covered with at least 2 feet of stockpiled bottom ash over the winter months, thus providing some freeze thaw protection. Saw cutting may not be necessary if random cracking can be tolerated.
• Future research and demonstration should explore in-situ mixed concrete using recycled concrete, bottom ash, Class C fly ash, and both Portland cement or slag cement to develop strength at a faster rate.

“Eco-pad” at Menomonee Falls Service Center

A second “eco-pad” pavement was constructed in the Fall of 2011 at Menomonee Falls Service Center (MFSC), located in Menomonee Falls, Wisconsin. The pavement was developed on an area of approximately 100 ft. by 165 ft. and was mixed in-situ with over a 90 percent recycled material content consisting of recycled concrete, bottom ash and cementitious Class C fly ash combined with Portland cement. The purpose of the MFSC “eco-pad” pavement was for support of heavy construction equipment (cable spools and transformers) and long term durability. Other locations at the MFSC have concrete slabs to serve the same purpose but were not as cost effective or environmentally friendly as an “eco-pad”.

A 9-inch thick uniform compacted bottom ash base grade was established for the “eco-pad” pavement of which 3 inches was later incorporated into the concrete by in-situ mixing and 6 inches remained as the base. A perimeter soil berm with a height of 6 inches and width of 12 inches was graded around the pavement area to contain cementitious powders placed with a vane spreader during the in-situ mixing operations. The in-situ mixing phase consisted of placing 5 inches of crushed recycled concrete across the proposed pavement area with dump trucks and using a road grader to create a uniform layer. The recycled concrete and bottom ash was pre-mixed to a depth of 8 inches with an asphalt reclaimer by making one pass over the pavement area.

The cementitious materials were pre-blended by Lafarge in a 50/50 blend of Portland cement and Class C fly ash and placed dry at a rate of 134 lbs/yd2 with a vane feeder truck over the previously mixed aggregate blend. Then with a second pass, the pavement area was re-mixed with a pavement recycler. After mixing, the aggregate and cementitious materials mixture was compacted with a large vibratory sheepsfoot compactor (minimum of 3 passes), graded, and final rolling was accomplished with a smooth drum roller in the static mode. A target mixture moisture content of 9 to 13% (± 2%) for optimum strength (3000 psi) was required. The compaction of the in-situ mixture was specified at 95 percent of the maximum dry density as determined by the modified Proctor method. Following the final rolling, compaction, and sealing; the next day, control joints were sawed into the pavement at 20 foot grids. An additional application of sealer was applied to control joints following saw cutting and surface washing. Finally, the pavement surface was undisturbed for a minimum of 7 days, where no vehicle or equipment traffic was allowed on the surface during that period.

Field Testing and Results

As mentioned in the first “eco-pad” pavement section, there is a three-stage analysis. The initial stage consisted of performing a grainsize analysis on samples of the field blended aggregates. A laboratory mixture analysis of the 34/66 blend of bottom ash and crushed recycled concrete treated with 20% of the blended cementitious material was also performed to establish laboratory moisture-density and moisture-strength relationship. The test results are shown in Table 6-17. Results of the moisture density relationship testing indicated a higher maximum dry density (130 lb/ft3) than expected (110 lb/ft3) with a lower optimum moisture content (9%).

Table 6-17: The Laboratory Results of the BA/RC (34/66) Blend With 20% Cementitious Material

The second stage of the testing was performed during the field mixing of the blended aggregate and cementitious materials. Using the nuclear gauge method, a field density test was performed during the compaction phase to assess the in-situ moisture content and percent compaction. Additionally, samples of the in-situ mixed concrete were obtained (sample size of 4 inch x 4.6 inch) and compacted in the field by the Modified Proctor method. The field molded specimens were delivered to the laboratory and cured for a period of 7 to 56 days to assess the compressive strength development of the mixture complying with the compaction method, ASTM D-1557. The results are shown in Table 6-18. In summary, by 7-days, the mixture in the cylinders had reached above the optimum strength and both the 28-day and 56-day compressive strengths were over 4,000 psi with the low water-to-cementitious ratio.

The final phase of the testing is to obtain in-situ core specimens which will be taken during Spring of 2012 (after one year of field curing) for testing of compressive strength.

Table 6-18: MFSC Eco-Pad Compressive Strength Data

Chapter 7 - Natural Mined Gypsum and Commercial Applications of We Energies FGD Gypsum

Introduction

Natural Gypsum (68)

During the Paleozoic Era, 600 million years ago when salt water oceans covered most of the earth, gypsum deposits were formed. Gypsum is a non-metallic mineral, found in rock form and among the most plentiful minerals in the world. It is composed of 79.1% calcium sulfate and 20.9% water, by weight. It has the chemical formula CaSO4·2H 2O. In its absolute pure form, gypsum is white. However, gypsum normally contains impurities (such as clay and other minerals and in some cases soluble salts) whose presence makes the rock appear gray, brown, pink, or sometimes almost black.

Gypsum has been known and used from the earliest times. The ancient Assyrians called this rock, Alabaster, and it was used for sculpturing. Five thousand years ago, the Egyptians had learned to make plaster from gypsum and they used it to line the walls of palaces and tombs. It can also be found inside the great Pyramids, still standing unchanged after fifty centuries. The ancient Greeks named this mineral, “Gypsos”, and now it is known in English, as “Gypsum”. They described Gypsos as a material that does not burn. In later years, this unique property of gypsum made it very valuable. In the late 1700s, a French chemist Lavoisier analyzed the chemistry of gypsum. He and the other chemists then ground up gypsum into powder and heated it (calcined) until most of its water content was evaporated. When water was added to the resulting white powder, it formed a pliable, plastic mass known as Plaster of Paris. With such development, the material allowed molding to any desired shape, after which it would harden and retain that shape. Gypsum is the only natural substance that can be restored to its original rock-like state by the addition of water alone.

Based on the fact that gypsum can be calcined when exposed to heat treatment at low temperatures, there were vast increases in utilization of natural gypsum. Some of the modern applications include use as a setting time regulator for Portland cement, as fertilizer, and for soil amelioration. Benjamin Franklin was one of the first individuals to introduce it in the United States, when he used ground raw gypsum on his farm soils and called it land plaster. The largest volume use of gypsum today goes into wallboard manufacturing. In 1894, Augustine Sackett invented the principle of a panel “sandwich” made up of a gypsum core with sheets of cardboard stuck to each side. These gypsum “boards” were formed by sandwiching a core of wet plaster between two sheets of heavy paper. When the core sets and dries out, the sandwich becomes a strong, rigid, fireproof building material to be used as wallboards in construction.

Using natural mined gypsum in the applications identified above involves multiple processing and handling steps. The first stage consists of the preparation of the raw gypsum (rock form) which covers such steps as mining, transporting, and storage, drying, crushing, and grinding. The second stage involves the calcination of this material using a variety of equipment, such as kilns. Before moving to the second stage the natural gypsum needs to meet the requirements of the calcination unit, which includes control of the moisture content and particle size distribution. Usually, the gypsum rock consists of relatively large pieces containing up to 4% free moisture content.

We Energies FGD Gypsum

We Energies began operating a state-of-the-art air quality control system at Pleasant Prairie Power Plant (PPPP) in 2006, at Oak Creek Expansion Generating Units (OCXP) in 2010 and at the Oak Creek Power Plant (OCPP) in 2012. One of the systems, the flue gas desulfurization (FGD) system uses limestone and water in a slurry to wet scrub over 95 percent of the sulfur out of the plant’s combustion gases. The chemical reaction between the limestone slurry and sulfur in the flue gas with the addition of oxygen produces calcium sulfate, also known as FGD gypsum, as a by-product.

As mentioned in chapter 3, FGD gypsum is similar to natural mined gypsum in overall composition, and hence can be used in many of the same manufactured products as mined gypsum. However, there are differences between the two which can either restrict or enhance the use of FGD gypsum in place of mined gypsum. For example, FGD gypsum has a higher moisture content which combined with fine grain size can affect handling and processing at existing manufacturing facilities designed for mined rock gypsum. On the other hand, FGD gypsum requires less grinding than rock gypsum due to its finer grain size. Most new plants for producing wallboard are designed to accommodate FGD gypsum, either solely or in combination with natural gypsum. Chlorides, ash, iron and manganese compounds in FGD gypsum can cause issues such as surface crystallization that can affect paper adherence and color variation that makes it undesirable for some products and applications (69).

In 2007, We Energies began marketing FGD gypsum, produced at PPPP, to produce wallboard building materials for construction use. Soon after, local farmers were interested in applying FGD gypsum to their fields to improve their soils, minimize surface water runoff, and increase crop production. After receiving approval from the Wisconsin Department of Natural Resources and a license from the Wisconsin Department of Agriculture in 2008, We Energies began actively marketing the FGD gypsum for agricultural use as a soil amendment to increase field productivity. Gypsum became a local source for amending soils for southeast Wisconsin famers and a lower-cost alternative when compared to out-of-state sources of mined gypsum. Figure 7 -1 shows the growth in agricultural use at We Energies since 2008 and Table 7- 1 shows a break-down of We Energies FGD gypsum utilization compared to national utilization.

Figure: 7-1: We Energies FGD gypsum utilization growth in agriculture.

Table 7-1: FGD Gypsum Utilization

FGD Gypsum Use in Wallboard Manufacturing

While wallboard has traditionally been manufactured with mined gypsum, the use of FGD gypsum has become increasingly popular due to economic, environmental and the proximity of wallboard manufacture to power plant sources. For states like Wisconsin and Illinois, which do not have gypsum mines, FGD gypsum provides an attractive local alternative to importing mined gypsum from other states such as Iowa or Michigan.

On June 21, 2010, the EPA proposed national rules to ensure the safe disposal and management of coal combustion residuals from coal-fired power plants. The proposed rules primarily address disposal but also mention good utilization practices such as gypsum use in wallboard manufacturing. According to the EPA, making wallboard with FGD gypsum is safe and environmentally friendly. Notably, the EPA’s own award-winning building in Arlington, Virginia is made using wallboard containing FGD gypsum1.

1 http://www.epa.gov/epawaste/conserve/rrr/imr/pdfs/recy‐bldg.pdf

The production of FGD gypsum is a controlled, engineered process designed for quality and consistency. By complying with the environmental regulations, the installation of wet scrubbers on coal-fired power plants reduces sulfur dioxide air emissions, and has resulted in an increase in production of FGD gypsum materials. Prior to using mined FGD gypsum in wallboard manufacturing, the gypsum is calcined. This refers to the removal of one and a half waters of hydration, resulting in calcium sulfate hemihydrate (CaSO4·½H2O). When water is mixed with the powdered hemihydrate, it rehydrates, recrystallizes, and hardens. There are two board types of hemihydrates (beta and alpha), which depend on the calcination process used. Beta hemihydrate is formed by heating the gypsum under atmospheric pressure conditions; and alpha hemihydrate is formed by heating the gypsum under induced pressure. Beta hemihydrate is commonly referred to as “Plaster of Paris” or “stucco”, which is commonly used in standard wallboard. Alpha hemihydrate is referred to as gypsum cement, which is more expensive to produce and is utilized in flooring and high strength materials. After calcining, slurry of beta hemihydrate, foam and other additives are blended in a mixer. Set retarders may be added to the mixer to prevent premature hardening of the plaster. The slurry is then placed between two continuously moving sheets, one above and one below, and allowed to harden, forming the board. Once the material hardens, gypsum crystals form and bond to the cardboard. After hardening, the still-wet boards are sent to a dryer, where additional moisture is removed, for about 45 minutes. After drying, the boards are cut to lengths typically ranging from 8 to 14 feet. Table 7-2 shows the general specifications for FGD gypsum used in wallboard which may vary by manufacturer and the product being produced (69) and Figures 7-2a and 7-2b show a typical FGD gypsum wallboard process.

Table 7-2: General Specification for FGD Gypsum Utilization in Wallboard

The following general guidelines are followed with respect to FGD gypsum characteristics to meet product specifications (70):

a. Gypsum Purity

High purity in FGD gypsum (> 95% calcium sulfate) is desired by the manufacturer because lower weight board can be produced.

Also, higher purity reduces the potential for deleterious effects on the wallboard. In 2010, We Energies FGD gypsum had a combined average of 97.5% purity from its two power plant sources (PPPP and OCXP).

Purity can be determined by several test methods including Differential Scanning Calorimetry/Thermal Gravimetry (DSC/TGA), X-Ray Fluorescence Spectroscopy (XRF) and SO3 analysis.

b. Free Moisture

The free moisture in FGD gypsum supplied by We Energies is approximately 8% compared to mined gypsum with 0% to 3% moisture. Sometimes wallboard manufacturers will blend FGD gypsum with mined gypsum depending on the thermal drying capacity of the system. The high moisture gypsum has the tendency to stick and build up on the conveying equipment. Hence, the high moisture reduces the usage of FGD gypsum and also its value.

Free moisture of mined and FGD gypsum materials are determined using a simple oven weight loss method per ASTM C-471.

c. Impurities

The type and quantity of impurities can have an impact on qualifying the FGD gypsum for uses and include:

1. Residual carbonates: Unreacted limestone (Ca/MgCO3) is the predominant impurity found in many FGD gypsum sources. Since limestone remains chemically inert during the board conversion process, there is no interference. However, increased wear on processing equipment results from encountering higher amounts of hard limestone (Mohs value 3 - 4) compared to gypsum (Mohs value 1.6 – 2).

Limestone quantity can be determined through XRF oxide analysis of calcium and magnesium in conjunction with CO2 analysis by coulometric titrimetry. Alternatively, CO2 can be quantified through Differential Scanning Calorimetry (DSC) scans.

2. Fly ash: The concern arises in the chemical variability associated with the burning of different coal sources. It can affect the paper to core bonding and cause increased wear on process equipment as it contains silica and iron. Another concern with fly ash are certain trace elements in raising question from some sources on industrial hygiene issues.

Fly ash can be detected using a scanning electron microscope (SEM). By using image analysis, an estimate of the amount of fly ash present can be established. Fly ash can also be calculated by determining the mass balance around the scrubber and dust collection system.

3.  Silica (SiO2): Silicon dioxide is an impurity found in clay, fly ash, or quartz which can raise a concern from an industrial hygiene perspective. It becomes an issue when high quantities of respirable silica of 0 to 4 microns are present. Additionally, low amounts (1-2%) of crystalline silica/quartz with Mohs value of 7 can cause wear on gypsum processing equipment.

Silica can be quantified using XRF. X-ray diffraction (XRD) is used to identify whether the SiO2 present is amorphous or crystalline in nature. In addition, ASTM C-471 describes a wet chemistry method to determine SiO2 and insoluble matter.

4. Calcium Sulfite (CaSO3 ·½H2 O): This is an unwanted impurity in FGD gypsum as it can cause cake washing and dewatering problems. Usually materials rich in calcium sulfite are landfilled.

Thermal analysis and XRF will detect sulfite above 0.1%. Titration procedures are also considered in determining sulfite.

5. Soluble Salts: Soluble salt impurities affect the physical properties of gypsum wallboard. Salts go into solution when calcined gypsum is mixed with water and other additives in the board mixer. During the drying process in the kiln, the salts migrate to the paper and core interface, which interrupts the paper and core bond. Since salts are very hydroscopic and cause moisture to deposit in the critical bond area of the board, the salts can cause detachment between the board paper and the core. The four soluble salt ions typically monitored are magnesium (Mg2+), potassium (K+), sodium (Na+) and chloride (Cl-).

Soluble salt analysis for FGD gypsum sample is determined on mathematical reconstruction based on the theoretical solubility of the ions.

6. Trace Elements: Due to industrial hygiene concerns, the trace elements and the pH of the FGD gypsum are evaluated. Trace elements are unwanted impurities that are found in limestone minerals, fly ash, and can also increase process equipment wear. We Energies FGD gypsum pH ranged from 7.6 to 8.2 in 2010.

Studies have been performed (71) to determine whether trace quantities of mercury were being released into the atmosphere as part of gypsum used as a feedstock for wallboard production. Another question was to evaluate the potential for leaching in groundwater when wallboard is disposed in municipal landfills. As per the Ontario Hydro method, the measured mercury loss mass rates from the FGD gypsum feedstock ranged from 0.01 to 0.2 pounds of mercury per million square feet of wallboard produced. On the other hand, according to TCLP methodology, the wallboard did not produce measurable mercury concentrations as a leachate (<0.25µg/L). The TCLP maximum concentration allowed for mercury is 200µg/L.

Trace elements can be determined through several methods including Atomic Absorption/Emission (AA) with a graphite furnace option, wet chemistry methods, Inductive Coupled Plasma (ICP) and XRF.

7. Organic Impurities: In the board conversion process, organic impurities can affect the gypsum rehydration step. It can easily cause the rehydration time to lengthen and cause the board line to slow down and reduce production. Also, organic impurities can affect the crystal growth and reduce the strength development of the gypsum core.

Organic impurities can be identified through several methods: coulometric titrimetry, Infrared spectroscopy (IR), Nuclear Magnetic Resonance (NMR) and High Performance Liquid Chromatography (HPLC).

d.  Physical Properties

Even though mined gypsum and FGD gypsum are chemically equivalent, they are physically different in particle size and shape. Hence, this factor has to be accounted for when blended together.

Figure 7-2a: Process of manufacturing wallboard using both Natural and FGD Gypsum. Illustration used from United States Gypsum Company.

Figure 7-2b: Photographs of process of wallboard manufacturing steps using FGD Gypsum. Illustration used from National Gypsum Company.

FGD Gypsum Use in Portland Cement2

Portland cement is a mixture of compounds formulated by burning limestone and clay together at high temperatures ranging from 1400ºC to 1600ºC. Portland cement is utilized throughout the construction industry in a variety of applications, one of which is as an ingredient in the production of concrete. Portland cement consists of five major compounds: Tricalcium silicate (50%), Dicalcium silicate (25%), Tricalcium aluminate (10%), Tetracalcium aluminoferrite (10%) and Gypsum (5%). When water is added to cement, each of these compounds undergoes hydration resulting in the final hardened product. Uncalcined FGD gypsum (as a replacement to mineral gypsum) is used as an additive to Portland cement to serve as a set retardant in the mixture. Figure 7-3 shows a simple flow diagram of Portland cement production.

Figure 7-3: A flow diagram of Portland Cement production with full-blown schematic diagram of the rotary kiln. Diagram provided by University of Illinois (Materials Science and Engineering).

2 http://matse1.matse.illinois.edu/concrete/prin.html

FGD Gypsum Use in Agriculture

For many centuries, gypsum has been used in agriculture as a soil amendment, conditioner and fertilizer. Due to its chemical make-up, it provides soluble sources of calcium and sulfur, supplying needed nutrients and improving plant growth. Without the use of gypsum, soil compaction prevents root penetration, aeration and water infiltration. Also, the loss of soil permeability causes saturation of the soil with salt or other elements that can be harmful to plant growth and health. Some of the physical benefits of utilizing gypsum include promoting clay flocculation for air and water movement, correcting for subsoil acidity by decreasing the toxic soluble aluminum, enhancement of root penetration and assisting in reclaiming sodic soils.

In 2007, the Electric Power Research Institute, the Ohio State University and various other electric companies in the United States initiated a research project using FGD gypsum in agricultural applications to evaluate the effect on soils chemical and environmental properties for comparison to natural gypsum used in agricultural applications (72). In 2009, We Energies joined this network to help acquire additional scientific information to demonstrate the benefits of the FGD gypsum. The company gained regulatory approval for land application of FGD gypsum from the Wisconsin Department of Natural Resources (DNR) in 2008 and the Illinois Environmental Protection Agency (IL EPA) in 2010. We Energies marketer of agricultural gypsum also obtained regulatory approval for use in Indiana.

