Fly Ash Composites as Wood Substitute - Coal Fly Ash Based Building Materials

ABSTRACT

To meet the shelter component of the people, timber, bricks, blocks and concrete are being used as major construction materials in the building industries and all such materials are exploited from natural resources. Due to depletion of available natural resources like forest reserves, minerals (clay, limestone, aggregates etc) and its environmental consequences accentuate the importance of utilisation of wasted resources suitably for developing alternative building materials. In this context, fly ash generated from coal combustion process for electricity generation is considered as a potential engineering material which can be recycled and used effectively in a technically feasible, economically viable and socially acceptable manner.

CSIR-AMPRI, Bhopal has developed expertise and created facilities for large scale utilisation of fly ash in making wood substitute composites, fired bricks, concrete blocks, coating materials/ paints and demonstrated by constructing proto type houses using fly ash based building materials for confidence and awareness generation. Furthermore, CSIR AMPRI has demonstrated the use of fly ash as soil modifier and micro nutrients to increase the agricultural productivity at different parts of India. This paper deals with the highlights of fly ash based building materials with special emphasis on fly ash composites as wood substitute.

For manufacturing wood substitute composites, processed fly ash was mixed with polymer and catalyst and synthesized using natural fibre reinforcement in moulds of required dimensions. The fly ash based polymer composite products exhibited better tensile, flexural and impact strength. Fly ash composites are weather and corrosion resistant, termite, fungus, rot and rodent resistant and fire retardant. This durable and abrasive resistant fly ash composite is stronger than wood and could be used as a substitute for timber. The timber substitute composite is cost effective & maintenance  free and has wider applications in construction industry for use as doors, windows, ceilings, flooring, partition, furniture etc.

INTRODUCTION

Universally, coal combustion based power generation is the most widely implemented source to full fill the electricity demand and as a consequence large quantity of fly ash is being released by thermal power plants as waste material and becomes a major environmental hazard. Presently, major fly ash producers are India (~200 million tons per annum (MTPA)), China (~160 MTPA) followed by United States of America (~ 150 MTPA). Universally, there is a large consensus on electricity requirements and coal based energy generation is expected to increase rapidly in the immediate future and fly ash generation is expected to increase many folds.

As per the ASTM standards, in India bituminous and sub-bituminous coal results in class ‘F‘ash and lignite coal produces class ‘C’ ash having high degree of self-hardening capacity. Physical, chemical and mineralogical, morphological and radioactive properties of fly ash in general vary as they are influenced by coal quality, combustion process, degree of weathering, particle size and age of the ash. The major constituents of fly ash consist of silica, allumina and iron oxides together is about 87%. The other chemical constituents in fly ash are Ca, Mg, Na and K. However, trace elements such as Mn, Zn, Cd, Pb, Mo, Ni, As, Se and B in fly ash are important concern for land disposal due to their environmental contamination.

Already accumulated and ever increasing quantity of fly ash leads to major disposal problems followed by other environmental issues. There are various methods available for the management of fly ash which primarily includes the prevention of waste at source, waste minimisation, recycling, and materials recovery. From the work done by other researches, it is apparent that fly ash has many potential to use in cement, bricks, cement, concrete, ceramics, adhesives, wallboard, and agriculture / soil amelioration, wood substitute, paint, road embankment, backfills and valuable metal extraction.

CSIR AMPRI, Bhopal has made significant efforts to evaluate various physical-chemical, engineering, mineralogical and morphological properties of fly ash and assessed its potential to effectively utilise it as a raw materials in developing alternative building materials such as wood substitute composites, bricks, concrete, roofing sheets, paints and as additives for immobilisation of hazardous jarosite waste . Various lab and bench scale experiments were conducted to synthesis and fabricate all such building materials and optimised the process parameters and demonstrated few of the technology in pilot scale and exploring for possible commercialisation. The highlights of the work done at AMPRI on fly ash utilisation is summarised as below.