We Energies collected samples of commercial sulfur containing fertilizers and soil amendments from three sources, mined gypsum (“Top Grow” brand by ASC Mineral Processing, which is a Pelletized Gypsum), and sulfur fertilizer (“Hi Yield” brand used for soil treatment - Aluminum Sulfate and Ammonium Sulfate) for comparison to PPPP FGD gypsum and performed analyses of the chemical content and leaching characteristics in accordance with the ASTM standards specified in Wisconsin NR 538 requirements for the beneficial use of industrial by-products. The results are shown on Tables 7-3 and 7-4 respectively. This information was used to support We Energies exemption request to allow beneficial use to proceed in Wisconsin. As shown in Table 7-4, it is important to note that sulfate is the primary leachate component and its useful presence in FGD by-products, FGD gypsum can be sought as a valuable source for farmers to treat soils as an alternative to mined gypsum. The results show that the mined gypsum sample exceeded the Category 1 bulk chemical guidelines for As (0.76 ppm) and Be (0.052 ppm). Further, the mined gypsum sample exceeded the Category 1 leachate guidelines for Cr, Fe, Pb, and Mn. As for both Aluminum sulfate and Ammonium sulfate, they also exceeded numerous Category 1 leachate guidelines. Category 1 leaching guidelines were exceeded for the PPPP FGD gypsum parameters fluoride and manganese. Thus, FGD gypsum has been exempted and licensed for use in agriculture in Wisconsin. Lastly, the very low volume filter cake produced by the FGD wastewater treatment process was also tested and found to be unsuitable for use in agriculture.

Farmers began adopting We Energies gypsum materials as they learned about the local source availability and the benefits of using a more concentrated form of FGD gypsum (>95%) thus providing more calcium sulfate per ton than mined gypsum. Due to the nature of the production process of FGD gypsum, it yields a consistent fine and uniform particle size, which not only provides rapid release of the calcium and sulfur into the soil, but can be easily applied with conventional lime spreading equipment. Wisconsin farmers now have the advantage of obtaining the material at a lower cost since the FGD gypsum is produced in southeast Wisconsin. Approvals for agricultural use of We Energies FGD gypsum have also been obtained for several counties in Illinois and Indiana.

Table 7-3: Chemical Contents of Mined Gypsum, Soil Fertilizers and We Energies FGD Gypsum and Filter Cake

Table 7-4: ASTM D-3987 Leachate Test Results of Mined Gypsum, Soil Fertilizers and We Energies FGD Gypsum and Filter Cake

Benefits of Using Gypsum for Agricultural Purposes (73)

Source of Plant Nutrients:

The composition of pure gypsum (CaSO4·2H2O) is 79% calcium sulfate (CaSO4) and 21% water (H2O). It consists of 23.3% calcium (Ca) and 18.6% sulfur (S) and provides an excellent source of soluble plant nutrients in the soil (Figure 7-4) 3. Calcium is an essential component of plant cell wall structures providing strength in the plant. It plays the role of counteracting the effects of alkali salts and organic acids within a plant. Sulfur is an essential plant food for production of protein, promoting activity and development of enzymes and vitamins. It improves root growth and seed production.

Figure 7-4: Soil Structural Difference – Control (left) and Gypsum (Right). Diagram provided by Agricultural Research Service 3.

Source of Improving Soil’s Chemical and Physical Properties:

Farmers with various crops face sulfur deficiencies due to a combination of factors. These factors include “increased crop yields that result in more sulfur removal from soil, reduced sulfur inputs contained as by-products in other nutrient fertilizers, and decreased sulfur deposition from the atmosphere” (72). Hence, gypsum is used as a sulfur fertilizer. Sulfate in the gypsum is the most favorable form for the plant roots to absorb sulfur to enhance crop production and increase resistance to environmental stress and pests.

Gypsum is also used as a calcium fertilizer to help improve the soil’s physical properties. Without adequate calcium, the biochemical uptake mechanism would fail. Soils that are Ca-deficient in the humid regions have the tendency to disperse and form a stable suspension of particles in water. In other words, highly hydrated ions, such as Na+ or Mg2+ repel the clay particles causing soil erosion. Thereby, adding the gypsum allows an amendment for sodic soil reclamation. This means, the Ca2+ that is provided by the gypsum is exchanging with Na+ and Mg2+, leading to clay flocculation in the soil. Clay flocculation is the “coagulation of the individual clay particles into micro-aggregates” (73), thus improving the soil structure for root growth and air and water movement as shown in Figure 7-5. The flocculation also prevents crusting of soil and aids in rapid seed emergence for no-till field crops. The crust formation is a result of rain or sprinkler irrigation on unstable soil.

3 http://library.acaa‐usa.org/5‐FGD_Gypsum_Influences_on_Soil_Surface_Sealing_Crusting_Infiltration_and_Runoff.pdf

Figure 7-5: Flocculation effect where soluble electrolyte, Ca2+ from the FGD gypsum overcoming the dispersion effects of highly hydrated ions, Mg2+ and promoting structural development.

Plants growing in acid soils can be chemically detrimental as they can be prone to high concentrations of soluble aluminum. “Subsoil acidity prevents root exploitation of nutrients and water in the subsoil horizons” (73). Even though the soil has low pH, the presence of high levels of exchangeable aluminum (Al 3+) makes it very toxic to most plant roots. Gypsum being a neutral salt and not a limiting agent, does not change the soil’s pH but rather enhances the root tolerance from acid subsoil. The addition of the FGD gypsum “can ameliorate the phytotoxic conditions arising from excess soluble aluminum in acid soils by reacting with Al3+, thus removing it from the soil solution and reducing its toxic effect on the plant roots” (73). This leads to an increase in calcium supply to lower depths for root uptake of water and nutrition from the subsoil layers as shown in Figure 7-6. Gypsum utilization can also improve the water-use efficiency of crops that are grown in dry areas or during times of drought.

Figure 7-6: FGD gypsum (CaSO4) forming soluble complex with Al³⁺ and reducing the soil toxicity with exposure to Ca³⁺ for root uptake.

The application of FGD gypsum has been shown to improve surface water infiltration rates and percolation by inhibiting and delaying surface seal formation. This also reduces soil erosion by flocculated clay particles which inhibit the soil to move offsite.

Phosphorus is an essential macronutrient required by the plants, to transfer energy from one reaction to drive another reaction within cells. Having adequate phosphorous available to the plants stimulates early plant growth and accelerates maturity. However, many soils are highly enriched with soluble phosphorus when manures or fertilizer phosphorous are heavily applied without proper soil testing. It gets very critical when eroded sediment are easily transported by storm water towards streams, ponds and wetlands as phosphorus is carried along with the sediment from the agricultural field causing eutrophication. “Eutrophication is defined as excessive nutrients in a lake or other body of water” (73). Hence, with the FGD gypsum application, the soluble calcium binds with the soluble reactive phosphate (SRP) forming an insoluble calcium phosphate precipitate (shown in Figure 7-7) improving the water quality with decreased runoffs.

Figure 7-7: Effect of FGD gypsum on Erosion. Illustration provided by Agricultural Research Service.

In summary, gypsum can provide many physical and chemical benefits to soil in addition to nutritional benefits.

• Improves soil structure with flocculation effect for root growth and air and water movement
• Prevents crusting of soil and aids in seed emergence
• Improves infiltration rates and hydraulic conductivity of soils to have adequate drainage
• Reduces erosion losses of soils and nutrients and phosphorus concentration in surface water runoff
• Corrects for subsoil acidity and aluminum toxicity.

Chapter 8 - Fly Ash Stabilized Cold In-Place and Full Depth Reclamation of Recycled Asphalt Pavements, Stabilized Soils, and Stabilized Coal Ash

Introduction

We Energies conducted studies in cooperation with Bloom Consultants, LLC and the Center for Highway and Traffic Engineering at Marquette University in Milwaukee, Wisconsin to evaluate the potential application of fly ash in asphalt pavement construction. In a typical cold in-place recycled (CIR) application, existing hot mix asphalt (HMA) layers are pulverized, graded, compacted and used as a base layer for a new hot mix asphalt surface. In most CIR applications, the existing HMA layers are pulverized to the full thickness, and in some cases through the top 2” or 3” or the entire depth of aggregate base. The CIR material is sprayed with water to get the desired moisture content. The material is graded and then compacted with vibrating steel drums and pneumatic tired rollers.

In recent years, stabilizers have been added into the CIR materials to improve the structural capacity of the CIR layers. In these studies self-cementing Class C fly ash was used to bond with CIR materials and the long-term performance of the final pavement section is being monitored.

In addition, Class C fly ash was used by We Energies to stabilize a coal ash fill surface to construct a commercial office building parking lot on top of the coal ash fill area.

We Energies also conducted a study that demonstrated the use of industrial by-products (Class C fly ash, bottom ash and cement kiln dust) with the recycling process known as full depth reclamation (FDR) of asphalt pavements. “FDR is a process of pulverizing a predetermined amount of flexible pavement that is structurally deficient, blending it with chemical additives and water and compacting it in place to construct a new stabilized base course” 1. This process was developed for road reconstruction with longer life than the traditional roads and uses fewer resources, making it more sustainable and economical.

Case Study I: Highland Avenue, Mequon

A 1.5 mile long section of West Highland Avenue, between Wauwatosa Avenue and Farmdale Road, was resurfaced in 1997. The existing pavement had a 5½” thick asphaltic surface with an aggregate base varying in thickness from 7” – 18”. This stretch of road is a two-lane cross section with an average annual daily traffic (AADT) of 1150. The pavement was constructed over a natural cohesive soil subgrade material.

A 1.5 mile length of the pavement was re-surfaced, two 800 ft. long test sections were stabilized with a fly ash binder and an asphalt emulsion binder respectively. The project was undertaken in August of 1997. The existing HMA surface was pulverized to a total depth of 8” then graded and compacted using standard procedures.

The 800 ft. asphalt emulsion stabilized test section was constructed by repulverizing the upper 4” of the CIR base, and incorporating emulsified asphalt at a rate of 1½ gal/yd2. The base was then graded and compacted. The 800-ft. length of fly ash stabilized section was constructed by applying 35 lbs/yd2 of Pleasant Prairie ASTM C-618, Class C fly ash over the pulverized CIR base and repulverizing the top 5” of CIR base. The pulverized layer was shaped with the grader and moistened with surface applied water, at the rate of 8 gal/yd2. The stabilized base was graded and compacted similar to the other test section.

The asphalt emulsion stabilized test section received a 3½” HMA surface, and the fly ash stabilized test section received a 4” HMA surface. The remaining portion of the pavement received a 4” HMA surface without repulverization of the base. Due to the lack of established procedures and equipment to transfer fly ash from the supply tank to the spreader truck and in spreading fly ash, some delays and dusting problems occurred. This problem has now been solved by using a vein feeder spreader for the fly ash and by addition of water to the reclaimer mixing chamber.

1 Wolfe, W., Butalia, T.S., and Walker, H., “Full-Depth Reclamation of Asphalt Pavements Using Lime-Activated Class F Fly Ash: Structural Monitoring Aspects”, The Ohio State University, Departement of Civil and Enviornmental engineering and Geodectic Science, 2009.

Figure 8-1: Fly ash being placed uniformly on the pulverized pavement.

Figure 8-2: Pavement being repulverized after fly ash application.

Pavement Performance

Representative sections, 500 ft. each in length, were selected from the asphalt emulsion stabilized, fly ash stabilized and control sections. Visual inspections performed on these three sections did not show any surface distress (i.e., cracking, rutting or raveling). Nondestructive deflection testing using the Marquette Falling Weight Deflectometer (FWD) was conducted prior to the construction, after initial pulverization, after one year, and after six years of service to establish structural integrity of each test section. This data was used to back calculate in-situ subgrade resilient moduli and the structural number of the pavement (74).

The preconstruction and post pulverization structural number (SN) results (back calculated) indicate general agreement between section uniformity of the upper pavement layers. The post construction testing and back calculation of SN shows that the fly ash stabilized section gave an 8.6% increase in SN (2.53 vs. 2.33) when compared to the control section. Also the fly ash stabilized section gave a 4.6% increase in SN (2.53 vs. 2.42) compared to the asphalt stabilized section, after making adjustments for the difference in thickness of the HMA surface.

Using the back-calculated SN values of the pavement sections, the structural coefficients of the stabilized and unstabilized CIR base material were calculated. The structural coefficient was found to be 0.11 for the untreated CIR base layer, 0.13 for the asphalt emulsion stabilized layer and 0.15 for the fly ash stabilized base layer.

Based on the 1993 edition of the AASHTO Guide for Design of Pavement, an estimate of the allowable number of 18,000 lb. equivalent single axle loads (ESALs) was determined. In this calculation, a design reliability of 85%, an overall standard deviation of 0.35 and a design serviceability loss due to traffic of 2.0 were used. Figure 8-3 shows the allowable ESALs vs. SN (structural number) for the range of subgrade resilient moduli exhibited within the test sections. By holding the subgrade resilient modulus constant and adjusting the asphalt layer coefficient to 0.44, the structural numbers were recalculated. The revised values of SN are as follows:

Control section = 2.65

Emulsion stabilized section = 2.74

Fly ash stabilized section = 2.85

Figure 8-3: Allowable Traffic Estimates

The allowable traffic estimate based on the revised SN provided a more meaningful comparison. Based on the revised SN, the fly ash test section provided a 58% increase in allowable traffic compared to the control section and a 28% increase in allowable traffic compared to the asphalt emulsion test section. Long term testing of the pavement is required to understand its behavior. However from the studies completed to date, the fly ash stabilized CIR section appears to have good potential.

Falling Weight Deflectometer tests were conducted again in October 2003, approximately six years after construction, within the control section, the emulsion stabilized section, and the fly ash stabilized section. Surface deflections were used to back calculate subgrade and pavement parameters including the flexural rigidity of the upper pavement layers and the effective structural number of the pavement (75).

Figure 8-4 provides a summary of the back calculated effective structural number (Sneff.) As shown, the Sneff of the fly ash stabilized section is greater than comparable control or emulsion stabilized sections with the exception of the westbound emulsion stabilized section with a stronger subgrade.

In general, after six years of service the structural integrity of fly ash stabilized section of Highland Road appeared to be equal or better than both the control and emulsion stabilized sections. From a condition standpoint, all sections are performing well with no observed surface cracking.

Figure 8-4: Comparison of Effective Structural Numbers (Sneff.) (75)

Case Study II: CTH JK, Waukesha

County Trunk Highway JK is located in Waukesha County, Wisconsin and the project segment runs between County Trunk Highway KF and County Trunk Highway K, with a project length of 3,310 ft. It is a two-lane road with an average daily traffic (ADT) count of 5,050 vehicles in year 2000 and a projected ADT of 8,080 in design year 2021. The existing pavement structure consisted of approximately a 5” asphalt concrete surface layer and a 7” granular base course.

The project scope included construction of a reinforced concrete pipe culvert. The contractor completed this task prior to starting the paving. The base course of the pavement section at the culvert for a length of approximately 50 feet was constructed using crushed aggregate, instead of fly ash stabilized CIR materials. Prior to construction of the road, undercutting was performed at places where severe pavement distresses existed. The pavement was excavated to a depth of 2 feet underneath the existing base course and was filled with breaker run stone. Initial pulverization started on October 9, 2001. The existing HMA pavement was first pulverized to a depth of 5”. After spraying water on the surface of pulverized materials, the pavement was repulverized to a depth of 12” and was graded and compacted by a Sheep’s Foot Roller.

Fly ash was placed on October 11, 2001. Fly ash was transferred from the supply tanker to the vein feeder spreader truck through a hose, which significantly reduced dusting. The vein feeder spreader truck applied the fly ash at an application rate of 8% by weight. The feed gates from the spreader truck provided a six ft. wide surface application. Water was sprayed to obtain the water content of the stabilized CIR materials to the desired 5.0% moisture content. The fly ash and moisture contents were controlled by an operator, based on field experience. The mixing operation commenced immediately after distribution of fly ash over a length of approximately 100 feet and was completed within one hour, using the pulverizer. Compaction of the mixture began immediately after mixing and was completed within one hour following spreading of fly ash. The compaction of the base course included 6 passes of the Sheep’s Foot Roller followed by 2 passes of the Vibratory Drum Compactor.

A laboratory mix analysis to evaluate the stabilization potential of recycled pavement material with Class C fly ash was conducted. A field sample of existing asphalt pavement and underlying aggregate bases was obtained from CTH JK. The results of the grain size analysis on the CIR material indicated a sand and gravel mixture with trace fines. The analysis showed that the sample contained 68% gravel (larger than #4 sieve), 26% sand (between #4 and #200 sieves) and 6% silt (between #200 sieve and a size of 0.005 mm) and clay (between 0.005 and 0.001 mm) size particles. Evaluation of fly ash stabilized CIR material was performed at two fly ash contents, 6% and 8% by dry weight of total mix. Laboratory analysis of the fly ash stabilized materials was in accordance with ASTM C-593, where the Moisture-Density (ASTM D-1557) and Moisture-Strength (ASTM D-1633) relationship of specimens compacted in a 4” diameter mold was obtained. Results of the moisture density relationship test on the recycled asphalt pavement indicated a maximum dry density of 141.7 pcf at an optimum moisture level of 5.0%. In addition, moisture density relationship tests on the recycled asphalt pavement material with 6% and 8% fly ash added indicated a maximum dry density of 142.3 pcf and 142.9 pcf at an optimum moisture content of 5.5%, respectively. A maximum unconfined compressive strength of 250 psi and 380 psi at an optimum moisture content of 5% were also obtained after seven days curing, respectively.

Pavement Performance

Pavement performance of CTH JK was evaluated using the FWD test in each year between October 2001 and 2008. The results of the testing indicate that the strength of fly ash stabilized CIR recycled asphalt base course developed significantly and the modulus increased from 179.7 ksi in 2001, to 267.91 ksi in 2002, and to 328.82 ksi in 2003. The layer coefficient of fly ash stabilized CIR recycled asphalt base course was 0.23 at time of FWD testing in 2002 and 0.245 in 2003, compared to 0.16 in 2001. No cracking and rutting was identified in the pavement distress survey. Compared to the pavement of CTH VV with untreated CIR recycled asphalt base course, the structural capacity of fly ash stabilized CIR recycled asphalt base course in CTH JK, with a layer coefficient of 0.245, is appreciably higher than that of untreated CIR recycled asphalt base course, with a layer coefficient of 0.13 (76). Figure 8-5 shows the structural number of CTH JK pavement for the first three years.

Figure 8-5: Structural Numbers of Pavement in CTH JK

In 2004, a few additional transverse cracks were observed, as well as some longitudinal cracks in the traffic wheel paths. In 2005, the longitudinal cracks were more severe, when compared to the 2004 survey. The surveys performed in 2007 and 2008 showed alligator cracking becoming an issue in some locations. However, a crack sealing operation was conducted in 2010 to address those cracking issues. In general, cracking in the pavement represents the most severe threat to this pavement, either due to fatigue represented by alligator cracking, or thermal distress due to weather conditions. Rutting does not present a challenge to this section of CTH JK, as measured in the field.

The most recent visual distress survey was conducted in October 2010 to evaluate the physical condition and distress of the pavement. The visual distress survey determined the type, size, location and degree of severity of distresses present at the time of the inspection. The subsequent evaluation of those distresses included comparing the current survey information with the results observed in previously conducted inspections.

The pavement received a crack sealing treatment in 2010. This report establishes the gains in the pavement physical properties due to the crack sealing treatment. In 2007, this study started collecting visual survey data complying with ASTM D-6433 (Standard Practice for Roads and Parking Lots Pavement Condition Index Surveys). Based on the reports for the years before 2007, it can be assumed that the pavement condition was fair as of 2005. Table 8-1 was prepared to compare the Pavement Condition Index (PCI) calculated according to the procedure, ASTM D- 6433. PCI is a numerical rating of the pavement condition that ranges from 0 to 100, with 0 being the worst possible condition and 100 being the best possible condition. The PCI value is then translated into a verbal rating that ranges from “Excellent” to “Failed”.

Table 8-1: Intensity of Distresses in the Pavement

Table 8- 1 shows that the longitudinal and transverse cracking is the dominant mode of cracking followed by the alligator cracking. For the longitudinal and transverse cracking, the table shows that in 2007 medium severity cracking exhibited an intensity of 14.1%. This value increased to 22.3% by 2008. After the cracking sealing treatment, this value dropped significantly to 0.4%. The table also shows that the rutting severity did not change since 2007. The average rut depth also did not change since 2007.

Figure 8 -6 shows the PCI rating as determined using the ASTM D-6433 standard. It is important to note that the PCI value and the corresponding rating represent a collective quality of the pavement physical characteristics. The pavement is assumed to have a PCI score of 100. Then a deduct value is determined based on the intensity and severity of the recorded distresses.

Figure 8-6: PCI rating for CTH JK since 2007

Figure 8-6 shows that the condition of the pavement was degrading since the pavement is rated poor in 2007 and very poor in 2008. The visual survey in 2010 was conducted after crack sealing maintenance was applied to the pavement. This upgraded the pavement condition to “Fair”. This big jump in the PCI value reflects the severity of the cracking experienced by the pavement. Alligator cracking observed in pavement is an indicator of fatigue failure, and longitudinal and transverse cracking are typically related to either thermal stresses or construction practices.

The rutting and alligator distresses are structurally-related distresses. These distresses are caused by the pavement deflecting under the given traffic loads. This deflection might be due to deformation in the surface layer, or deformation in the pavement system of base and surface layers. The FWD is capable of determining the weak points in the pavement through back calculating of the layers modulus.

It is important to note that the average rut depth measured for this pavement section since 2002 is within the low severity range according to the ASTM D-6433. However, the rut depth recently showed a significant increase. The following plot (Figure 8-7) shows the progression of the rut depth measured since 2002.

Figure 8-7: Rut depth progression since 2002.

The plot shows that between the years 2005 and 2007, the rut depth jumped by 0.26 inch (6.6mm). This represents about 79% of the total accumulated rut depth occurring in just two years. The average rut depth has not changed since 2007, which is recorded at 0.33in (8.82mm). According to the ASTM standard, a rut depth less than 0.5 inch is considered low severity. This indicates that the pavement structure is able to resist the accumulation of severe levels of permanent deformation.

The results of this pavement condition survey are providing an improved rating due to the recent maintenance treatment. It is important to note that this improvement in the rating is due to the mechanism by which the PCI rating is conducted, where the severity of some of the cracks were downgraded to a low severity level as a result of the crack sealing. Yet, this does not mean that the cracks decreased in opening size, or the pavement resistance to traffic loads improved. The best way to evaluate the pavement structural capacity is to conduct testing using the FWD to calculate the layer modulus and structure number. Once this is conducted, the information can be incorporated in the AASHTO Mechanistic Empirical Pavement Design Guide (MEPDG) to conduct further analysis and predict pavement performance in the coming years.

Case Study III: Commercial Office Building Parking Lot

The surface parking lot is located at 3600 S. Lake Drive, St. Francis, Wisconsin. The lot area contained a capped coal ash fill. The coal ash was placed there more than 30 years ago by We Energies. The Class F fly ash and bottom ash were by-products from the former Lakeside Power Plant operation. Due to the large quantity of coal ash, the cost to remove, transport and dispose of the coal ash is prohibitive. Therefore, it was decided to build the parking lot on the existing coal ash fill. Because the coal ash fill did not contain any Class C fly ash, the coal ash was graded and stabilized with Class C fly ash to a depth of 12”. Upon compaction, a 5” asphalt pavement was placed directly on top of the compacted self-cementing fly ash mixture, without the need to use crushed aggregate base course. For the parking lot ramp, a 12” Class C fly ash stabilized sandy clay was used as sub-base directly underneath the asphalt pavement. The construction was done in August 2002.

A significant cost savings of approximately $400,000 was achieved by avoiding the costs associated with removal and hauling of the existing coal ash off site and the need to import crushed aggregate for base course. The life expectancy of the parking lot using Class C fly ash stabilization is expected to be equal to or better than the standard practice of using a crushed aggregate base course material. Figure 8-8 shows the parking lot. The parking lot has been inspected regularly since installation and has performed very well, showing little sign of distress over the years. The last pavement inspection was made in April 2012.

Figure 8-8: Commercial office building parking lot.

Case Study IV: PIPP Haul Road Pavement, Marquette, Michigan (77)

A 3.6 mile length of the landfill access roadway at Presque Isle Power Plant (PIPP) in the Town of Marquette, Michigan, is used by PIPP operations and a portion is used by an iron mine to access its coal and limestone stockpiles. This Marquette haul road was rebuilt during October 2006 using the full depth reclamation (FDR) process. The existing two-lane road was approximately 26 feet in width, with a narrow shoulder in each direction. This road had an asphaltic pavement section and two unpaved gravel sections, which had been exposed to 30 years of extreme weather conditions and heavy wheel loading of plant equipment that often exceeded 195,000 pound gross weight loads. Over time, the paved areas had potholes and widespread cracking and rutting, and the base was degrading. The unpaved sections had potholes, severe rutting, and was slippery during wet weather conditions and dusty during dry periods. The paved section was designed with 3.5 in to 4.0 in of asphalt and 6 in to 10 in of stabilized base course on top of granular subgrade. The objectives of this project were to reduce hauling and maintenance costs, improve safety, improve storm water management and dust control, conserve natural resources, and demonstrate the economics and structural performance of using CCP in road construction.

For this project, We Energies used substantial amounts of CCP for the full depth reclamation of a deteriorating paved roadway and in-situ stabilization of an unpaved roadway at PIPP. The base course layer was stabilized with the introduction of coal combustion by-products from PIPP that included cementitious fly ash (FAC) meeting ASTM C-618 Class C, granular bottom ash (BA), and cement kiln dust (CKD), which is produced by Lafarge North America located in Alpena, Michigan. In addition, reclaimed asphalt material (RAM) and recycled gravel material (RGM) were also used in some sections to improve the base course layer. Eleven mix compositions of BA, FAC and CKD were used and evaluated in this project as shown in Table 8-2.

Table 8-2: Tested Road Base Composition

Soil stabilization was essential for the new asphaltic concrete pavement to support the heavily loaded trucks that haul materials to the power plant’s landfill and to a nearby mining operation. As the cementitious fly ash binder is mixed with RAM or RGM and compacted, it improves the dry density by filling in the voids which, controls the shrink-swell properties by cementing the soil grains together much like Portland cement bonds aggregates together. “By bonding the soil grains together, soil particle movements are restricted and instead solidify into a dense monolith which improves the structural properties of the treated base or sub-base material by spreading the loads over a greater area.” (77). CCP and CKD were both used in stabilizing the base coarse and pulverized with asphaltic pavement.

The pavement system was designed using the AASHTO method of flexible pavement design with a structural number of 4.2. To decrease the cost and thickness of the asphaltic concrete, the team targeted a compressive strength of 300 psi (2.1MPa) based on the past project test results of full-depth reclamation. Prior to construction, samples of the gravel and asphaltic concrete from the existing structures were taken to determine the maximum dry density and optimum moisture for the different mixture combinations. Based on the lab results, the target moisture range and binder content of FAC and CKD were established for each stabilized base mixture.

Typically in-situ stabilized full depth reclamation projects would have two pass processes. The initial pass, using the pulverizer machine, pulverizes the asphaltic concrete and mixes it with the road base and added water. Then the composite is shaped and graded. The cementitious binder is then placed over the prepared material and a second pass with the pulverizer machine mixes both the materials. However, to reduce the costs, the project team decided to mix all the materials with a single pass process. BA from PIPP units 1-6 and 7-9 was an agglomerated ash that was used as a sandy aggregate within the structural pavement. BA was loaded from a stockpile at the landfill and hauled via a live bottom dump truck and placed to a thickness of 4” at the predetermined locations on the existing road sections. Dry FAC was loaded from the PIPP storage silo directly into the vane spreader truck. The vane spreader truck spread the fly ash at an application rate of 110 lbs/yd2 (540 kg/m2) which is approximately 11% by mass of the total stabilized base. The CKD was pneumatically conveyed from a bulk tanker truck to the vane spreader truck at a similar application rate as FAC. The distributer bar on the vane spreader truck was maintained at the lowest position to minimize the drop height of the cementitious powders and to minimize the amount of fugitive dust during the placement period.

After the placement of BA and the cementitious binder (FAC or CKD) on the existing paved and gravel road sections, the materials were pulverized in-situ to a total depth of 6 -10 in. (15.2 – 25.4 cm). The depths were adjusted at some road sections due to the presence of large rocks that would break cutting teeth on the pulverizer machine. A water truck followed the pulverizer and increased the moisture in the mix when needed. The base was immediately compacted with a vibratory sheepsfoot compactor with five to eight passes to achieve 95% of the proctor density. The material was then fine graded and the road crown was shaped using the swell in volume of the mixed materials. Due to high traffic flow, the fly ash stabilized and non-stabilized sections were surfaced with 3.5 in. (9 cm) and 4 in. (10 cm) of HMA (MDOT 13A) surface, respectively. The stabilized base extended one-foot (30 cm) beyond the edge of the pavement to facilitate distribution of wheel loads and protect the HMA pavement from shear failure. The base was stabilized to a width of 28 ft (8.5 m), while paving to a width of 26 ft (7.9 m).

Pavement Performance

Field samples of RAM and RGM, combined with FAC, CKD and BA, were obtained from various designated test sections. Laboratory mix analysis, proctor numbers, optimum moisture contents and compressive strengths were provided by local testing firms. The physical properties of the stabilized base course layers with the combination of different by-products are shown in Table 8-3. The moisture-density relationship was determined using the Proctor test method for each composite material in accordance with ASTM D-698 Method C. Moisture content was monitored during the construction with a nuclear density meter. The nuclear density tests resulted in a compaction range varying from 89.6% to 98.8% of the maximum dry density while the moisture content ranged from 7.7% to 14.9%. Generally, a lower water-to-cementitious material ratio yields higher compressive strength but not necessarily the highest density. The objective of soil stabilization is to optimize the moisture that will yield the highest density and highest compressive strength.

Table 8-3: Summary of the Field Measurements of Physical

The field samples of fresh stabilized base material were compacted into 4 in. (10 cm) diameter by 4 in. (10 cm) high cylinders using the ASTM D-698 Method C, wrapped in plastic and cured at room temperature in the laboratory. All the field samples had compressive strength exceeding the target strength of 300 psi (2.1 MPa) within 56 days and were over 90% of the target within 7 days. The results demonstrated that the FA mixture had the highest initial compressive strength, but at 28 days the BA+CKD mixture surpassed the FA mixture, having the highest compressive strength as shown on Table 8-3 and Figure 8-9. Overall, the composite mixtures for the stabilized base acquired good compaction and early strength development which allowed truck traffic at reduced speeds on the stabilized base after 24 hours of curing thus minimizing disruption to the plant and mining company operations. The early traffic loading did not have any apparent detrimental effects on the base. Within 48 hours, HMA (MDOT 13 A) was laid on top of the stabilized base.

Figure 8-9: Average compressive strength at 3, 7, 28 and 56 days curing period for different composite mixtures. Note: 145 psi = 1 MPa

Figure 8-10 shows the reconstructed Marquette haul road after 3½ years of service. To determine the performance and the structural capacity of the Marquette haul road under traffic, distress identification surveys and FWD testing were conducted on all the test sections approximately 3½ years after reconstruction and four winter seasons. None of the sections exhibited rutting (using straight edge assessment) despite load differences from loaded dump trucks (westbound) going to the landfill and coming back (eastbound) empty, but this may become distinctive as the road ages. Reflective transverse cracking was generally observed in stabilized base areas that had a resilient modulus exceeding 300,000 psi (2,070 MPa). The cores at the cracks indicate that it is not a structural failure of the base material. Longitudinal cracks were generally observed at the center line of the road between differing base mixtures.

Longitudinal cracks were observed within the lanes at hilly areas where there are switch back curves, at intersections, and in the area where there is a higher frequency of heavy truck traffic from mining operations. The cracks were filled with sealer after the third winter season. It is evident that some extension cracks propagated after the fourth winter season. Table 8-4 shows the distress survey and resilient moduli test results.

Ten auger borings were completed with a truck mounted drill rig along the haul road in areas near the midpoint of the test sections or areas suspected of having shallow bedrock. Thicknesses of the pavement layers were measured. Bedrock depths exceeded 15 ft (4.6 m) except for borings within test sections EB6, WB3a, EB10a and WB5 which had an 11 ft (3.3 m) depth, and approximately 13.5 ft (4.1 m) in depth at sections EB2, WB2, and WB8. The flight auger soil samples were classified visually in general conformance with the Unified Soil Classification System (USCS).

A silt layer was observed at each boring location except in the area of EB10b. The depth to the saturated silt was recorded. The thickness of the silt layer ranged from 2 ft (0.6 m) thick to 12 ft (3.7 m) thick. Most of the test sections that had longitudinal and transverse cracking had saturated silt within 3 ft (0.9m) of the pavement surface. The Marquette area typically has frost penetrating over 6 ft (1.8 m) under paved roads. Plant personnel observed heaving conditions in the areas of the shallow silt layer. No heaving was observed at the time of the distress condition survey and the FWD testing, and there was no observable frost remaining in the soil. It is very likely that the frost heave contributed substantially to the transverse and longitudinal cracking.

Figure 8-10: Reclaimed haul road with a HMA surface. (A) after one year, and (B) after four winter seasons.

FWD testing was performed by Engineering & Research International, Inc. (ERI). ERI performed FWD tests at a total of 241 points. A KUAB FWD was used in the test with a two-mass falling weight system to create a smoother rise of the force pulse on pavements with both stiff and soft subgrade. Deflections were measured by nine sensors. The test points were selected based on the sections length and with an intention to reduce the standard deviation of the test results. The FWD test points were equally spaced within the test sections except where cracks were encountered. A FWD survey was conducted in the eastbound and westbound lanes.

An elastic layer analysis was performed to back calculate the pavement layer moduli from the FWD test results by employing the ELMOD 6 software. The pavement temperature during the FWD test was recorded to be 45oF (7.2oC). The back calculated elastic moduli values for asphaltic concrete were corrected to a standard temperature of 70oF (21oC). The layer moduli were back calculated by assuming a three-layer pavement system. The three-layer system consisted of an asphalt bound layer, the base layer, and the subgrade layer. Iterations of the back calculations were performed to yield the lowest combination of standard deviations for the three layers. The bedrock depth did not influence the calculations. Figure 8-11 shows the average pavement layer modulus for the eighteen test sections. Based on the FWD test results, shown in both Table 8-4 and Figure 8-11, the subgrade layer in section EB 8 shows the comparably higher modulus of 25,553 psi (176 MPa) and EB 11 show the comparably lower modulus of 8,847 psi (61 MPa) within the project limit. As for the stabilized base course layer, section EB 6 shows the comparably higher modulus of 486,907 psi (3,357 MPa) and WB1 shows comparably the lower modulus of 41,397 psi (285 MPa). Finally, the asphalt concrete layer in section EB 3b and WB 3b show the comparably higher modulus of 485,480 psi (3,347 MPa) and EB 9a shows the comparably lower modulus of 133,434 psi (920 MPa).

The results (Table 8-4) show that FAC and CKD blended with RAM, RGM and BA improved the stiffness of the base in sections where they were used. The westbound lanes (loaded truck traffic) had moduli that were about 25% to 60% lower than the eastbound lanes (unloaded traffic). The unstabilized BA+RAM base had a higher than expected resilient modulus that warrants additional study. All the test sections had moduli values that were significantly higher than the conventional gravel base (EB11) except for WB1 as shown in Figure 8-12. It should be noted that WB1 did not exhibit any cracking or rutting.

Overall, the utilization of BA, FAC, CKD with the full depth reclamation process for soil stabilization demonstrated important benefits for the road construction industry. The construction of a stabilized base material (with a one pass process) attained high early strength development with minimal impact on traffic while also performing well for over 3½ years of service.

A typical cost for full depth reclamation ranges from $3.00 to $4.25 per square yard. The variation of the cost depends on the depth of pulverization, the amount of binders used, utilization of one or two pulverization passes, water truck requirements, the thickness of asphalt concrete being utilized without compromising the structural performance and finally the location of the road construction. Figure 8-13 shows a brief overview of the environmental benefits in utilizing full depth reclamation versus a new base.

Table 8-4: Distress Condition Survey and Mean Layer Modulus, April 2010

Figure 8-11: The Falling Weight Deflectometer (FWD) test results for the 18 test sections for the individual layer. See Table 8-4 for mixture composition for the individual section number.

Figure 8-12: The FWD – Base layer test results for the 18 test sections.

Figure 8-13: The environmental benefits of full depth reclamation (in-place asphalt recycling with self-cementing fly ash) vs. new base by comparing the use of energy and materials. Note: The above statistics are based on 1 mile (1.6 km) of 24 foot (7.3 m) – width of 2-lane road, 6-inch (150 mm) base.

Chapter 9 - Fly Ash Metal Matrix Composites

Introduction

Metal matrix composites (MMCs) are engineered materials formed by the combination of two or more materials, at least one of which is a metal, to obtain enhanced properties. MMCs tend to have higher strength/density and stiffness/density ratios, compared to monolithic metals. They also tend to perform better at higher temperatures, compared to polymer matrix composites.

Though MMCs have been in existence since the 1960s, their commercial applications have been limited due to their higher cost and lack of proper understanding. More developed MMCs, especially cast aluminum-fly ash composites, have shown the potential of being cost effective, ultra light composites, with significant applications (78). Such composites, if properly developed, can be applied for use in automotive components, machine parts and related industries.

Figure 9-1: Brake drum cast with aluminum ash alloy material in Manitowoc, Wisconsin

Aluminum and magnesium are lightweight materials, when compared to iron and steel. However, they do not have the strength requirements necessary for several applications. Metal matrix composites manufactured by dispersing coal fly ash in common aluminum alloys im-prove mechanical properties such as hardness and abrasion resistance.

Processed fly ash is estimated to cost about $0.10 per pound (including the cost of mixing the ash into the aluminum melt). Aluminum alloy 380 costs approximately $0.70 per pound. An alloy blend containing 40% fly ash would cost about $0.50 per pound, compared to $2.40 to $2.60 per pound for similar conventional aluminum-silicon carbide composites (79).

Preparation of Ash Alloy Metal Matrix Composites

Ash alloy metal matrix composites can be prepared using various techniques. The following methods were studied at the University of Wisconsin-Milwaukee to prepare ash alloys using We Energies fly ash.