WOOD SUBSTITUTE COMPOSITES

CSIR- AMPRI Bhopal, India has developed process know-how and optimised the parameters for utilisation of fly ash as a filler and reinforcing materials in manufacturing alternative wood products. Manufacturing wood substitute products includes: (i) selection of fly ash quality and natural fibre, (ii) processing of fly ash and natural fibre, (iii) fly ash and natural fibre characteristics, (iv) natural fibre fire retardant treatment, (v) calendaring of natural fibre woven fabric with fly ash and polymer matrix, (vi) casting / fabrication of composites and (vii) curing, cutting and staging of composite panels.

Raw materials

Fly ash, natural fibre and polymer are the major raw materials required for fabrication of wood substitute composites. Fly ash can be obtained from thermal power station and is available in abundance in all parts of India. For the present study, fly ash from the Satpura Thermal Power Station, Sarni (M.P) was collected and a fly ash sample was used after drying and sieving it through 150-micron sieve. Jute woven fabric is one of the natural fibre can be procured indigenously and is available in most parts of India, nevertheless, jute woven fabric is more prominently available in Kolkata, Odhisa, and Andhra Pradesh. For the present study, jute fabric was procured from local market in Bhopal. Fire retardant grade polyester resin is commercially available. For the present study, polyester resin and catalyst was procured from M/s Agro polymer New Delhi. It is to note that fly ash can also be replaced suitably with other industrial wastes such as red mud and marble waste  to achieve required quality of wood substitute composites.

Fabrication of fly ash polymer composite panels

The composite materials were synthesized by mixing the red mud/ fly ash / marble waste in the polymer with the help of a mechanical stirrer. In order to accelerate the polymerisation process, cobalt naphthonate (2-5%) and methyl ethyl ketone peroxide (mekp) (2-5%) as catalyst was used. Unsaturated polyester resin (Acrolyte 572), from Acro Polymer Pvt. Ltd. New Delhi, India, having viscosity as 15 poise at (25±2°C) and acid No 2mg KOH/g with slow flammability potential was used along with Cobalt Naphthonate as accelerator and Methyl Ethyl Ketone Peroxide (MEKP) as catalyst.

Fire retardant treatment was done to the jute cloth before using it for making composite panels. To impart fire-resisting potential, the jute fabric was treated with 10 % solution of ammonium phosphate and ammonium sulphate after its initial washing and drying. The composites in sheet form were prepared using jute cloth as reinforcement in polymer matrix. Composite panels were synthesised and fabricated by dispensing this matrix over the jute cloth in a hydraulic press. According to the requirement of the thickness, the number of layers of jute cloth can be placed. The laminates were prepared in hydraulic press using suitable moulds and pressed at a pressure of 0.5 MPa for one hour at room temperature and post cured in an oven at 80°C for 24 hours. A schematic representation of different steps involved in synthesizing the composites is shown below. The compositions of different constituents are: 50% industrial waste, 15% natural fibre and 35% polymer.

Physical and mechanical characterisation of fly ash polymer wood substitute composites

Physical and mechanical properties of the fly ash composite materials were done according to ASTM D792-91 (Density), ASTM D571-88 (Water absorption), ASTM D790-92 (Flexural strength), ASTM D638-91 (Tensile strength). The tensile and flexural strengths of the specimens were measured at a cross head speed of 5mm /min using universal testing machine (Instron 1185, England and UTM (5 KN), LRX Plus, Lloyd, UK. Abrasion tests were conducted using Suga Abrasion Tester, Japan as per ASTM D 1242-92. Fly ash polymer composite samples were prepared in required size and tested following the procedures recommended in the ASTM D256-92 using impact tester (Model IT504) Tinius Olsen, UK. Accelerated weathering studies was done as per ASTM G-26-92 using a Weatherometer (Atlas, ES 25, US). The composites reinforced with jute fibres were exposed to ultra violet radiation, water spray and humidity.