• Stir Casting
• Powder Metallurgy Pressure Infiltration
• Stir Casting

Aluminum-silicon alloys (A356.2 and Al 6061) were used in this work which was conducted at the University of Wisconsin-Milwaukee. In the stir casting process, the alloy is melted at a controlled temperature and the desired quantity of fly ash is added to the molten aluminum alloy. The molten alloy is stirred continuously to create a vortex to force the slightly lighter particles into the melt. Stirring continues to disperse the fly ash particles as uniformly as possible in a short time.

The matrix is then transferred into a preheated and precoated transfer ladle. The material is stirred again and then poured into preheated permanent molds. It is then cooled, cut to shape, and surface cleaned.

Photomicrographs of aluminum alloy (A356.2), with a 10% volume of precipitator fly ash showed that fly ash particles tend to segregate along the aluminum dendrite boundary due to particle pushing. Fly ash particles tend to float to the top of the cast ingots due to their lower density. However, the distribution is reasonably uniform except for the top layer.

Powder Metallurgy

Commercially pure aluminum (99.9%) and We Energies fly ash were used in this work. Oven-dried at 110°C, aluminum and fly ash powders were well-blended by using a rotating drum. The amount of fly ash varied from 5 to 10 percent by weight in the mixtures.

Aluminum fly ash samples were compacted at different pressures (20,000 psi to 60,000 psi) using a uniaxial hydraulic press (80). Aluminum and aluminum fly ash compacts were sealed in a transparent silica tube under pure nitrogen and sintered at 625°C and 645°C for 2.5 and 6 hours at both temperatures.

The green density of the aluminum fly ash powder compacts increased with the increase in compacting pressure and decrease in fly ash content. Fly ash particles did not change shape significantly even when sintered at 625°C for 2.5 hours.

The morphology of aluminum powders changes during compaction due to plastic deformation. When the quantity of fly ash in the composite increased above 10% by weight, the hardness significantly decreased, and thus it was concluded that powder metallurgy did not seem very promising for producing ash alloy composite parts.

Figure 9-2: Connecting rods produced at the University of Wisconsin-Milwaukee with aluminum ash alloy material.

Pressure Infiltration

Commercial aluminum-silicon alloy (A356.2) and We Energies fly ash were used in this study. Preforms were prepared by mixing cenospheres and precipitator ash with MAP (mono-aluminum phosphate). The slurry was poured into a mold, dried at 204°C for 24 hours and then cured at 815°C for five hours. The preforms were placed in a graphite die followed by preheating at 815°C for two hours. The aluminum alloy was poured into the die at 840°C. A pressure of 1,500 to 2,500 psi was applied on top of the molten alloy for a period of 10 minutes.

When higher percentages of fly ash are used in ash alloy materials, the pressure infiltration casting technique is preferred. The distribution of fly ash particles is uniform in the pressure-infiltrated casting. The volume percentage of fly ash in the composite can be controlled by controlling the porosity in the fly ash preform, which can be controlled by adjusting the quantity of foaming agent in the preform. The pressure infiltration method gave better castings than the other techniques developed earlier.

Properties of Ash Alloy

In order to determine the suitability of fly ash composites in the manufacture of various automobile and other components, abrasive wear behavior and forging characteristics of composites containing We Energies fly ash were also studied at the University of Wisconsin-Milwaukee.

Abrasive Wear Behavior

Standard Al - 7Si casting alloy (A356) and We Energies fly ash were used in wear tests in the laboratory. Composites were prepared in the lab by stir casting containing 3% fly ash by volume, and composites were also prepared by the squeeze casting technique containing 56 % fly ash by volume. Wear tests were carried out on a FALEX machine. The details of the test procedure can be obtained from reference 80.

The study concluded that:

• Fly ash improves the abrasive wear resistance of aluminum alloy. Specific abrasive wear rate of aluminum alloy with 3% fly ash composites was decreased with increasing load and increasing sliding velocity.
• The aluminum alloy - 3% fly ash composite showed better resistance than the base alloy up to 24N.
• Specific abrasive wear rates of the composite (aluminum alloy with 3% fly ash by volume) decreased with decreasing size of the abrading particles.
• Friction coefficients of the above composites decreased with increasing time, load and size of the abrading particles.
• Observation of wear surface and wear debris shows that fly ash particles in the composite tend to blunt the abrading SiC particles, thus reducing the extent of ploughing.

Forging Characteristics

The hot forging behaviors of Al 6061- fly ash composites were compared with that of the Al 6061 matrix alloys, Al 6061-20% (by volume) SiC and Al 6061 - 20% Al2O3 composites made by Duralcan and Comalco, respectively.

The Al 6061 - fly ash composites were made at the University of Wisconsin-Milwaukee using sieved precipitator fly ash particles obtained from We Energies and cenospheres from another source. The fly ash composites were made using the stir casting and squeeze casting techniques. Table 9-1 is a list of alloys and samples tested in the laboratory.

Table 9-1: Alloy Samples Tested in the Laboratory

Three-inch thick blocks were cut from the ingots and slightly turned to clean up imperfections. The blocks were then coated with either boron nitride or graphite paste to lubricate the ends.

The pieces were then forged in a 150-ton (1.34 MN) hydraulic press at a forging rate of 0.5 in/minute, under a vacuum of 13MPa (97508 torr). The forgings were made at The Ladish Co., Inc., in Milwaukee, Wisconsin. Table 9-2 lists the defects found in each forging.

The study at the University of Wisconsin-Milwaukee led to the following conclusions:

• The A1 6061 fly ash composites containing 5% or 10% fly ash performed similar to the A1 matrix alloys containing no fly ash during forging.
• All castings had porosity which affected forgeability.
• The A1 6061 alloy containing 5% and 10% fly ash forged without cracking. Under similar conditions, A1 6061- 20% SiC and A1 6061-20% A12O3 showed peripheral cracking. A1 6061- 20% fly ash composite showed some cracking. This may be due to non-uniform distribution of fly ash.
• A1 6061- fly ash composites had significant segregations in the forgings due to segregations in the billets. Despite the non-uniformity in the microstructure, these composites can be forged.
• The fly ash particles remained integrated to the alloy particles, showing good microstructure and no debonding. However, during forging some cenospheres collapsed leading to a layered structure of aluminum and collapsed cenospheres.

The University of Wisconsin-Milwaukee study suggests that We Energies fly ash can be used to make composites suitable for forging. However, additional work is being conducted to perfect this technology.

Table 9-2: Defects of Forging Samples Tested

Cenospheres

Cenospheres are hollow, gas-filled glassy microspheres, which normally represent a small portion of fly ash. Cenospheres are formed when primarily CO2 and N2 fill the semi-molten material in a coal-fired boiler. Cenospheres are generally about 1-3% by weight of the total fly ash produced. They are generally gray to buff in color, inert, and primarily consist of silica and alumina. Cenospheres are hard and rigid, light, waterproof, and insulative. Due to their hollow structure, cenospheres have low density (e.g., some cenospheres have a density below 1 g/cm3 and/or have a density as high as 2.9 g/cm3 depending upon the degree of hollowness, the size and the wall thickness of cenospheres) as compared to solid fly ash particles with densities as high as 3.2 g/cm3.

Cenospheres have valuable applications as fillers in the manufacture of paints, plastics, ceramics, adhesives, metal alloys, low density concrete, and lightweight composite materials such as syntactic foams. Cenospheres are also excellent thermal insulators, which is a direct result of their low density.

Cenospheres were harvested from fly ash by other electric power utilities utilizing wet separation methods in fly ash ponds. However, coal ash management regulations are currently under development by the United States Environmental Protection Agency that may eliminate wet handling and disposal methods, thus diminishing the supply of fly ash cenospheres to the market. This change has provided a potential economic opportunity for We Energies to develop processes for the separation of cenospheres using dry handling technologies. Furthermore, cenospheres in Class C fly ash (sub-bituminous coal) cannot be easily harvested using wet methods due to cementitious properties causing rapid solidification and hardening of the remaining fly ash. Therefore, dry separation methods would overcome this limitation on separating cenospheres from both bituminous and sub-bituminous based fly ash (U.S. Patent on “Separation of Cenospheres from Fly Ash”, 8,074,804 B2).

Identifying and Quantifying Cenospheres in Fly Ash (81)

We Energies has utilized methods using a stereomicroscope, polarized light microscopy and a Ferroscope to identify and semi-quantify cenospheres in fly ash samples.

In one method, fly ash samples were mounted on a slide with Fryquel, an organic liquid of known density and refractive index. Cenospheres were distinguished from the fly ash by their “bullseye” pattern (as shown in Figure 9-3), indicating a hollow particle. The quantification was based on volumetric optical classification rather than gravimetric measurements. This method allows for limited density separation of the fly ash particles on the slide and quantification of the density of the cenospheres particles with respect to the mounting fluid (Fryquel).

Figure 9-3: Using dry separation method, the “bullseye” pattern represents the cenospheres (hollow particle) in fly ash samples.

Sedimentation and centrifuging were also examined on a limited basis as an attempt to identify a simpler method to both separate and quantify the amount of cenospheres in a fly ash sample. Keep in mind that water cannot be used as a sedimentation fluid due to the reactivity of sub-bituminous fly ash (PPPP fly ash meets the Class C requirements of ASTM C-618) with water, as mentioned earlier. However, water was utilized successfully in centrifuge tests due to high dilution and the limited fly ash contact time. Other fluids were utilized in both sedimentation and centrifuging tests which included glycerin and synthetically derived heavy fluids.

Properties of Cenospheres in Pleasant Prairie Fly Ash

The observed particle size distribution of cenospheres in fly ash from PPPP is summarized in Table 9-3. The data shows that particle size distribution (measured in volume) can vary from sample to sample. The size range is based on the cenospheres alone. In other words, if 20% by volume of the 09/22/2008 sample shown on Table 9-3 contained cenospheres and 40% are in the 10-30 micron range, then the total fly ash sample contained 8% cenospheres in the 10-30 micron range.

Table 9-3: Particle Size Distribution of Cenospheres in PPPP Fly Ash

The cenospheres produced at other power plants are being marketed worldwide within the size ranges of 10 to 600 microns. Most of the size ranges being marketed are above 70 microns. As seen in Table 9-3, the cenospheres from PPPP fly ash are smaller than most of the cenospheres and have the highest concentration in the 10 to 30 micron size range. The larger size cenospheres are a reflection of the floatation separation method which limits collection to particles that float on water. Smaller hollow particles are less buoyant due to a smaller volume of encapsulated gases. The cenospheres in Class C fly ash are much smaller with diameters as small as one micron, which are anticipated to have greater utility and value.

The density of cenospheres was previously assumed to be less than 1 g/cm3, because they have been only harvested by wet methods. Recent observations have shown cenospheres with particle densities greater than 1 g/cm3, via transmission optical microscopy. The density of cenospheres in PPPP fly ash has been observed to vary from approximately 0.6 g/cm3 to 2.9 g/cm3, with density largely dependent upon the wall thickness of the particle.

The distribution of cenospheres in fly ash captured at various points within the electrostatic precipitators at PPPP was also evaluated. Fly ash captured in the inlet section of the precipitators had a greater percentage of cenospheres and a wider particle size distribution than fly ash and cenospheres captured in the outlet section of the precipitators. This suggests that fly ash from the inlet section of precipitators may be targeted to maximize cenospheres content and particle size distribution. However, such segregation of fly ash from inlet versus outlet hoppers may be impractical during routine power plant operation and the relatively small quantity of fly ash collected in the outlet hoppers.

Since little has been done in characterizing the variability of cenosphere size concentration and properties within PPPP fly ash and other power plants, We Energies concentrated on the fly ash cenospheres from PPPP. This way the variations in coal sources, combustion processes and NOx control additives would be one less concern on the impact of the effectiveness of the dry separation technologies that are being evaluated for a large scale process. Also it is generally understood that reburning fly ash at PPPP enriches the concentration of cenospheres since the previously burned fly ash particles bloat when injected and fired in the furnace a second time.

Recovery of Cenospheres from Fly Ash (US Patent 8,074,804 B2) (82)

Cenospheres can be recovered from fly ash by several methods, all of which take advantage of cenospheres’ low density property. Some of these methods include addition of fly ash to a pond of water, and skimming off cenospheres from the water surface, dry screening of fly ash into coarse and fine particles followed by addition of water to the coarse particles, skimming off cenospheres from the water surface, and drying and storage (U.S. Patent No. 4, 652, 433).

These processes do have disadvantages. For example, these methods only collect cenospheres with a density of less than 1.0 g/cm3 as only cenospheres of these densities float on water. Also, the fly ash that is produced from burning subbituminous western coal includes significant amounts of calcium compounds. For example, fly ash may include 10% or more lime. High calcium fly ashes such as ASTM C-618 Class C fly ash have cementitious properties and therefore, when mixed with water can rapidly harden and the remainder cannot then be easily saved for other purposes such as for use as a cementitious material in the production of concrete. In the case of Class F fly ash, a dry method is also appropriate because it provides the advantage of not expending energy to dry the remainder after separation of cenospheres for other uses. Another disadvantage is that many cenospheres are entrapped in agglomerated and/or hardened masses before floatation occurs. These methods also do not allow the recovery of cenospheres of controlled sizes and densities. As a result, the properties of polymeric composites that include cenospheres cannot be optimized due to the lack of availability of cenospheres with narrowly controlled sizes and densities.

In the We Energies dry process to recover cenospheres from fly ash, size separation followed by the density separation (or vice versa) with methods such as air classification, dynamic air classification, conventional vibratory screening, ultrasonic screening, fluidized bed classification, or a combination of these methods are used.

Air Classification or Fluidized Density Separation

This process involves using a dynamic centrifuge to separate cenospheres from fly ash by density and separation into narrow size fractions. First, the ultrafine fractions of the particles are screened out (which have less cenospheres) using a Micron Air Jet Sieve with 38 or 45 microns sieve (73.40% or 85.68% passing, respectively). The coarser fraction can be classified by mechanical screening and air jetting using a Hosokawa Air Classifier for narrow size ranges. The final classification screening would separate the hollow cenospheres from the fine solid fly ash particles. However, ultrafine fly ash contamination (ie., < 25 microns) was still observed retaining in the various sieve screens utilized to separate the fly ash into narrow size fractions due to agglomeration of the ultrafine fly ash by static charges induced during handling.

Density Classification

This process involves first separating fly ash in terms of density, followed by screening to separate the hollow from the solid particles with a further benefit of establishing narrow particle size ranges.

Use of Microscopy for Identification of Cenospheres

Cenosphere particles are identified by first spreading the particles on a layer of fluid. This is followed by viewing the particles under a light microscope. The particles having a central “bullseye” area are hollow (See Figure 9-3). The total number of particles in the sample are then counted. A ratio of the number of hollow particles to the number of all particles in the sample is then calculated to provide a percentage of cenospheres identified in the sample.

Using various methods of dry separation of cenospheres conducted at a lab-scale (detailed description of the methods in U.S. Patent 8,074,804 B2) resulted in a conclusion that a single step fluidized bed or other classification process is not likely to lead to the separation of cenospheres due to the overlap in size of solid and hollow particles of the same weight of fly ash, and the wide variation in density combined with a tendency to agglomerate. However, fluidized bed classification (density separation) after size separation into different narrow size fractions by screening did yield both solid fly ash and hollow cenospheres particles. The narrow size fraction recovered high volume percentages (above 90%) of cenospheres. Additionally, the usage of transmitted light and reflected light microscopy work, heavy media density separation, centrifugal and settling work quantified the size, weight or volume percentage and density of cenospheres during the different separation processes. With the heavy media density separation, the resulted cenospheres revealed higher densities of up to 2.9 g/cm3 when compared to the usual density less of than 1.0 g/cm3 as they were separated by floatation in water.

Applications of Cenospheres in Manufacturing Products

Cenospheres have been used for more than 30 years and were first used in the United States as an extender for plastic compounds, as they were found to be compatible with plastisols, thermoplastics, latex, polyester, epoxies, phenolic resins, and urethantes. Cenospheres are primarily used to reduce the weight of plastics, rubbers, resins, cements, extensively used as filler lubricants in oil drilling operations under high heat and high stress conditions, and also used in oil well cementing, mud putty and similar applications. The application of cenospheres in gypsum board jointing compounds, veneering plasters, stuccos, sealants, coatings and cast resins take advantage of reducing the material’s weight, increasing filler loadings, providing better flow characteristics, less shrinkage and reduced water absorption.

Listed below are some of the various applications where cenospheres are extensively used:

• Ceramics: Refractories, Castables, Tile, Fire Bricks, Aluminum Cement, Insulating Material and Coatings
• Plastics: BMC, SMC, Injection Molding, Moulding, Extruding PVC Flooring, Film, Nylon, High density Polyethylene, Low Density Polyethylene, Polypropylene.
• Construction: Specialty Cements, Mortars, Grouts, Stuccos, Roofing Material, Acoustical Panels, Coatings, Shotcrete, Gunite
• Recreation: Marine Craft Floatation Devices, Bowling Balls, Surf Boards, Kayaks, Golf Equipment, Footwear, Lawn and Garden Décor.
• Automotive: Composites, Undercoating, Tires, Engine Parts, Brake Pads, Trim Mouldings, Body Fillers, Plastics, Sound Proofing Materials.
• Energy & Technology: Oil well Cements, Drilling Muds, Industrial Coatings, Grinding Materials, Aerospace Coatings & Composites, Explosives, Propeller Blades.
• Concrete Countertops (new and growing application): Cenospheres being a lightweight aggregate and its variability in particle sizes, it can replace the normal-weight and size of the sand used in the concrete. For example, one pound of cenospheres is equivalent to the same absolute volume of about 3.8 pounds of sand. Additionally, it enhances the workability when these small spherical particles act like microscopic ball bearings in the concrete mixture and due to its spherical structural shape, the cenospheres improve the concrete’s density and strength by providing better packing. Finally, cenospheres can be a bulk filler where they can be used in cement grout slurry to replace other ingredients. Therefore, not only the grout increases in volume with cenospheres, the fine aggregate gradation of the particles also helps to reduce shrinkage.

We Energies, along with the Electric Power Research Institute (EPRI) and several other agencies, have been funding research projects aimed at developing technology for manufacturing ash-alloy automobile components. Moreover with the invention of dry methods to separate the cenospheres from fly ash and provide both larger quantities and a wider variety of qualities; the future applications in composite materials look promising.

We Energies holds patents for manufacturing methods with ash-alloy (U.S Patent 5,897,943 and 5,711,362). The first step of one method is to prepare a solid, porous, reinforcing phase preform combined with an aqueous medium comprising a binder, such as sodium silicate and polyvinyl alcohol. The ratio of the reinforcing phase to aqueous medium ranges is from 1:1 to 3:1. The ratio of binder to water in the aqueous medium generally ranges from 1:1 to 1:9, more usually 1:1 to 1:2. Following introduction into the mold, the slurry produced by the combination of the aqueous medium and reinforcing phase is dried to produce a porous, reinforcing material preform at temperatures ranging from 194°F to 482°F for one hour or two. The molten metal is then infiltrated into the porous preform by pressure ranging from about 2000 to 2500 psi. After infiltration, the resultant metal matrix composite is cooled using air drying or low temperature.

Ash alloys containing a volume of over 40% hollow cenospheres are extremely light. It is possible to develop magnesium composites with the density of plastics by proper addition of cenospheres and the use of controlled processes.

The metal matrix composites are produced with an excess of 50% of reinforcing phase. The reinforcing phase is comprised of fly ash combined with an aqueous medium comprising a binder to produce slurry. The slurry is then dried to produce a solid, porous, reinforcing phase preform. Molten metal is then introduced into the preform, resulting in metal matrix composites.