Fire behaviour characteristics

Fire retardance/ resistance tests were conducted to measure various properties like non- combustibility (IS: 3808 –1979), Ignitibility(BS 476 : part 5-1968), fire  propagation index (BS 476: part 6-1981), surface spread of flame (BS 476 : part 7-1981), and maximum specific optical density (flaming and non flaming) as per procedures given in ASTM E 662-79. Non-combustibility studies of fly ash based composites were carried  out using an electrical muffle tube furnace. The specimens of required size (dia 45mm, height 50mm) were prepared and inserted in the furnace for 20 minutes. Parameters determined were rise in temperature, duration of sustained flaming and mass loss of the specimen. The specific optical density of smoke (non flaming and flaming) for all the varieties of samples were evaluated using a smoke density chamber wherein smoke generated from the specimen was allowed to accumulate in the chamber equipped with tubular electrical furnace, providing external radiant flux on the exposed specimen surface during the test. The specimens of length 75 mm, breadth 75 mm and thickness 10 mm were used. In accordance with the standard procedure, the external radiant heat source is heated to yield a heat flux of 25kW/m2 in the plane of the exposed surface of the specimen. The photometric system was adjusted for 100% transmittance initially when there was no smoke inside the chamber. Two types of tests (non-flaming and flaming) were conducted on each of the materials. Specific optical density of smoke is calculated from the percent transmittance and its maximum value corresponds to the minimum value of percent transmittance.

Microstructure

Microstructural studies were carried out using scanning electron microscope (SEM JEOL 35 and JOEL JSM-5600, Japan). The samples were cold mounted in polyester resin and polished according to standard metallographic techniques. The samples were placed on brass studs and sputtered with gold prior to their SEM examination. Morphology of the fly ash particles and interfacial bonding of jute fibres with the polymer matrix were studied. The fractured surfaces were also examined to understand the fibre fracture and interfacial adhesion.

RESULTS AND DISCUSSIONS

Properties of raw materials

Table 1 shows the chemical composition of fly ash and jute fibre used in making composites. Fly ash from the Satpura Thermal Power Station, Sarni (M.P) was collected and fly ash samples were used after drying and sieving it through 150-micron sieve. X- ray diffraction of fly ash showed -quartz and mullite as the major crystalline phases. These constituents impart abrasion resistance to the fly ash. The major constituents in fly ash are SiO2 (57.64) and Al2O3 (30.94). Among the other constituents, Fe2O3 (6.47%) was present together with the oxides of sodium, calcium and magnesium in smaller proportion

Table 1 Chemical composition in fly ash

The chemical composition and mechanical properties of jute / fibre fabric is shown in Table 2. Major constituents in jute fibre were cellulose (~ 72.45%). Tensile strength of jute fibre showed as high as 654 MPa. The quality of Jute woven fabric was 300 GSM (g/m2) grade which was treating with fire retardant materials.

Table 2 Chemical composition in jute fibre / fabric

Properties of fly ash and natural fibre reinforced wood substitute composites

Properties of the developed fly ash based wood substitute composites are shown in Table 3.The density of the composites are varied from 1.60 to 1.76 gm/cc, which was found to be relatively higher than that of conventional related materials such as wood, rice husk board, medium density fibre board etc. The water absorption was found to be in the range of 1.10 to 1.50% after 24 hrs of soaking in water. The extent of absorption of water in the wood was much higher (12-20%) than that of the fly ash composites. The range of the swelling for the woods was significantly higher (as high as 8.6%) than the fly ash composites. Resistance of the composites to burning (firing) was also much improved over the conventional wood material. The composites were observed to be self extinguishing (within 15–22 second) in nature. Resistance to chemical attack and weathering was also much better in the case of the fly ash composites than the wood.

The fly ash composites developed in this study attained much improved physical and mechanical properties as compared to those of their conventional counterparts and the results are shown in Table 3 and Figs. 1 and 2. However, the density of wood substitute composite was higher than wood and the specific strength was still better than teak and other agro-waste materials. The addition of fly ash reduced the abrasion rate by almost 43%. Additional strengthening provided by the fibres might have led to improve abrasion resistance to the fibre composites. These results are compared with the unreinforced polyester matrix and PVC which is used for flooring. The extent of abrasion was found to be minimum in the fly ash composites. Addition of fly ash to polyester reduces the impact strength of laminates because fly ash reinforcement imparts brittle failure, whereas the reinforcement of fibers increases the impact strength because of the extra energy needed for fiber pullout and debonding of fibres from the polyester matrix. The accelerated weathering studies showed better performance of red mud and fly ash polymer composites as compared to the composites without fly ash. The improved behaviour may be due to the highly stabilizing ceramic oxides present in fly ash.

Table 3 Mechanical properties of fly ash composites Vs Conventional Materials

Fig.1 Comparison of tensile and Flexural strength fly ash composites with other related materials

Fig. 2 Comparison of water absorption and density of fly ash composites with other related materials

Fire ignitability behaviour of fly ash polymer composites indicate that these are not easily ignitable. The fire propagation index shows that fly ash composites are better in comparison with medium density fibre board (MDF), expanded polystyrene (EPS) and wood (Table 4). It is due to the combined affect of fire retardant additives and inorganic nature of fly ash. An appraisal of the above clearly suggests that polymer matrix composites so developed in the present study using fly ash and plant fibres as reinforcement/ fillers are potential wood substitute materials.

Table 4 Fire Behaviour of industrial waste-Jute polymer composites vis-à-vis conventional materials

Uniqueness of the wood substitute products:

• Durable and stronger than wood
• Weather resistant and durable
• Corrosion resistant
• Termite fungus, rot and rodent resistant
• Fire retardant, self-extinguishing nature
• Cost effective & maintenance free
• Use in variety of applications: doors, ceilings, flooring, partition, furniture, etc.

Technology Enabling Centre for manufacturing Wood substitute composites

For up-scaling and customization of timber substitute composites, a Technology Enabling Centre (TEC) was setup at CSIR-AMPRI Bhopal for manufacturing wood substitute composite products in semi batch process and to serve as a training center for budding entrepreneurs (Fig. 3). This technology was developed in view of National Forest Policy of Government of India for alternative to wood for building application so that consumption of timber in building and house construction can be minimised. Furthermore, this will be a potential solution for effective fly ash utilisation leading to solving various environmental threats associated with mismanagement of fly ash, deforestation and ecological imbalance.

Machinery / equipment requirement

The machineries/ equipment required for manufacturing of wood substitute composite products are as follows:

• Pulverising system
• Oven for drying raw materials
• Jute cloth dressing and handling system
• Fire retardant treatment mixing tank
• Wet / dry cloth winder, and re-winder / bobbins
• Mixing system for industrial wastes and resin
• Coating tank to impregnate resin mixed with industrial wastes into the cloth matrix
• System for laying, thickness building and cutting of wet sheet
• Hydraulic press
• Transfer trolley and multi deck trolley
• Curing oven

The utility of the concept developed for manufacturing wood substitute composites was successfully demonstrated both in lab scale and pilot scale and is expected to find a socio, techno-economic contribution to the construction and other industries. The developed composite products can be used in construction sector as an alternative to the conventionally used wood. Photographic view of wood substitute composites and their applications are shown below (Fig. 4, 5, 6, 7).

Fig. 4 Fly ash Jute fibre fabric reinforced polymer composite panel

Fig. 5 Fly ash -polymer composites decorative panel and Door

Fig. 6 Fly ash polymer composites in partitions and wind energy blades

Fig. 7 Batteries recharge station / house constructed using fly ash based composite sheets at New Delhi

Ready-made house was constructed using fly ash polymer composite panels as walling and roofing elements which is being used for re-charging Solar Auto Risksha’s batteries at Chandhini Chock, New Delhi. The advantages of use of such wood substitute composites are (i) Light weight structure, approximately 40 - 45 kg/m2 floor area, (ii) Easy to transport: 10-20 houses at a time, (iii) Easy to assemble and de-assemble with folding options, (iv) Fire resistant and corrosion resistant fly ash composite sheets.

Yet, there are possibilities and challenges for further research and development of light weight and high strength advanced composite materials for high end use application, which may be accomplished by CSIR- AMPRI Scientists with the potential funding support of industries, if found challenging.