For fly ash preforms, both cenosphere fly ash (density <1) and precipitator fly ash (density >1) were combined with monoaluminum phosphate solution (MAP solution) to produce a pourable slurry for the preparation of aluminum- fly ash metal matrix composites. The preparation used was the squeeze casting or pressure infiltration technique as mentioned above. From the resultant aluminum-fly ash composites, the fly ash was evenly distributed throughout the composite with a percentage of 60% fly ash.

When in preparation of lead-fly ash composites, 40% in volume, cenosphere fly ash was used and the resultant characteristics have shown that the hardness of the material significantly increased and decreased in density compared to a pure lead composite. The cenosphere’s density is 0.48 g/cm3 and lead density is 11.27 g/cm3, and the observed density of lead-40 volume percent cenosphere composite was 7.75 g/cm3.

Extended corrosion tests were conducted for a period of 470 days on lead-fly ash (cenospheres) composites to determine the applicability for use in batteries. 13 mm diameter rods of lead-fly ash specimens were immersed in the electrolyte to a depth of 7.5 mm. The anode specimens were immersed in 5M sulphuric acid and subjected to electrical potentials controlled to stimulate the condition of a lead-acid battery anode under stand-by conditions. All specimens were subjected to the same constant applied potential where the measurements were performed at ambient

room temperature of 20ºC. The current readings for each cell were taken every 24 hours. Observed measurements of current densities of several lead samples and lead-fly ash composites showed close proximity to each other. It was concluded that there may be a little higher corrosion current in the fly ash composites in initial stages due to acid exposure of some surface fly ash particles, but, over time the corrosion current decreased. However, in long term exposure at room temperature, the anode corrosion behavior of lead-fly ash composites was as least equal to, if not better than, pure lead samples. This has shown a potential use in lead-lite batteries (US Patent 5,711,362).

Since the development of new methods of producing metal matrix composites involves reinforcing phase preforms (comprising of fly ash – cenospheres), the percentage of reinforcing material and metal in the composite can be readily controlled, the distribution of the reinforcing phase throughout the matrix can be controlled, the strength of the composite can be enhanced, and the shape of the composite can be readily controlled through the shape of the preform resulting in a wide range of potential applications.

Advantages of Using Ash Alloys

The significance of developing and marketing ash alloys can be fully understood only if we consider the overall benefit to various industries and to the environment. The process of developing an ash alloy matrix with excellent properties is very involved, expensive and lengthy. The following are a few of the benefits that hold promise in providing a significant impact on the community:

• Economics: Ash alloys are at least 10-30% lower in cost than other alloys available in the market. Hence, foundries and auto part manufacturers can potentially realize significant savings that can be shared with consumers.
• Reduced Energy Consumption: With a projected annual displacement of 225,000 tons of aluminum with ash, the savings in energy costs for aluminum production is about $156 million annually.
• Availability of Lightweight Material: The U.S. auto industry has a goal to reduce vehicle weight. Ash alloys are significantly lighter when compared to steel.
• Improved Gas Mileage: Due to the projected significant weight reductions, the gas mileage of U. S. vehicles will improve and the savings will be significant. The Department of Energy’s Light-Weight Materials Program has predicted that a 25% weight reduction of current vehicles would result in a 13% (750,000 barrels/day) reduction in U.S. gas consumption.
• Avoided Ash Disposal Cost: Electric utilities generate approximately 60 million tons of coal fly ash per year, which are landfilled. If fly ash can be sold as a metal matrix filler, utilities would avoid disposal costs and simultaneously generate revenue from the sale of ash. The anticipated market value of processed fly ash is $100/ton.
• Reduced Greenhouse Gases: Greenhouse gases are produced during the two stages of aluminum production; bauxite processing and alumina reduction. Carbon dioxide (CO2) and perfluorocarbons (PFCs) are generated in significant amounts during these processes. Decreasing the production of aluminum or other metals by fly ash substitution will significantly reduce the production of these gases. CO2 emissions would also be reduced by approximately 101 million tons per year.
• U.S. Competitiveness: The U.S. auto parts manufacturers are losing market share to overseas competitors who benefit from low-cost labor. The competitive edge of the United States is its research and development facilities and technical expertise. Development and commercial use of a superior ash alloy matrix at less than half the cost of conventional materials can boost the competitive edge of U.S. parts manufacturers.

These benefits are not limited to the automotive industry. The commercial applications of lighter weight materials, if adapted, can benefit foundries, manufacturers, transportation, construction, electrical and consumer goods industries.

Chapter 10 - Environmental Considerations of We Energies Coal Combustion Products and Regulatory Requirements

Introduction

Fly ash and bottom ash consist primarily of residual inorganic components in coal that are not vaporized or emitted as volatile gases when coal is burned. The ash contains smaller amounts of other non-combustible constituents that are not inorganic such as small amounts of unburned coal. The most common mineral elements found in coal ash in the form of oxides are primarily silicon, aluminum, iron and calcium (60). During coal combustion, FGD residue is produced during the SOx removal process from generated gases. Even though FGD residue does not contain significant quantities of heavy metals, mixing with fly ash can contribute trace elements such as boron, arsenic and selenium to the material making its utilization more challenging. We Energies has employed FGD systems after the fly ash is collected.

Oxidation takes place in the furnace due to the heat of combustion. Coal ash contains trace quantities (in the parts- per-million/billion range) of many other naturally occurring elements in their oxidized form. Coal ash composition and mineralogy, including trace element contents, vary primarily based on the source of coal and the combustion conditions.

The major chemical constituents of both fly ash and bottom ash obtained from the same power plant are essentially the same. However, the availability of minor and trace elements can vary between fly ash and bottom ash. The chemistry of coal ash is very similar to many naturally occurring soils and natural aggregates. The availability of trace elements from all of these materials is directly related to the particle size. Therefore, the leaching potential of fine fly ash is higher than sand to gravel size bottom ash due to the exponentially higher total surface area available in samples of the same mass.

After reviewing research work on the environmental and health risks associated with coal ash utilization, the U.S. EPA determined that coal ash is nonhazardous in 2000. Current Wisconsin and Michigan regulations require lined landfills with leachate collection, covers and a network of monitoring wells when either fly ash, bottom ash and/or FGD material (gypsum or filter cake) is placed in solid waste disposal sites or other non-contained applications to prevent trace elements from reaching drinking water sources. The use of a respirator is also recommended when handling dry fly ash, which is the same for other finely divided siliceous materials.

Precautions are generally taken to prevent ash from blowing or dusting during handling. We Energies material safety data sheets (MSDS) for coal ash and gypsum are included in Appendix A

The utilization of CCPs has several benefits. For example, the controlled emissions from a typical cement plant producing 245,000 tons of cement (which is similar to the quantity of We Energies fly ash used as a cementitious material) are 12,000 lbs. of HCl; 54 lbs. of Hg; 220 lbs. of HF; 171 lbs. of Pb; and 49 lbs. of Se. This is in addition to approximately one ton of CO2 emissions for every ton of cement produced.

Hence, if the entire 245,000 tons of cement is replaced by Class C fly ash (produced anyway from coal combustion), we are reducing CO2 emissions that would otherwise be released into the atmosphere by 490,000,000 pounds. About 11 million tons of greenhouse gas emissions were avoided by using coal ash to replace cement in 2010 alone (ACAA, December 13, 2011). This is a large step in reducing greenhouse gas emissions and preserving our virgin raw materials for future generations (sustainable development).

Chemical Elements in Coal Ash

Coal ash contains many of the naturally-occurring elements, most of them in trace quantities. Table 10-1 gives the list of commonly found elements in coal ash.

Table 10-1: Chemical Elements in Coal Ash

The type and quantity of trace elements in the ash primarily depends on the source of coal. The presence of trace elements in coal ash is a reason that good judgment is required for utilization especially when considering new applications. Many states have regulations that provide guidelines for safe utilization practices.

Leaching From Coal Ash Land Applications

We Energies fly ash, bottom ash and FGD material (FGD gypsum and filter cake) have been successfully used in several varieties of land applications. FGD gypsum is used for wallboard material for construction, cement manufacturing and as a soil amendment in agriculture. FGD filter cake can be used as an admixture for road base material but recently, it is being stored for future landfill construction applications. Bottom ash is commonly used as a replacement for conventional sand, gravel and crushed stone base material for roads, parking areas and building floor slabs, structural fill, backfill, in manufactured soils and recently as a fine aggregate for EcoPads. Fly ash is also sometimes used in the production of CLSM, for soil stabilization, cold in-place recycling (FDR) of asphalt pavements, and as a raw feed material for the production of Portland cement.

We Energies performs total elemental analysis by the Test Method for Evaluating Solid Waste Physical/Chemical Methods (SW-846) and Proton Induced X-ray Emission Spectroscopy (PIXE) methods and leaching tests of ash samples in accordance with the ASTM distilled water method (ASTM D-3987). These tests are used to assess the elemental composition and leaching potential of the ashes as well as to categorize each combustion product source for permitted applications under the State of Wisconsin rules.

The Wisconsin Department of Natural Resources (WDNR) adopted NR 538 in January, 1998, with the purpose of encouraging the beneficial use of industrial by-products. NR 538 also requires generators to provide certification information on their by-products to the WDNR. The results of the total elemental analysis by SW-846 and PIXE methods on We Energies fly ash, bottom ash and FGD material are shown in Tables 10-2, 10-4 and 10-6, respectively. The results of the ASTM D-3987 extraction analysis on We Energies fly ash, bottom ash and FGD material are shown in Tables 10-3, 10-5 and 10-7. NR 538 has defined limits for several categories of industrial by-products based on the concentration of certain specified parameters.

There are five categories in total with Category 1 having the lowest concentration of the listed parameters. Category 1 by-products also have the lowest level of regulatory requirements in terms of beneficial utilization. It can be seen from the following tables that the concentration of elements leaching from fly ash, bottom ash and FGD material is very low. We Energies fly ash, bottom ash and FGD materials contain only very limited quantities of the trace elements.

Most of these parameters meet the requirements set for Category 1 or Category 2 material. The WDNR can grant an exemption to be classified in a particular category if the concentration of one or two elements is slightly in excess of the set limits. However, this is done on a case-by-case basis. If no exemptions are granted, We Energies bottom ash is primarily a Category 2 material and FGD gypsum and fly ash are primarily Category 4 materials (with a few exceptions for both fly ash and bottom ash).

Table 10-2: NR 538 Fly Ash Analysis - Bulk Analysis Data Summary (2010)

Table 10-3: NR 538 Fly Ash Analysis – ASTM D-3987 Leachate Test Result Summary (2010)

Table 10-4: NR 538 Bottom Ash Analysis – Bulk Analysis Data Summary (2010)

Table 10-5: NR 538 Bottom Ash Analysis – ASTM D-3987 Leachate Test Result Summary (2010)

Table 10-6: NR 538 FGD Materials Analysis – Bulk Analysis Data Summary (2010)

Table 10-7: NR 538 FGD Materials Analysis – ASTM D-3987 Leachate Test Result Summary (2010)

Leaching From Products Containing Coal Combustion Products

Fly ash has found great applications in construction products like concrete, CLSM and in the manufacture of Portland cement. It is well established that leaching of trace elements from concrete and fly ash-stabilized clay is negligible. Concrete is very dense and impermeable, making it hard for water to penetrate into the interior of a concrete structure. The reaction products in concrete are stable, dense and do not leach significantly in the natural environment.

The composition of CLSM material is different from that of concrete. It is a low-strength material, often with a compressive strength of less than 300 psi. When prepared with large amounts of fly ash, the permeability is also very low. However, the potential for future removal and handling could allow the material to be broken up into smaller particles with more leachable surface area. Hence, ASTM D-398 Extraction Analysis has been performed on this material to determine the amount of trace elements leached out of high fly ash content CLSM. Table 10-8 shows the total results of total elemental analysis for CLSM produced with PWPP Units 2 and 3 fly ash. Table 10-9 gives the results of ASTM D-3987 Extraction Analysis for the same material. The extract meets all requirements for Category 2 per NR 538.

Table 10-8: Total Elemental Analysis – CLSM Produced with Port Washington Power Plant Units 2 & 3 Fly Ash

Table 10-9: ASTM D3987 Extraction Analysis – CLSM Produced With Port Washington Power Plant Units 2 & 3 Fly Ash

In 2010, a study was performed to evaluate the surface water runoff from OCPP Class C fly ash-stabilized and non-stabilized clay soil exposed surfaces. The primary objective was to determine the potential leaching of fly ash constituents into surface water runoff over fly ash-stabilized clay soil and to assess any adverse environmental impacts to surface water from such runoff. This study involved laboratory simulations using natural soil and fly ash-stabilized soil as test pads exposed to simulated rainfall over varying lengths of time. Then the measurements of analyte concentrations in the runoff water were compared to established benchmarks for surface water quality protection (84).

Table 10-10 shows the results for the analysis performed on the control synthetic precipitation samples collected immediately prior to each simulation. Analytical results for recirculated runoff water with all simulations using stabilized and non-stabilized clay are shown in Table 10-11 and 10-12, respectively.

Table 10-10: Summary of Analytical Results for Unused (Control) Runoff Water (mg/L)

Table 10-11: Summary of Analytical Results for Recirculated Runoff Water for Stabilized Clay under Various Test Simulations

Table 10-12: Summary of Analytical Results for Recirculated Runoff Water for Non-Stabilized Clay under Various Test Simulations

Additionally, a comparison between the OCPP composite fly ash leachate testing and the maximum dissolved concentration measured in runoff water from stabilized clay is shown in Table 10-13.

The synthetic precipitation was tested for the total and dissolved metal concentrations prior to its use in the simulations. The detected analytes and their measured concentrations were generally consistent with naturally occurring levels of trace inorganic constituents in natural waters, hence providing an appropriate simulation of field conditions. Moreover, a comparison of results of the analysis of control water used for stabilized versus non-stabilized simulations indicated very low variability in the constituent make-up of the runoff water used in the test simulation, suggesting that the runoff simulations did not constitute any addition to the leachable fraction of fly ash-stabilized soil. Both the fly ash-stabilized and non-stabilized clay samples for recirculated runoff water resulted in a high degree of consistency between dissolved and total analysis for each simulation, indicating that the trace metals are present as dissolved constituents in runoff water and are not associated with suspended solids or colloidal matter in the water. This indicates that the fly ash has little effect on the dissolution chemistry for the trace metals from the soil matrix. The results indicated by the tables have shown that a relatively small number of analytes are likely to be present in runoff water generated from the surface of clay soils stabilized with OCPP fly ash.

Table 10-13: Summary Results (mg/L) for 2005 Leachate Test of OCPP Bulk Fly Ash

Radioactivity of Coal Ash (85)

Based on the elimination of combustible materials and concentration as a result of coal combustion, the Ra-226 concentrations in ash can be on the order of 1-30 pCi/g. Analyses of various ashes and ash products produced at We Energies plants in 1993 and 2003 found Ra-226 concentrations in the range of 1 – 3 pCi/g. This is comparable to the concentrations in soil (0.2 – 3 pCi/g) and within the range of 1 – 8 pCi/g found in ash from analyses of other fly ash in the US (Cement and Concrete Containing Fly Ash, Guideline for Federal Procurement, Federal Register, Vol. 48 (25), January 28, 1983, Rules and Regulations; Zielinski and Budahn, Fuel Vol. 77 (1998) 259-267).

Given that the ash may be landfilled or may be used in building materials as an aggregate or cementing material, the doses resulting from these applications have been studied to determine if there is any risk. The British Nuclear Radiation Protection Board conducted a detailed evaluation of the doses from fly ash released to the air to people living within 500 meters (547 yards) of a plant stack, to landfill workers burying fly ash, to workers manufacturing building products from fly ash, and to people living in a house built with fly ash building products. The maximum doses determined from this evaluation were 0.15 mrem/yr for the person living near the plant, 0.13 mrem/yr from releases from the ash landfill, 0.5 mrem/yr for workers manufacturing building products, and 13.5 mrem/yr to a resident of a home constructed with fly ash building materials. The latter is close to the 13 mrem/yr from living in a conventional brick/masonry house.

The levels of radioactivity are within the range found in other natural products. The doses resulting from using the ash in various products are comparable to doses from other human activities and from other natural sources. The doses from the radionuclides in ash are much less than the 300 mrem/yr received from normal background radiation. See Appendix B for the report prepared by Dr. Kjell Johansen for We Energies.

Radiochemistry Tests Performed on We Energies Coal Combustion Products (86)

Radiochemistry tests were performed on fly ash and bottom ash samples from MCPP, VAPP, OCPP, PPPP, PIPP, PPPP gypsum, PPPP filter cake, and PIPP spent powdered activated carbon (TOXECONTM) sorbent. Using γ-ray spectroscopy, the concentrations of radionuclides 226Ra, 232Th, and 40K in We Energies CCP was determined. Table 10-14 shows a summary of activity concentrations of radionuclides and the effective dose equivalent (ede). The test results ranged from 4.57 x 10-8 mSv/yr ede for gypsum to 1.73 x 10-6 mSv/yr ede for filter cake. These results were six to eight orders of magnitude lower than both the maximum allowable ede exposure from radiation in consumer products (0.1 mSv/yr) and exposure beyond the natural background radiation (3.0 mSv/yr) in North America. Additionally, all We Energies’ CCP except filter cake met the EPA’s water quality radionuclide limits for radium (5 pCi/g), thorium (5 pCi/g) and potassium (40 pCi/g).These results indicates that We Energies’ CCP are safe in terms of both primordial and cosmogenic radionuclides, and fall within both national and international recommendations. The other parameters that can be extracted from the radiochemistry test results are external activity concentration index, internal activity concentration index and radium equivalent activity. These parameters are used to evaluate the compliance of specific building materials with international recommendations.

Table 10-14: Summary of Activity Concentration of Radionuclides 226Ra, 232Th, and 40K and Ede for We Energies CCP (2010)

Coal Ash Exemptions

The WDNR monitors the beneficial utilization of CCPs. NR 538 was adopted to categorize by-products and to recommend self-implementing rules to be followed for utilization. However, CCPs have been beneficially utilized for a long time and the WDNR has granted We Energies specific exemptions for many proven applications such as use in concrete, asphalt, CLSM, soil amendment and various aggregate applications.

With increased understanding of coal combustion products and its relationships with the natural environment, We Energies continues to perform research and seek exemptions for additional beneficial use applications.

Table 10-15 provides data on some of the metals that can be typically found in fly ash and soil as compounds along with typical ranges. Of course, one would expect to find higher natural concentrations in area geology where specific metals are mined.

Table 10-15: Typical Metals Found in Fly Ash and Soil

Regulations of Ash Utilization - Wisconsin Department of Natural Resources

The Wisconsin Department of Natural Resources has the authority to regulate the utilization of individual by-products, including coal combustion products, in the State of Wisconsin. It encourages the use of industrial by-products as an alternative to sending these materials to solid waste landfills. Chapter NR 538 has been an important step in the evolution of using industrial by-products in a beneficial way. The NR 538 sets rules for 13 predefined industrial by-product utilization applications.

The purpose of Chapter NR 538 is “to allow and encourage to the maximum extent possible, consistent with the protection of public health and the environment and good engineering practices, the beneficial use of industrial by-products in a nuisance-free manner.” NR 538 does not govern hazardous waste and metallic mining waste, nor does it apply to the design, construction or operations of industrial waste water facilities, sewerage systems and waterworks treating liquid wastes.