The outcome the process and product know-how is three fold:

• The addition of fly ash to polyester increased the resistance to abrasion, density, modulus of rupture and other mechanical properties of composites
• Suitable combination of polyester, fly ash or other industrial wastes and natural fibres can be used to get the desired properties of the laminates / composites.
• The developed composites have the potential to be used in a variety of  engineering applications.
• This composite is an eco friendly material and the environment benefits are twofold: (i) this helps to prevent further deforestation and (ii) this leads to effectively utilising fly ash contributing to reduction in pollution hazards. The Building Materials Development Group at AMPRI Bhopal has expertise and facilities for large scale recycling fly ash in building materials. The facilities have been in use for the development of technologies of direct relevance to the construction industries and waste producers.

Recent advances: Process and product improvement on wood substitute composites

It is to note that there was a constraint associated with wood substitute composite’s density (1.45-1.8 gm/ cc). By carrying out various experiments during recent years, composites have been synthesized and fabricated with density varying from 0.35- 1.2 gm/ cc using polyester resin and polyurethane resin system (Table 5). Further, sandwich composites and light weight composites using fly ash and polyurethane resin system ha(av)e also been fabricated which has showed improved aesthetic and glassy finish. The photographic view and the mechanical properties of the light weight fly ash composite and sandwich composites are shown below (Fig. 8 a,b,c):

Fig. 8 Fly ash based sandwich composites and solid light weight composites

Table 5 Mechanical properties of Fly ash based light weight composites

Mostly, composites have been manufactured using glass, carbon and other petroleum based products and such composites are being used in a variety of applications. Emission of undesirable pollutants during manufacturing of petroleum based fibre is major  problem which prioritizes the need of exploitation of renewable resources and recycling waste materials for the development of new materials that could be used as an alternative to the conventionally used wood for construction industry and other commercial purposes. The outcome of the work done at CSIR AMPRI Bhopal will significantly contribute for utilisation of fly ash and natural fibres in making a composite wood substitute materials leading to deforestation and save our environment.

CLAY- FLY ASH BRICKS AND THEIR DURABILITY CHARACTERISTICS

Based on the lab scale studies, predetermined quantity of industrial wastes weree added to the soil depending upon the characteristics of the soil and wastes and thoroughly mixed with required quantity of water. Casting of the bricks were done manually and by machine. After drying bricks were placed in the kiln for firing at 900 ±10°C and then removed for use in building construction. Fig. 9 shows fly ash bricks manufacturing at CSIR AMPRI Bhopal.

Salient features:

• Less breakage
• Better shape
• Saving of soil and fuel
• Comp. Strength : 70- 40kg/cm2
• Water absorption < 18% and
• Shrinkage < 10%.

Fig. 9 Fly ash bricks manufacturing at CSIR AMPRI Bhopal

The performance of clay bricks and clay fly ash bricks exposed under different accelerated environmental conditions such as UV radiation, temperature & moisture, wetting & drying, SOx, NOx, CO2, Cl2, acidic, alkaline and saline treatments were assessed. The result revealed that over a period of 50, 100, 200, 300, 400 and 500 days of these treatments, there was no significant variation on the properties of both types of bricks. The durability of bricks developed using clay fly ash is comparable to that of clay bricks and can be used effectively even in polluted environment. This study has further strengthened that clay fly ash bricks can be effectively utilised in the construction sector without compromising the quality even under degrading environmental conditions.

FLY ASH BASED CONCRETE BLOCKS AND ROOFING SHEETS

For the faster construction and as a substitution of bricks, AMPRI, Bhopal has developed precast concrete blocks from fly ash, stone dust and coarse aggregate. Both solid and hollow blocks can be manufactured in suitable sizes and exhibited a compressive strength 40 –60 kg/cm2 and water absorption below 12%. These blocks can be used as walling material in houses/building construction. The fly ash based concrete blocks are cost effective, consistent quality, showed good appearance and does not require plastering and can be used for speedy construction with less labourers input.

CSIR- AMPRI, Bhopal has developed a process to effectively utilise fly ash along with organic fibres for the production of corrugated roofing sheets. This roofing sheets can be used as an alternative to carcinogenic asbestos cement sheets. Its strength is comparable with asbestos cement sheets. Sheets can be made manually or mechanically and thus have great potential for use in rural areas. The advantages of fly ash –cement roofing sheets  are use of renewable natural fibres, eco friendly, sheets are repairable, no health hazards, cost effective and manufacturing led to employment generation in rural sectors. For demonstration, 24 twin houses were constructed using fly ash based concrete blocks and roofing sheets at the CSIR AMPRI premises. Even after 20 years of their construction, these houses are remain intact and no water seepage, cracks or any damages could be recorded.

Fig. 11 Proto type houses constructed used fly ash based building components at CSIR AMPRI, Bhopal

PAINT

Cost effective and better quality paints have been developed using fly ash as an extender. The properties such as chemical inertness, low oil absorption and specific gravity value of fly ash contributes for better quality of fly ash based paints. This study opens a new avenue for fly ash utilisation leading to partial or complete replacement of conventional extenders. The paint prepared with Fly Ash has shown improved abrasion resistance over other commercially available formulations and also good protection against corrosion. Also, the incorporation of fly ash does not affect film properties like brushability, drying time and gloss. There is a wide scope of utilising Fly Ash as an extender in paints. This fly ash paint can be used in industrial maintenance coatings on metallic structures for corrosion protection in moderate to severe corrosive environments, as anti-abrasive coatings, as marine coatings

FLY ASH IN CONSTRUCTION OF HOUSING

Alternative building materials such as clay fly ash bricks and fly ash wood substitute composites and precast RCC roofing were developed and demonstrated by constructing 16 numbers of prototype houses (Fig. 11). AMPRI scientific, technical and administrative staffs are residing in these houses. for the past 16 years They were constructed for generating confidence and awareness among common mass on fly ash utilization in housing sector.

Fig. 11 Proto type houses constructed used fly ash based building components at CSIR AMPRI, Bhopal

CSIR-AMPRI, Bhopal has been conferred with the National Award for Fly Ash Utilization, jointly awarded by Ministry of Power, MoEF and DST, Government of India for the R & D on “Use of Fly ash for Building Components, Agriculture & Value added products” on 4th December 2005.

CONCLUSIONS

CSIR AMPRI, Bhopal has developed cost effective alternative building materials from fly ash and has expertise and facilities. The salient features of the fly ash bricks are less breakage, better shape, use of waste material, saving of fuel, increase in strength and durability of fly ash bricks are as good as clay bricks. The paints prepared with fly ash and blue dust have shown improved abrasion resistance over other commercially available formulations, good corrosion resistance. The fly ash wood substitute products are stronger than wood, weather resistant and durable.

Worldwide, scientific innovations on fly ash utilisation have resulted in the introduction of alternative materials as substitutes to cement matrix composites, polymer matrix composites and metal metric composites. Fly ash is indeed wasted resources and it is imperative to leverage all R&D organizations’ expertise, technologies and research strengths to (i) maximise the use of fly ash (ii) up-scale lab scale technologies and (iii) commercialize existing technologies to achieve quantum cost, environmental & social benefits on fly ash based alternative composite materials and (iv) perform advanced R&D innovation to cope-up with the future needs of the society.

The technologies developed at CSIR AMPRI Bhopal have shown major utilisation potential of fly ash and setting-up of secondary industry especially for manufacturing of alternative building materials which can significantly contributes towards increasing the employment opportunity and economy of the urban / rural people. However, yet there are several impediment for effective implementation of such technologies for optimum benefit. Especially lack of awareness, interaction and confidence among the general community, lacking of comprehensive technological package and initial investment, intellectual perception and co-ordinated participation from research organisation entrepreneurs and funding agency from the inception of the project are the major barrier to be alleviated for successful commercial exploitation.

Source: Asokan Pappu, Mohini Saxena#, RK. Morchhale - Principal Scientists CSIR- Advanced Materials and Processes Research Institute (AMPRI), Bhopal

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