Figures 10-1 to 10-5 give flowchart guidance for beneficial use of industrial by-products in accordance with NR 538. This flowchart can be used as a ready-reference to help understand the various requirements and beneficial applications governed under NR 538. In the State of Wisconsin, the NR 500 series of rules cover all aspects of operation, maintenance and post closure monitoring of landfills; and includes NR 538 on the Beneficial Use of Industrial By-products.

Regulations of We Energies Ash Utilization - Michigan Department of Environmental Quality

The Michigan Department of Environmental Quality (MDEQ) is responsible for regulating ash utilization in Michigan. The regulations in Michigan are different than in Wisconsin. Fly ash has been used in concrete widely. However, other land applications have been limited. In the State of Michigan: Act 451 of 1994 (as amended) Part 115: Solid Waste Management, Section 324, covers all aspects of landfill design, permitting, construction, operation, maintenance and groundwater monitoring. The Section 324.11514, Promotion of Recycling and Reuse of Materials, covers beneficial use of industrial waste materials

Readers are referred to the following web location for Michigan statutes and rules:

http://www.deq.state.mi.us/documents/deq-wmd-swp-part115.pdf

Figure 10-1: NR 538 Beneficial Use of Industrial By-Products

Figure 10-2: Flow Chart for General Usage of Industrial By Products

Figure 10-3: Flow Chart for Application of Industrial By-products in Transportation Embankments

Figure 10-4: Flow Chart for Application of Industrial By-products in Unconfined Geotechnical Fill

Figure 10-5: Flow Chart for Application of Industrial By-products as Surface Course Material and Road Abrasive

Ammonia Removal-Ash Beneficiation (US Patent 6,755,901)

Coal-fired power plants are utilizing several proven technologies to improve the quality of air emissions through the reduction of nitrogen oxides (NOx). These include Low NOx burners, Selective Catalytic Reduction (SCR), Selective Non-Catalytic Reduction (SNCR), and Amine Enhanced Lean Gas Reburn (AEFLGR) . These modifications and additions to coal-fired combustion systems normally result in additional residual carbon and/or ammonia compounds. We Energies has developed the ammonia liberation process (ALP) as a way to overcome the far reaching effects that the installation of NOx reduction technologies may have. The process developed by We Energies employs the application of heat to liberate the ammonia compounds from the ash, consume undesirable carbon and render the ash a marketable product. The process design employs few moving parts to keep wear and maintenance low. The system is adaptable to meet the different ash characteristics generated by the various NOx reduction systems as well as the quantity of ash needing beneficiation.

Ammonia Removal Process

The type of NO x reduction process used typically determines the type and characteristics of the ammonia contaminants. In general the most common and abundant species are the bisulfate and sulfate forms. These species have the required removal temperatures of 813°F and 808°F, respectively. The ammonia liberating process preheats the ash and then feeds it to a processing bed where its temperature is increased to about 1,000°F with hot fluidizing air. The fluidizing air is supplied by a burner and forced through a porous metal media. This high temperature media provides support for the ash and distribution for the air flow. The heat breaks down or consumes the contaminants and the air flow carries the contaminants away from the ash. The ash leaves the processing bed and is cooled with a heat exchanger. This reclaimed heat can be used to preheat the incoming untreated ash. The clean ash is transferred to storage for subsequent use. The contaminated air flow leaving the processing bed is passed through a baghouse where any fugitive ash is captured and returned to the ash exiting the processing bed. The dust free ammonia laden gas may then be passed into a wet scrubber for removal of the contaminants for disposal or passed back into the combustion process or NOx reduction process.

ALP Pilot Plant Test

We Energies has assembled and tested a small-scale prototype ALP unit. The unit is operated under the parameters described above. The properties of fly ash before and after the tests are shown below. The amount of ammonia in the ash was significantly reduced. The resulting fly ash is a marketable ash that could be beneficially utilized as a “green” construction material.

Mercury Removal-Ash Beneficiation (Patent 7,217,401)

The emission of mercury compounds from all sources, including coal-fired power plants, has drawn national and international attention due to the fact that certain forms of mercury have deleterious effects on wildlife and can be toxic to humans. Activated carbon injection (ACI) is by far the most effective and widely accepted technology to remove mercury from the flue gas of power plants. However, the implementation of ACI ahead of the primary electrostatic precipitator (ESP) or baghouse will inevitably increase the mercury concentration and carbon content in coal ash.

We Energies conducted a study to develop and demonstrate a technology to liberate and recapture the mercury adsorbed onto activated carbon and fly ash, and provide high quality fly ash for reuse in concrete applications or to recycle sorbents used for mercury removal (88).

A bench scale study was done to select an optimum removal and combination of temperature and retention time to maximize mercury (Hg) recovery. Fly ash samples taken from Presque Isle Power Plant (PIPP) were used in the experiments. The total Hg concentration in the sample was determined by cold-vapor generation atomic fluorescence spectroscopy (AFS). Samples were treated in a muffle furnace in an inert atmosphere at different temperatures ranging from 371°C to 538°C for retention times of one to five minutes. A nitrogen atmosphere was maintained to keep the carbon from igniting. The percent of Hg liberated from the ash samples was determined by measuring the total Hg left in the ash after thermal treatment. PIPP fly ash Units 5 & 6 was derived from western bituminous coal and collected using a precipitator. The original total Hg concentration in the sample was 0.42 ppm. The results indicated that both temperature and retention time are important parameters in the thermal desorption process. At temperatures lower than 482°C, the maximum Hg removal was 40% even with prolonged thermal treatment. More Hg can be removed with higher temperature and longer treatment. At 538°C, 90% of the Hg was liberated from the fly ash within four minutes. Figure 10-6 shows the rate of Hg removal from PIPP fly ash in the muffle furnace using different combinations of temperature and retention time.

Based upon the test results obtained from the bench scale study, a test program was designed to generate experimental data from a pilot apparatus. The pilot test apparatus is comprised of seven main components: a cone-shaped hopper, air slide, baghouse, burner, collector underneath the air slide, Hg condenser, and wet scrubber. During each fly ash processing run, samples were fed into the air slide through the cone -shaped hopper. The speed of sample going through the system was controlled by a rotary feeder. Inside the air slide, samples were heated by hot air coming from the burner. The temperature inside the air slide was controlled by adjusting the air flow rate of the burner. A data logger connected to five thermocouples located at the burner, baghouse inlet, and the inlet, midpoint and outlet of the air slide, were used to record the temperature readings. After traveling through the air slide, part of the sample went to the collector at the discharge end of the air slide and the rest of the sample went to the baghouse. Hot air that exited the baghouse passed through a mercury condenser and wet scrubber before being emitted into the ambient environment. Fly ash samples from Presque Isle Power Plant (PIPP), Valley Power Plant (VAPP) and Pleasant Prairie Power Plant (PPPP) were used for the pilot study. Hg concentration and carbon content were measured before and after thermal treatment for comparative purposes. Loss on ignition was used as the indicator of carbon content.

Figure 10-6: Effect of temperature and retention time on mercury removal from PIPP fly ash.

A total of ten fly ash samples from three different power plants were used in the pilot study. The pilot study was conducted in two phases: first, ash samples (two from PIPP, one from PPPP and one from VAPP) were treated in the pilot scale apparatus under fixed temperature and rotary feeding rate (retention time); second, fly ashes (three split samples from PIPP and three split samples from PPPP) were tested under different temperatures and rotary feeding speed. The Hg concentrations in these fly ash samples ranged from 0.11 ppm to 1.00 ppm. For each test in phase one, the initial temperature of the air slide inlet was set at 538°C and the rotary feeding speed was set at 1000 rpm. The results of these tests are shown in Table 10-16. All four initial tests indicated that Hg could be liberated from various ash samples at 538°C using the pilot scale apparatus. The majority of the sample passing through the air slide discharged to the collector under the air slide with very low concentrations of Hg detected in these samples. A small proportion of the sample passed with the air flow to the baghouse and contained a higher Hg content.

Table 10-16: Phase I Pilot test data of Mercury Liberation from PIPP, PPPP and VAPP Samples at 538ºC and Rotary Feeding Speed of 1000rpm

Further experiments were performed to determine how temperature and rotary feeding speed would impact the Hg desorption process using PIPP and PPPP samples. Three experiments were run with the rotary feeder speed set at 800, 1000 and 1200 rpm and the air slide inlet temperature set at 538°C using PIPP samples. The initial Hg content in these samples was around 0.14 ppm. PPPP samples were treated with different heating temperatures, 538°C, 593°C and 649°C and the rotary feeder speed fixed at 1000 rpm. The results are shown in Table 10-17.

Data analysis shows no obvious correlation between the rotary feeding speed and Hg removal. The Hg content in fly ash samples collected under the air slide was 77.3% to 89.3% lower than that found in the original samples. It is possible that rotary feeder speed does not significantly impact the retention time of samples in the air slide.

Table 10-17: Effects of Temperature and Retention Time on Mercury Liberation (Pilot Test Phase II)

Multi-Pollutant Control Using TOXECON Process

We Energies demonstrated the EPRI Toxecon process system at the Presque Isle Power Plant (PIPP) located in Marquette, MI. The Toxecon process (shown in Figure 10-7) captures high particulate matter (PM) in pulsed-jet baghouses coupled with activated carbon injection (sorbent technology) to achieve high mercury capture. It can capture over 90% of the mercury contained in the combustion process emissions with mercury fixation on the activated carbon.

The separation of mercury from the powdered activated carbon (PAC) would potentially allow for mercury sorbent regeneration and reuse the exhausted activated carbon through thermal desorption (90).This research used both a pilot-scale high temperature air slide (HTAS) – U.S. Patent 7,217,401 (104) and bench-scale thermogravimetric analyzer (TGA) demonstrating this thermal removal of mercury in spent powdered activated carbon from the Toxecon process. “The HTAS removed 65, 83, and 92% of mercury captured with PAC when ran at 900ºF, 1000ºF, and 1200ºF, respectively, while the TGA removed 46 and 100% of mercury at 800ºF and 900ºF, respectively. Scanning electron microscopy images and energy dispersive X-ray analysis show no change in PAC particle aggregation or chemical composition. Thermally treated sorbents had a higher surface area and pore volume than the untreated samples indicating regeneration. The optimum temperature of PAC regeneration in the HTAS was 1000ºF. At this temperature, the regenerated sorbent had sufficient adsorption capacity similar to its virgin counterpart at 33.9% loss on ignition. Consequently, the regenerated PAC may be recycled back into the system by blending it with the virgin PAC” (90).

Figure 10-7: Incorporation of the Toxecon equipment to an existing plant’s particulate control device

U.S. EPA Mercury Emission Regulation

Based on the Federal Requirements to regulate mercury emissions from power plants, the U.S. EPA issued a maximum available control technology rule (MACT) under Section 112 of the Clean Air Act for mercury requiring a 91% reduction in mercury emissions from coal-fired power plants.

Use of Ash Landfill Leachate (Mineral Water) in Concrete (U.S. Patent 8,236,098)

Coal combustion products (CCP) such as fly ash, bottom ash, boiler slag, and flue gas desulfurization (FGD) materials are beneficially used extensively in the construction and building industries. Excess CCP are commonly stored today in monofill landfills that are lined, covered, and constructed with leachate collection systems. Once collected, the leachate is typically trucked or piped to a wastewater facility for treatment at a cost. The leachate is composed of water from stabilization, precipitation, dust control, and compaction and is enriched with minerals from the CCP placed in the landfill. The monofill landfill leachate is therefore rich in CCP constituent elements and minerals such as calcium, sodium, potassium, boron, magnesium, sulfate, chloride, aluminum, silica, and other trace elements, many of which can be helpful as activators in cementitious reactions during concrete production. Consequently, the landfill leachate can be used as mixing water for concrete production with beneficial effects of increased compressive strength, reduced shrinkage, and accelerated hydration. For example, a building material such as “green” bricks could be manufactured using mineral rich leachate water as an ingredient while conserving normally used drinking water. Benefits include conservation of fresh water sources, reduced loading on wastewater treatment facilities, beneficial effects to concrete products, and eliminating the costs associated with purchasing drinking water as an ingredient as well as the cost of wastewater treatment. We Energies has a patent pending for the use of monofill CCP leachate in concrete production.

Materials for Making Concrete

The materials for making concrete and other building materials may include Portland cement, pozzolan, specialty admixtures, aggregates and liquid landfill leachate. The composition of these materials will depend on the function and the properties of the finished product but will have a compressive strength suitable for various construction applications.

Concrete mixtures generally use only sufficient water to make the mixture workable for placement and to yield hardened concrete having a compressive strength of greater than 8.3 MPa (1200 psi) after 28 days of curing. Portland cement is a well-known cement that upon mixing with water binds the other materials present in the mixture into concrete. Typically, fresh concrete has mixing water exceeding the amount needed for hydration for purposes of workability, handling, and finishing. Sulfate in small prescribed amounts can yield a shrinkage compensating effect, and actually reduce shrinkage cracking in concrete. In fact, specialty Type K cements are produced specifically to yield this effect.

A concrete may also be made from a composition including pozzolan. A pozzolan is a siliceous or aluminosiliceous material, which independently has few or fewer cementitious properties, but in the presence of an activator such as a lime-rich medium such as calcium hydroxide, shows better cementitious properties. Examples of pozzolans include fly ash, silica fume, metakaolin, ground granulated blast furnace slag, and some finely divided natural minerals.

Various activators are suitable for use with pozzolans in a composition. For example, the activator can be selected from alkali metal carbonates, alkali metal silicates, alkali metal hydroxides, alkali metal oxides, alkali metal fluorides, alkali metal sulfates, alkali metal carboxylates, alkali metal nitrates, alkali metal nitrites, alkali metal phosphates, alkali metal sulfites, alkali metal halides, alkaline earth metal carbonates, alkaline earth metal silicates, alkaline earth metal hydroxides, alkaline earth metal oxides, alkaline earth metal fluorides, alkaline earth metal sulfates, alkaline earth metal carboxylates, alkaline earth nitrates, alkaline earth metal nitrites, alkaline earth metal phosphates, alkaline earth metal sulfites, alkaline earth metal halides, and other mixtures. Sodium, potassium and lithium are examples of alkali metals, whereas magnesium and calcium are examples of alkaline earth metals. The activator can also be selected from calcium oxide, calcium hydroxide, calcium silicate, and calcium carbonate. In addition, activators can be either Portland cement or cement kiln dust or an organic acid such as citric acid.

The amount of Portland cement, pozzolan, activator, aggregate, and landfill leachate can be varied depending on the physical properties desired in the building materials. The compressive strength of a concrete can be controlled by varying the weight ratio of Portland cement to pozzolan (e.g., fly ash). The liquid landfill leachate may replace all or part of the tap water to produce a concrete. The pH and conductivity of the liquid landfill leachate may range from 8 to 11, and 1000 to 100,000 micromhos, respectively. Air-entrainment of 5% or higher by weight may be included to provide workability and increased resistance to deterioration of the concrete due to freezing and thawing cycles.

Water is needed in the production of concrete to provide a media for hydration reactions, and to facilitate the production of a material which is workable and easy to place and compact. The landfill leachate provides both the media for hydration and elements and minerals which can act as activators in cementitious reactions. Higher performance concrete is made when hydration reactions are accelerated and shrinkage is reduced with increased compressive strength when compared to concrete made in a conventional way.

Acceleration of hydration of cement in concrete results in a more rapid increase in setting time and compressive strength. Liquid landfill leachate can be utilized in a sufficient amount such that the composition sets in the desired timeframe with the specified compressive strength.

Materials used in the examples

Type I Portland cement with a specific gravity of 3.15 and a Blaine fineness of 380 m2/kg was used. The Portland cement complied with ASTM C 150-07. Fly ash conforming to ASTM C 618-05 Class C, and having physical and chemical properties in Table 10-18 and Table 10-19 was used. The coarse aggregate used was crushed quartzite of 25 millimeters (about 1 inch) maximum size with a specific gravity of 2.65, and water absorption of 0.15%.The fine aggregate used was local natural sand with a fineness modulus of 2.71, a specific gravity of 2.65, and water absorption of 0.5%.The air-entraining admixture used was MB-AE 90 with a recommended dosage ranging from ¼ to 4 fl oz/cwt (16-260 mL/100 kg) of cementitious material. It was supplied by BASF to provide air-entrainment of 5% or higher.

Table 10-18: Chemical Properties of Fly Ash

Table 10-19: Physical Properties of Fly Ash

Tap water from the Milwaukee Water Works, Milwaukee, Wisconsin, USA was used for the base case. Based on a 2008 Annual Water Quality Report, this tap water had (among other things) a maximum pH of 7.80, a maximum conductivity of 335 micromhos, a maximum boron concentration of 0.022 mg/L, maximum concentration of calcium of 38.0 mg/L, a maximum concentration of iron of 0.076 mg/L, a maximum concentration of magnesium of 13 mg/L, a maximum concentration of potassium of 1.6 mg/L, a maximum concentration of sodium of 13.0 mg/L, and a maximum concentration of sulfate of 27.0 mg/L.

Landfill leachate from a landfill having coal combustion products such as fly ash and bottom ash was used for comparison. The landfill leachate had a pH of 10.1, a conductivity of 21,100 micromhos, boron concentration of 53 mg/L, calcium concentration of 220 mg/L, iron concentration of 0.035 mg/L, magnesium concentration of 72 mg/L, potassium concentration of 80 mg/L, sodium concentration of 5,300 mg/L, and sulfate concentration of 11,000 mg/L.

Mixture Proportions Used in “Proof of Concept” Examples

Four concrete mixtures were tested. These included two reference mixtures based on tap water and fly ash, designated as RFA below and a tap water and fly ash-Portland cement blend (56:44), designated as RBC below. These concretes (RFA/RBC) were compared with concretes of similar composition based on landfill leachate in place of tap water (designated as WFA/WBC). The specified concrete mixture proportions (in lbs per yd3) are presented in Table 10-20.

Table 10-20: Summary of the Mix Proportions Used in the Examples

Normally, the application of at least three aggregate types is recommended to meet the requirements for optimal aggregate proportioning. With the aggregates used, the 55:45 coarse aggregates – fine aggregates (sand) mix provides the best particle size distribution that matches the optimal 0.45 power curve (as shown in Table 10-21). The concrete mixtures were designed for a relatively low w/c of 0.44 and a water content of 275-290 lbs/yd3. The water content was adjusted to provide a slump of 5.5±2.5 inches. The air-entraining admixture content was also adjusted for mixtures RFA, RBC, WFA, WBC, respectively. The resulting (corrected for the yield) concrete mixture proportioning per yd3 is presented in Table 10-22.

Table 10-21: Particle Size Distribution of Aggregates

Table 10-22: Concrete Mixture Proportions

Casting and Curing of Test Specimens

All the concrete mixtures were mixed for 5 minutes in a laboratory drum mixer. The ASTM C-192 “Standard Practice of Making and Curing Concrete Test Specimens in the Laboratory” was used for the preparation of concrete specimens. Tests were conducted on fresh concrete mixtures to determine slump, temperature, air content, unit weight (density), yield of fresh concrete, and setting times (initial and final). From each concrete mixture, 21 (three for each age tested) 100 x 200 mm (4” x 8”) cylinders were cast to determine compressive strength at the age of 1, 3, 7, 14, 28, 56 and 91 days. The specimens were cast in two layers with vibration. Linear shrinkage or expansion of concrete was investigated using 3” x 3” x 11” beams, using three specimens for each mix. After casting, all molded specimens were covered with plastic sheets and left in the curing room for 24 hours. They were then demolded and the specimens were returned to the moist-curing room at 73.4 ± 3°F (23.0 ± 1.7°C) and 96 ± 1% relative humidity (RH) until they reached the testing age.

Testing of Fresh Concrete

The following properties of fresh concrete were investigated: Slump in accordance with ASTM C-143 “Standard Test Method for Slump of Hydraulic Cement Concrete”; Fresh Density (Unit Weight) in accordance with ASTM C-138 “Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete”; Air Content in accordance with ASTM C-231 “Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method”; and Setting Time in accordance with ASTM C-403 “Standard Test Method for Time of Setting of Concrete Mixtures by Penetration Resistance”. The results are presented in Table 10-23.

Testing of Mechanical Properties

The compressive strength of each concrete mixture was determined following ASTM C-39 “Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens” on three cylinders at 1, 3, 7, 14, 28, 56, and 91 days. The mean value of the three cylinder strengths at a particular age was reported as the compressive strength value. The results are also presented in Table 10-23.

Table 10-23: Fresh Properties, Compressive Strength and Shrinkage of Investigated Concretes

Figure 10-8: Compressive strength comparison of the four different mixtures of concrete. RFA –reference mixture with fly ash. RBC -reference mixture with blend of fly ash and Portland cement. WFA and WBA have similar composition with mixture of special processed water

Figure 10-9: Deformation strain of the four different mixtures of concrete. Positive and negative deformation strain corresponds to the expansion (swelling) and shrinkage respectively.

Testing of Shrinkage

Water content, mortar paste fraction, admixture selection, cement and pozzolan types and quantities, coupled with aggregate characteristics and mix design proportions have the most significant impact on concrete’s drying shrinkage characteristics. Careful selection of these variables is critical. ASTM C-157, “Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete” was used to determine the length change of hardened fly ash concrete. The results are presented in Table 10-23 and Figure 10-9 above.

Results and Discussion

Fresh Concrete Properties

The initial slump of the investigated concrete mixtures was in the range of 5.5 to 8 inches. The Class C fly ash (only cementitious material) concretes demonstrated very quick slump loss, after 15 minutes reaching 56 and 38% of initial slump value or 4.5 and 3 inches for the RFA and WFA concretes respectively. All investigated concrete mixtures presented had air content between 5.5% and 6.5%. However, it should be noted that to provide the above -mentioned air content, the fly ash-based mixtures required a relatively high dosage of air-entraining admixture.

The average temperature of the concrete mixtures was 74 ± 2 ºF. The Class C fly ash (only cementitious material) concretes demonstrated very quick setting, with an initial setting time of 30 and 50 minutes for the RFA and WFA concretes respectively. Because of low strength and specimen rupture, it was impossible to determine the final setting time for these types of concretes; thus, final time values were obtained by extrapolating the experimental data. For practical application of such concrete, the addition of retarding admixture is required to extend the setting times to acceptable levels. Specimens RBC and WBC had extended initial/final setting times of 16:40/24:55 and 11:50/15:50 respectively, which is a clear sign of retarded hydration of cement. Consequently, it can be noted that the application of landfill leachate (specimen WBC) allows considerable acceleration of hydration resulting in shorter setting times.

Mechanical Properties

The compressive strength of the investigated concrete is shown in Table 10 -23 and Figure 10-8 above. The control concrete based on fly ash developed higher compressive strengths as compared with Portland cement-fly ash concrete at an age of one day. Concrete based on fly ash-Portland cement blend had a higher compressive strength (vs. RFA) after three days of hardening. At the age of 91 days, such concrete had more than a four-fold increase of compressive strength as compared with the reference fly ash concrete. The application of landfill leachate in fly ash concrete (specimen WFA) reduced strength as compared with RFA in all ages of hardening. However, the application of landfill leachate (specimen WBC) in concrete based on fly ash-Portland cement blend considerably improves early (seven days and less) strength and provides better strength through the 56-day age and near the same strength at the 91 day age (6991 psi vs. 7084 psi).

Shrinkage Properties

The results of shrinkage tests are reported in Table 10-23 and Figure 10-9. At the age of 91 days, the control fly ash-based concrete (RFA) demonstrated only 65% of shrinkage as compared with Portland cement-fly ash concrete, 211 vs 323 microstrain, respectively. The application of landfill leachate (specimens WFA and WBC) reduces shrinkage as compared with its corresponding reference concrete. For example, for concrete based on fly ash-Portland cement blend, the shrinkage reached 323 and 300 microstrain, for tap water and landfill leachate specimens respectively. Interestingly, the application of landfill leachate also resulted in reduced swelling (due to exposure to 95% RH until 28-day age). Fly ash concrete with landfill leachate (specimen WFA) had a very low drying shrinkage at the ages of 56 and 91 days (105 and 95 microstrain) respectively.

Conclusions

It can be seen from the tests above that the use of landfill leachate to produce concrete can be beneficial. The application of landfill leachate does not cause any pronounced difference in workability of investigated fly ash concrete. Fly ash concretes demonstrated very quick setting, with initial setting time of 30 and 50 minutes for RFA and WFA concretes respectively. The use of landfill leachate on fly ash-Portland cement-based concrete (specimen WBC) allows considerable acceleration of hydration that result in shorter setting times as compared with the reference (RBC).

The conducted investigation demonstrates that the replacement of tap water with landfill leachate results in fly ash-Portland cement-based concrete with significantly enhanced strength, especially in early ages of hardening. The observed performance improvement is a very important feature related to the application of landfill leachate, which can be effectively used in many practical construction applications such as producing concrete for precast products, highways, and cast-in-place applications.

It has been demonstrated that the application of landfill leachate results in reduction of shrinkage as compared with corresponding reference concrete. For concrete based on fly ash-Portland cement blend, the 91 -day shrinkage reached 323 and 300 microstrain for tap water and landfill leachate specimens respectively. The application of landfill leachate also resulted in reduced swelling, suggesting the formation of a less porous microstructure, hindering shrinkage-swelling deformation of concrete.

It can therefore be seen that the application of landfill leachate provides an economically advantageous means of using coal combustion products sustainably. The methodology described herein, produces concrete with increased compressive strength, reduced linear shrinkage, and accelerated hydration of cement. Fresh water, as a sometimes scarce natural resource is also conserved.

Chapter 11 - Carbon Dioxide is a Coal Combustion Product

Introduction

Carbon dioxide (CO2) gas is used for various commercial applications, including food grade food processing and industrial gases. In the case of food processing, soft drink beverage carbonation processes are volumetrically small applications and require conformance with a stringent purity standard. Production of food grade carbon dioxide is relatively inexpensive, generated by air stripping methods with a high level of control over contaminants and undesirable constituents affecting health, odor and taste.

Industrial CO2 uses include refrigeration and cooling applications, metals treatment (hardening agent), welding (shield gas to prevent oxidation), sand-blasting (solid form), propellant (aerosol cans), dry cleaning (replacing solvents), cold sterilization (with ethylene oxide), coffee de-caffeination, fire extinguishers and pH adjustment (reduction) in sewerage treatment plants.

Virtually all of the commercial or industrial uses for CO2 involve a process which ultimately releases the unreacted or excess gas to the atmosphere. Energy used to capture CO2 during refinery operations, the largest source of CO2 production, is incidental to the overall refining process and is subsidized by the higher value gas and chemical stocks produced therein. Power plants as an alternative source for CO2 could substitute for these sources, but would result in a displacement of volume without reducing the net CO2 emissions.

A well-known use for CO2 is in tertiary oil well flooding for secondary oil recovery – extracting additional oil from depleted oil fields. It increases the yield from oil fields by boosting pressure in the formation and by thinning the oil to increase flowability (dissolves into oil and decreases viscosity). Pipeline systems are required to convey the gas in compressed form to western oil production regions. This approach serves a beneficial function in allowing additional oil recovery, and sequesters the gas in deep geological formations where it is not part of the atmosphere.

Reports from the oil transportation industry indicate that economics of CO2 gas transportation for oilfield use is directly related to proximity to pipeline hubs or networks.

Southern Illinois is the practical limit of the pipeline network in the Midwest. However, additional sources indicate that there are options for sequestering CO2 in Illinois with existing oil and gas wells or geologic formations which do not exist in Wisconsin.

The feasibility of underground storage of CO2 has been researched extensively by We Energies – Gas Control. This effort has been in conjunction with a search for natural gas storage in local geological formations, with the conclusion that there is very limited local capacity available for underground storage. Historically, the company has obtained underground storage for natural gas in lower Michigan. The need for natural gas storage and the price structure for this commodity preempt CO2 issues today.

Another issue which is currently under review by regulators and the industry regarding underground CO2 sequestration is the viability and permitting of different types of geological formations and well networks. In review, different types of formations exhibit different characteristics affecting their ability to retain CO2 in a permanently fixed condition. This involves porosity parameters, confining geology, pressures and material reactivity. Deep geologic sequestration is the most viable option for management of CO2 because large potential volumes are involved, but there may be a significant restriction placed on locations where it can be safely and efficiently implemented.

There are several large sources of CO2 in industry which supply the bulk of demand for the gas. Ethanol plants and bio-diesel plants produce large amounts of CO2. These types of production facilities have greatly increased in number in recent years, outstripping commercial demand for CO2. The effect of commercial and industrial reuse of CO2 therefore, is not expected to reduce CO2 emissions to the atmosphere in any appreciable way since the scale of CO2 utilization is small in comparison to the anthropogenic CO2 generation in industry, without even considering natural sources. The industrial utilization of CO2 therefore cannot break-even with production. For example, according to industrial gas distributors, ethanol production generates over 30,000 tons of CO2 per day alone. There are 35 bio-diesel plants currently operating and producing CO 2, and new plants are under construction. Petroleum refineries also produce a minimum of 3% of liquefied CO2 from cracking a given volume of petroleum at refineries. This means that there is a potential for large amounts of the CO2 gas being in the market in the near future. There is therefore an opportunity for practical measures to mitigate excess CO2 in the atmosphere. Some of these measures include but are not limited to algae farming, carbon capture, and CO2 mineralization using various by-product materials such as cement kiln dust (CKD), lime kiln dust (LKD), recycled concrete fines (RCF), Class C fly ash (CFA), and blast furnace slag (Slag).

Algae Farming

Algae farming has been considered for CO2 capture and use, as a recycling method rather than a sequestering strategy. In essence, a highly active algae consumes CO2 from a power plant flue gas and converts it to a carbon fuel (oil) oxygen and a biomass byproduct. The biomass, with a high BTU content (9,000 BTU/lb) can be dried and burned as power plant fuel or refined to produce commercial products such as bio diesel fuel and ethanol.

CO2 Capture

The CO2 capture pilot project at Pleasant Prairie Power Plant (P4) was designed to test the economics and efficiency of the ALSTOM chilled ammonia process using a small slip stream of flue gas from the plant. The site was selected in part because of the presence of a wet scrubbing system for sulfur, providing a clean and cooled flue gas (130 °F), and the presence of an ammonia stock at the site due to the selective catalytic reduction (SCR) unit. The ALSTOM system, while designed to produce a liquefied CO2 which could be collected and transported via tanker for use or sequestration, was built in pilot scale to only produce gaseous CO2. Additional compression and associated costs would have been required to convert this gas to liquid for easier transport. This fact greatly affected the ability to find utilization options for CO2 in current applications.

CO2 Mineralization Using Various By-Product Materials

We Energies has developed and patented processes for the mineralization of CO2. Initial work involved the use of Class C fly ash and the proof of concept testing was performed at the Center for By-products Utilization at the University of Wisconsin-Milwaukee. This work was followed by a large-scale field demonstration at the Pleasant Prairie Power Plant landfill site in 2007. The process utilized a proprietary foaming agent (Elastizell, Inc.) which produces a high level of discrete bubbles which are mixed with the Class C fly ash and water. The lime reacts with CO2 producing carbonates which are incorporated into the concrete. The resultant solid has very low weight, in the range of 70 pounds per cubic foot which can be crushed and used as a lightweight aggregate. The quantity of CO2 utilized is relatively small, requiring only the normal CO2 levels in the atmosphere. A high temperature source of CO2 such as a power plant flue gas at approximately 300 °F would require cooling to be effective but could be helpful in winter for production and curing.

Further work was conducted on lime containing by-product materials including cement kiln dust (CKD), lime kiln dust (LKD), recycled concrete fines (RCF), class C fly ash (CFA), and blast furnace slag (Slag) to produce additional materials foamed with CO2 containing gases (ambient air, pure CO2 , and power plant flue gas) (shown in Table 11-1). Test cylinders were made and cured for 7, 14, 28, 56, 91, and 182 days from which carbonation potential and compressive strength were determined at each curing age. The results are shown in Figures 11-1 to 11-3 and Tables 11-2 and 11-3 for carbonation and Figures 11-6 to 11-8 and Table 11-4 for compressive strength.

Laboratory Mixture Proportions and Data

The amount of ingredients used, for each 1.8 cubic feet (0.051 cubic meter) batch of foamed material produced are shown on Table 11-1. The wet cylinder weight was targeted at 4.8 pounds +/- one pound (2.2 kg +/- 0.45 kg) for consistency of density. Additional quantities of carbon dioxide based foam were required with pure CO2 gas to obtain the desired range of density due to the instability of the foam after formation.

Table 11-1: Mixture proportions and Data for 1.8 ft3 (0.51 m3) Batches (91)

Carbonation Testing per RILEM (92)

Carbon dioxide in the presence of moisture may penetrate the surface of mortar or concrete, and react with alkaline components in the cement paste, in the mortar or concrete, mainly Ca(OH) 2. This process (carbonation) leads to a reduction of the pH value of the pore solution to less than 9 (92). The reduction of the pH-value can be made visible by the color change of an indicator solution such as phenolphthalein which turns non-carbonated mortar or concrete dark pink or fuchsia, and the carbonated mortar or concrete remains colorless. All of the test specimens made with various types of foamed mortar were stored together for three days indoors in the laboratory where they were made. The plastic cylinder molds were removed from the test specimens and they were moved to a curing chamber and subjected to roughly a 0.03% CO2 concentration found in the ambient air, with the lab temperature of 20ºC +/- 2ºC (70ºF +/- 3ºF), and relative humidity of 65% +/- 25%. A humidifier was used to add moisture to the curing chamber air during storage when the indoor air was dry in the room where the curing chamber was located. Conditions of storage such as time, humidity, and temperature were recorded. The test cylinders were spaced in the curing chamber so that air was able to reach the test surfaces unhindered at all times. For this reason, a free space of at least 0.8 inch (20 mm) was left around the specimens. “Carbonation occurs at the highest rates at relative humidity from about 40 to 70 percent. Near 0 or 100 percent, there is little or no carbonation.” (92)

Table 11- 2 and Figure 11-1 show the depth of carbonation for each of the five by-product materials when produced with integral foamed CO2 contained in ambient air and carbonation advancement of Class C fly ash mortars respectively. Figure 11-2 shows the graphical representation of carbonation depth progression as a function of time. The CFA cast cylinders carbonated most rapidly attaining full thickness carbonation at an age of 56 days. RCF cast cylinders carbonated to full thickness at 6 months. All of the other by-product materials did not attain full carbonation within the 6 month period with the foam formed with ambient air. CKD attained a carbonation depth of 1.0 inch (44 mm), LKD with 1.25 inches (54 mm) and Slag with 1.25 inches (54 mm) of carbonation at the age of 6 months.

If the CFA hardened foamed material were crushed into a ¾ inch (19 mm) aggregate at an early age, and carbonation occurred inward from outside edges similar to the cylinders tested, the aggregate would achieve carbonation in less than 7 days of storage. Similarly, carbonation of up to a 3/4 inch (19 mm) nominal size aggregate could be accomplished for RCF in 14 days, Slag in 28 days and CKD and LKD in 56 days.

Figure 11- 1: Carbonation Depth Advancement for Class C Fly Ash mortars (91)

Table 11-2: Carbonation Depth for Hardened By-Product Materials Foamed With Ambient Air (91)

Figure 11-2: By-Product Mixtures Carbonation Depth Progression versus Time (91)

Quantification of CO2 Mineralization

A portion of the carbonated cylinder (as determined from the phenolphthalein indicator test), from each type of mortar material, was sealed in a plastic food-grade bag for testing of carbon dioxide content in accordance with ASTM C-25, “Standard Test Method for Chemical Analysis of Limestone, Quicklime, and Hydrated Lime” (93) as shown in Figure 11-3. Immediately after compression testing and carbonation testing, samples were vacuum- sealed in the plastic bags. The samples were removed from the vacuum-sealed plastic bags at the time of testing for CO2 by mass determination. Later, the carbonated portion of the sample was removed from the vacuum-sealed bags and prepared for quantification of CO2. The samples were filed to the full depth of carbonation, and the filings were collected for further preparation by grinding in a mortar and pestle, shown in Figure 11- 3. The ASTM C-25 (93) gravimetric test method was used for quantifying the amount of carbon dioxide contained within a mineral sample. The samples were decomposed with hydrochloric acid and the liberated CO2 was passed through a series of scrubbers to remove water and sulfides. The CO2 was absorbed with Ascarite, a special sodium hydroxide absorbent, and the gain in weight of the absorption tube was determined and calculated as percent CO2. The balance, calibration weights, Ascarite absorber, and standard sample (reagent grade Na2CO3) are shown in Figure 11-4. The wet chemistry apparatus is shown on Figure 11-5.

Figure 11-3: Sample Preservation and Preparation for CO2 Content Tests (91)

Table 11-3: CO2 Content of Various By-Product Carbonated Materials by Percent Mass with Ambient Air at Different Curing Ages (91)

Figure 11-4: Balance, Calibration Weights, Ascarite Absorber and Standard Sample (Reagent Grade Na2CO3) (91)

Figure 11-5: ASTM C-25, “Standard Test Method for Chemical Analysis of Limestone, Quicklime, and Hydrated Lime,” (93) Gravimetric Method Wet Chemistry Apparatus (91)

Table 11-3 provides the results of ASTM C-25, “Standard Test Method for Chemical Analysis of Limestone, Quicklime, and Hydrated Lime,” (93) gravimetric testing method for CO2 by mass for various by-product materials. The CO2 content determined for these materials at an age of zero days represents the mass percentage of CO2 in a sample of the raw material, before it was used in production of the mortar. The CKD was provided by Lafarge North America from the cement manufacturing facility located in Alpena, Michigan. The LKD was provided by Western Lime Corporation from the lime manufacturing facility located in Eden, Wisconsin. The RCF was provided by A.W. Oakes and Son, Inc. from the recycled concrete crushing facility located in Racine, Wisconsin. The CFA was provided by We Energies from the Pleasant Prairie Power Plant located in Pleasant Prairie, Wisconsin. The power plant uses sub-bituminous coal from the Powder River Basin of Wyoming. The slag was provided by Holcim (US) Inc. from the slag manufacturing facility located in Chicago, Illinois. All other values given in Table 11-3 are from the carbonated portion of the cured cylinders and are the values obtained from ASTM C-25 testing of the by-product materials foamed with the ambient air. The initial amount CO2 (by mass) contained in the by-product materials powder before testing was 9.5%, 22.2%, 14.6%, 0.1% and 1.4% of CO2 for CKD, LKD, RCF, CFA and Slag, respectively. These amounts increased after 182 days to 20.7%, 25.9%, 1.6%, and 5.2% for the CKD, LKD, CFA and Slag based materials foamed with ambient air respectively. However, CO2 content for the RCF based materials decreased to 12.6%. This decrease may actually represent the greater variability that is inherent in a crushed concrete sample consisting primarily of a composite of coarse and fine aggregate materials, various cementitious materials, and also by the relatively small samples used in analysis. The total amount of CO2 was highest in the LKD aggregates followed by CKD, RCF, Slag and CFA aggregate materials for all of the CO2 containing gases used.

The phenolphthalein indicator tests are intended to record the depth of carbonation at points in time, and these percent CO2 by mass tests are intended to record the mass percentage of CO2 embodied within the full-depth of the carbonated zone as indicated by the phenolphthalein indicator test at these same points in time.

Compressive Strength Testing

Four-inch (100-mm) diameter by eight-inch (200-mm) long specimens were cast, cured, and tested in a compression testing machine located in the University of Wisconsin-Milwaukee Concrete Laboratory, see Figures 11-6 and 11-7. The compressive strength of the five by- product -based materials at the ages of 7, 14, 28, 56, 91, and 182 day ages was determined (Table 11-4). An average compressive strength was obtained for the three cylinders tested in compression for each of the five test mixture materials at the ages indicated. The by-product based materials tested are low-strength compared to concrete and fall in the range of CLSM, with a required compressive strength of 1200 psi (8.3 MPa) or less. The variations in strength test results appear amplified on the graphs (Figure 11-8) because of the smaller y-axis scale for compressive strength compared to conventional concrete, but actually fall in the expected compressive strength variability range for cylinder specimens tested in compression.

Figure 11-6: Compressive Strength Cylinder Storage (91)

Figure 11-7: Compressive Strength Cylinder Testing (91)

Table 11-4: Average compressive strength test results (91)

At the conclusion of testing (day 182), the CKD mixture had the highest compressive strength of 216 psi (1.49 MPa) followed by the slag mixtures at 173 psi (1.19 MPa). The compressive strength of the other three by-product materials concluded below 75 psi (0.52 MPa).

Figure 11-8: Compressive Strength versus Time of By-Product Material Mixtures Foamed with Ambient Air (91)

Crushing and Screening

The fractured test cylinders after strength testing were saved from each type of material and mixture associated with the different types of foamed controlled low strength materials (CLSM) produced with different gases. These materials were later crushed into aggregates. Initial crushing feasibility testing was performed with the Los Angeles (LA) Abrasion testing apparatus that was available at the UW-Milwaukee Concrete Laboratory. The LA Abrasion tumbler was equipped with steel balls similar to a ball mill. The tumbling and rotating balls impacted on the fractured cylinders essentially converting the hardened low-strength material into rounded shapes, and a powdery material as shown in Figure 11-9. This was not satisfactory for production of a crushed-stone type of aggregate.

Figure 11-9: LA Abrasion Equipment Crushing of CLSM Test Cylinder Samples after Strength Testing (91)

A laboratory-scale double-roller aggregate crusher, located at Payne and Dolan Incorporated’s crushed stone quarry operations in Waukesha, Wisconsin, was used for further evaluation of CLSM as aggregates. This equipment is routinely used for crushing rock cores. The fractured test cylinders were placed directly into the double rollers, at the top of the crusher. This resulted in crushed aggregate pieces that were collected in a pan located below the double-rollers, see Figure 11-10. This crusher produced a typical crushed stone type of angular shaped aggregate. The amount of fines was larger than would typically be found for natural crushed stone. This is likely due to the significantly lower compressive strengths of these low density by-product CLSM materials.

Aggregate Testing

The crushed aggregate materials were tested for the following characteristics that would allow for comparison to other materials:

• Dry Bulk Density, by ASTM C-29, “Standard Test Method for Bulk Density (Unit Weight) and Voids in Aggregate.” (94)
• Absorption, by ASTM C-127, “Standard Test Method for Density, Relative Density (Specific Gravity), and Absorption of Coarse Aggregate.” (95)
• Specific Gravity, by ASTM C-127, “Standard Test Method for Density, Relative Density (Specific Gravity), and Absorption of Coarse Aggregate.” (95)
• Gradation by ASTM C-136, “Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates.” (96)
• Staining by ASTM C-641, “Standard Test Method for Iron Staining Materials in Lightweight Concrete Aggregates.” (97)

Figure 11-10: Double Roller Crushing of Fractured Compressive Strength Test Cylinder Samples. (Upper Left: Feeding cylinder fragments into crusher. Upper Right: Double-roller crusher unit. Lower Left: Top view of the double-roller crusher. Lower Right: Collection pan with collected crushed by-product aggregates) (91)

The physical properties test results for the five different by-product-based aggregates are shown on Table 11-5. The aggregate grain size distribution curves for each material and other supporting aggregate classification data are shown in Figures 11-13 to 11-17 and Tables 11-6 to 11-10. Figure 11-11 shows photographs of the equipment used for the aggregate testing including the oven drying of the aggregate, dry-rodded unit weight test, sieve analysis, and the emptying of a sieve for the weighing process on a laboratory-scale. Figure 11-12 shows the 24-hour soaking of the aggregates, drying with towels to the saturated surface dry condition, and obtaining an underwater weight for the aggregates. Some of the aggregate samples had a density below that of water and therefore they floated.

Table 11-5: Physical properties of By-Product-Based Aggregates (91)

The bulk density in a dry-rodded state is shown in Table 11- 5. It is defined in ASTM C-29, “Standard Test Method for Bulk Density (Unit Weight) and Voids in Concrete,” as “the mass of a unit volume of bulk aggregate material, in which the volume includes the volume of the individual particles and the volume of the voids between the particles.” (94). The bulk density relationship is important for planning packaging and transportation commercial arrangements. The density compared to other materials is also helpful in planning geotechnical and concrete product applications. The dry-rodded bulk densities for the by-product-based aggregates fall in the range of 44.1 to 69.4 lb/ft 3 (707 to 1113 kg/m3). Absorption and specific gravity are also shown in Table 11-5. Absorption is defined in ASTM C-127, “Standard Test Method for Density, Relative Density (Specific Gravity), and Absorption of Coarse Aggregate,” as “the increase in mass of aggregate due to water penetration into the pores of the particles during a prescribed period of time, but not including water adhering to the outside surface of the particles, expressed as a percentage of dry mass.” (95) The aggregates were soaked in water for 24 hours (plus or minus 4 hours) and then dried in towels to the saturated surface dry (SSD) condition.

Figure 11-11: Photographs of Aggregate Physical Properties Tests. (Upper left photograph shows the oven drying of the crushed aggregate, upper right photograph depicts the dry-rodded unit weight test, lower left photograph depicts the grain size distribution sieve apparatus and the lower right photo graph depicts the emptying and weighing of the sieved samples) (91)

Figure 11-12: Additional Photographs of Aggregate Physical Properties Tests (91)

The absorption values ranged from a low value of 18.1 percent for the RCF aggregate material to a high of 41.6 for the CKD aggregate material. The absorption property is important in calculating the change in density from a dry to SSD condition for storage, packaging and transportation purposes. Absorption is also an important property for performing water content calculations in concrete mixture proportioning, and in calculating internal curing potential for lightweight aggregate concrete. Specific gravity is defined in ASTM C-127, “Standard Test Method for Density, Relative Density (Specific Gravity), and Absorption of Coarse Aggregate,” as “the ratio of the density of the aggregate (oven dry) to the density of distilled water at a stated temperature” (95) which in this case was 73°F (23°C). Specific gravity is commonly used for volume calculations in concrete, asphalt and other mixture proportioning purposes. The fineness modulus property of the by-product-based aggregates are shown in Table 11-5. Fineness modulus is defined in ASTM C-136, “Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates,” and calculated “by adding the total percentages of material in a sample that is coarser than each of the following sieves (cumulative percentages retained), and dividing the sum by 100: 150-μm (No. 100), 300- μm (No. 50), 600-μm (No. 30), 1.18-mm (No. 16), 2.36-mm (No. 8), 4.75-mm (No. 4), 9.5-mm (⅜-in.), 19.0-mm (¾-in.), 37.5-mm (1½-in.) and larger increasing in the ratio of 2:1.” In this study, the sieves conclude at 19.0-mm (¾-in.) because this was the planned top-size of the aggregates. The fineness modulus property is used primarily as an indication of aggregate relative fineness or coarseness. Fineness modulus ranged from a low of 1.74 for RCF aggregates to a high of 6.42 for the Slag aggregates, and the higher the fineness modulus value, the coarser the aggregate

Figure 11-13: Laboratory Test Results of Physical Properties of Aggregate: Grain Size Distribution Curve for CKD + Ambient Air Crushed Aggregate (91)

Figure 11-14: Laboratory Test Results of Physical Properties of Aggregate: Grain Size Distribution Curve for LKD + Ambient Air Crushed Aggregate (91)

Figure 11-15: Laboratory Test Results of Physical Properties of Aggregate: Grain Size Distribution Curve for RCF + Ambient Air Crushed Aggregate (91)

Figure 11-17: Laboratory Test Results of Physical Properties of Aggregate: Grain Size Distribution Curve for Slag + Ambient Air Crushed Aggregate (91)

Table 11-6: Laboratory Test Results of Physical Properties of Aggregate: Mechanical Analyses for CKD + Ambient Air Crushed Aggregate (91)

Table 11-7: Laboratory Test Results of Physical Properties of Aggregate: Mechanical Analyses for LKD + Ambient Air Crushed Aggregate (91)

Table 11-8: Laboratory Test Results of Physical Properties of Aggregate: Mechanical Analyses for RCF + Ambient Air Crushed Aggregate (91)

Table 11-9: Laboratory Test Results of Physical Properties of Aggregate: Mechanical Analyses for CFA + Ambient Air Crushed Aggregate (91)

Table 11-10: Laboratory Test Results of Physical Properties of Aggregate: Mechanical Analyses for Slag + Ambient Air Crushed Aggregate (91)

Staining

A “temporary” blue-green staining was observed at the center of cured ground granulated blast furnace slag cylinders when fractured during strength testing. The temporary staining is described as “greening” in a publication of the Slag Cement Association (100). “The blue-green color is attributed to a complex reaction of sulfide sulfur in slag cement with other compounds in the Portland cement. The degree and extent of the coloration depends on the rate of oxidation, the percentage of slag used, curing conditions, and the porosity of the concrete surfaces. … If greening does occur, it usually appears within a week of concrete placement and typically disappears within a week after oxidation starts. Surface greening diminishes as oxidation progresses and does not need to be treated” (100). It should be noted that the slag cement used in this project was used without Portland cement or other cementitious materials and still produced the temporary blue-green staining color. The staining was very bright in color, and indeed temporary, eventually disappearing from the surfaces of the aggregates produced from slag. ASTM C-641, “Standard Test Method for Iron Staining Materials in Lightweight Concrete Aggregates” provides a visual colorimetric method that was used to determine the staining potential of the five by- product-based aggregates. The test involves selecting two-100 gm samples of aggregate with a specific gradation passing the ⅜ inch (9.5 mm) sieve and retained on the No. 30 (600 μm) sieve. The sample is then placed at a uniform thickness on a white- filter paper that is then wrapped with cheesecloth. The wrapped sample was saturated in distilled water, and placed in a steam bath for 16 hours of continuous exposure.

The sample was then removed from the filter paper, and the filter paper was oven dried on a watch glass. ASTM C-641 provides examples of filter paper staining for comparison. In this case, all of the samples tested had no staining with a rating of zero. These results are not entirely surprising as the iron content indicated in the oxides (Fe2O3) analysis of the by-product was not too high, even the highest in the Class C fly ash, had a Fe2O3 content of only 5.5%. All of the other by-product materials had measured values below 2.0% for Fe2O3. Photographs of an example of the staining test apparatus, cheesecloth-filter paper wrapped samples, and the filter paper results for each CKD aggregate material prepared are shown on Figure 11-18.

In summary, all five of the by-product material based aggregates did not demonstrate staining effects when tested per ASTM C-641, “Standard Test Method for Iron Staining Materials in Lightweight Concrete Aggregates.”

Comparison to Commercial Aggregate Materials

The by-product-based crushed aggregate material properties were compared to published values for natural or lightweight aggregates. It should be noted that these materials were expected to be unique and were not necessarily expected to mirror the properties of natural or manufactured -lightweight aggregates. Pumice and expanded shale were identified as lightweight aggregates used commercially with similar physical density properties. The oxide compositions differ for the by-product-based materials. They all had significantly larger amounts of CaO than the pumice and expanded shale aggregates. The pumice and expanded shale aggregates had significantly larger amounts of SiO2. The pumice also demonstrated the lowest values for MgO, K2O, and TiO2. The published values for the other mineral compounds found in pumice and expanded shale, with the exception of Fe2O3 which was sometimes higher, all fell within the range of the values found for the by-product materials tested. The oxides analysis and other chemical properties comparison of the by-product-based aggregates and commercial aggregates are shown on Table 11-11.

The by-product-based crushed aggregates range of values for physical properties are compared to published values for natural crushed gravel, two sources of pumice, and a source of manufactured expanded shale aggregate. An attempt was not made to optimize grain size distribution but rather to show the as-crushed condition for each of the new aggregate materials. Specific gradations could be managed with conventional commercial aggregate screening plant equipment.

Table 11-11: Oxides Analysis and Other Chemical Properties Comparison of By-Product-Based Aggregates to Commercial Materials (%) (91)

The Witelite pumice physical properties fell within the ranges measured for the five by-product-based aggregates properties with the exception of absorption which was lower for this source of pumice at 16.3%. The volcanic pumice had a lower density of 37 lb/ft3 (593 kg/m3) and specific gravity of only 0.76. Staining was not reported for this aggregate source but is very important from an aesthetic perspective. Iron oxides that stain can provide unwanted black, brown, red, orange or pink discoloration in concrete and concrete products. The expanded shale properties shown in Table 11-12 for this source of lightweight aggregate fell within the ranges shown for the by-product-based aggregates with the exception of absorption which was slightly lower at 18% and the specific gravity was higher at a value of 1.73. The crushed gravel physical properties shown in Table 11-12 were significantly different when compared to the by-product aggregates with the highest density of 113 lb/ft3 (1812 kg/m3 ), absorption at only 1.6%, and a specific gravity of 2.57. Staining test performance was not reported. The physical properties comparison of by-product-based aggregates and commercial aggregates shown in Table 11- 12 confirm that these aggregates are similar to expanded shale aggregates which fall in density below normal crushed gravel aggregate and just higher than pumice lightweight aggregate.

Table 11-12: Physical Properties Comparison of By-Product Mineral Based Aggregates to Commercial Materials (91)

In summary the oxides analysis and physical properties of the by-product-based aggregates produced were determined, and shown to be similar to expanded shale and pumice aggregates as shown in Tables 11-11 and 11-12. All five of the by-product-based aggregates also did not show any iron staining effects. This information can be helpful in identifying potential construction uses for these new carbonated aggregate materials.

Feasibility Analysis of a Scaled Up Commercial Process

A carbonated-foamed material can most efficiently be manufactured at a site located adjacent to a by-product material producer. The following data indicates that the potential revenue from aggregate sales plus the estimated future value of CO2 sequestration credits would be sufficient to support a further detailed evaluation of an actual commercial, carbonated-foamed slurry to aggregates manufacturing facility. The volumes of industrial by-products produced, which were used in this project, are large. Many producers incur an expense for disposal in an environment with increasing environmental regulations for landfill facilities. Landfills are unpopular and can be difficult to permit with an estimated lead time in some areas of seven years or longer. Landfills can also present industries with ongoing expenses for treatment of leachates, and property maintenance long after the by-products have been landfilled and the landfill is closed. The prospect of constructing a facility for a carbonate mineralization process with a valuable and useful end product adjacent to a coal-fly ash power plant, LKD, CKD, Slag, and/or RCF source, to recycle by-product materials, that may otherwise be destined for disposal can be environmentally, economically, and sociologically attractive. Depending on market pricing for these commodities, it is possible that a carbonate-mineralization to aggregate production process could become commercially attractive because the aggregates produced can be easily stockpiled and stored outdoors to meet seasonal demands for such aggregates. The advantages and disadvantages of a commercial by- product aggregate production process, from the social, environmental and financial perspectives are discussed below.

Benefits of Commercial By-Product Aggregates

Social Perspective:

• No loss of jobs. Existing landfill construction and disposal activities become carbonate-mineralization to sand/gravel/crushed stone stockpiling and material handling functions.
• Land use does not increase and existing dedicated property is converted to a higher value manufacturing use.
• If lightweight aggregates are manufactured and more widely used, there could be a safety benefit for workers with fewer injuries from lifting and handling lighter concrete blocks and other building materials.
• There  would  be  lower  fuel  usage  for  delivering  lower  density  materials conserving a valuable and dwindling energy resources for other societal purposes.
• There would be potential energy efficiency from the additional insulating value of lower density building materials.

Environmental Perspective:

• The need for additional landfills and associated environmental impacts could be reduced or eliminated for these industrial by-product materials.
• Existing natural mineral resources can be preserved for future generations.
• The process makes beneficial use of industrial by-products in producing carbonate mineralized aggregates.
• CO2 is sequestered and stored for the geologic long term as carbonate at the amounts shown, see Table 11-13.
• A useful “green” building material becomes available at many new locations for a variety of purposes, such as: lower density geotechnical applications, insulating material, green roof rooting media, lightweight concrete and concrete products such as masonry units, and also provide potentially increased fire resistance and protection.

Table 11-13: Average CO2 Sequestered by Percent Mass at 182 Day Age (91)

Financial Perspective:

• The overall life cycle cost of industrial by-product disposal in a landfill in Wisconsin is estimated at approximately $35.00 to $45.00/ton.
• Lightweight aggregate pricing varies based on the quality and end-use application between $24.00 to $38.00/ton.
• Normal-weight aggregate pricing varies based on quality and processing requirements between $3.00 and $12.00/ton.
• CO2 credits were valued in Europe at as much as $31.50/ton during 2006 (103) and are projected to increase in the future, as new laws regarding lowering CO2 production become prevalent in the U.S.A.
• Aggregate is the largest volume ingredient in concrete, making up to 80% of the concrete volume. Therefore, the use of these by-product-based aggregates could significantly increase the recycled “green” content of concrete building materials

The following conservative economic assumptions for feasibility come from the author’s personal experience with other similar activities:

• Industrial by-product source manufacturing plants typically have an existing landfill, and space for production and storage of carbonate-aggregates on site.
• Dry fine powder industrial by-products can be moved from plant collection silos to an on-site batch plant silo for $5.00/ton (short haul or pneumatic transport line).
• Contractors can supply foaming agent, water and equipment to process industrial by-product materials at $30/ton.
• Hardened foamed material can be picked up, crushed and stockpiled, which provides additional surface area and pathways to absorb CO2, for $15.00/ton.
• Normal material handling economics apply for supplying stockpiled materials to users, although a fuel savings may be possible if replacing normal weight materials with lightweight materials.
• CO2 credits are available for sequestered CO2 at $30.00/ton. Figure 11-19 was prepared as an example, to calculate the CO2 credit in dollars per cubic yard, based upon percent CO2 mass sequestered for an aggregate with a dry-rodded density of 55 lb/ft3 (882 kg/m3) (91). For example, if the manufactured aggregate has a density of 55 lb/ft3 (882 kg/m3), and CO2 sequestration credits are valued at $30.00/ton, and the percentage by mass of CO2 sequestered within the aggregate are known, then a manufacturer could easily reference this chart to find the dollar value of the CO2 sequestration credit available for each cubic yard of aggregate produced and sold. This information is important in establishing a competitive selling price for the manufactured aggregate product (91).

Figure 11- 19: CO2 Credit Value Example for 55 lb/ft3 Dry Density Aggregate (91). Note: 1.00 lb/ft3 = 16.0356 kg/m3, 1.00 ton = 0.91 metric ton

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Source: We Energies

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