It is the action of human beings that determines the worth of any material. Materials having potential for gainful utilization remain in the category of waste until its potential is understood and put to right use. Fly ash is one such example, which has been treated as a waste material in India, till a decade back, and has now emerged not only as a resource material, but also as an environment saviour. The Indian market is extremely receptive to clean development mechanism (CDM), with a fair amount of appreciation both by the government and the industry. Developed countries like US account for 30 % of global emissions, while India contributes about 3 % of the global Green House Gases (GHS) against the global average of 5.2 %.  Use of fly ash in various products and partly substituting cement at current annual levels in India saves generation of CO2 by 25 million tonnes, good quality lime by 35 million tonnes and coal by 15 million tonnes a year. The potential that is yet to be tapped is multi-fold of the current levels. Notwithstanding the above, it is also acknowledged that there are large regional variations in both production (as can be seen from Figure 1) and use of the material, with the perception of fly ash and attitudes to its use, being continually redefined. 

Supplementary cementing materials (SCM) are widely used in India to partly replace cement in concrete. The SCM’s most commonly used include fly ash, ground granulated blast furnace slag and silica fume. We have selected fly ash as SCM for the manufacturing of fibre cement sheets due to its availability, cheaper than cement and protection to environment from pollution. This paper describes in detail how the use of fly ash in fibre cement sheets on a massive scale (35 to 40% in formulation) has been a success story, meeting all the requirements, as specified. 

ABOUT FLY ASH 

Fly ash is a burnt and powdery derivative of inorganic mineral matter that generates during the combustion of pulverized coal in the thermal power plant. The burnt ash of the coal contains mostly silica, alumina, calcium and iron as the major chemical constituents. Depending on the burning temperature of coal, the mineral phases in crystalline to non-crystalline structures such as quartz (SiO2), mullite (3Al2O3 2 H2O), hematite (Fe2O3), magnetite (Fe3O4), wustite (FeO), metallic iron,  orthoclase (K2O Al2O36 SiO2) and fused silicates usually occur in the burnt coal ash.

 Silica and alumina account for about 75 to 95 % in the ash. The classification of thermal plant fly ash is considered based on reactive calcium oxide content as class-F (less than 10 %) and class-C (more than 10 %). Indian fly ash belongs to class-F. The calcium bearing silica and silicate minerals of ash occur either in crystalline or non-crystalline structures and  are hydraulic in nature; they easily reacts with water or hydrated lime and develop pozzolanic property. But the crystalline mineral phases of quartz and mullite present in the ash are stable structures  of silica and silicates, and are non-hydraulic in nature. Usually the fly ash contains these two mineral phases as the major constituents. Therefore, the utilisation of fly ash in making building materials like fibre cement sheets largely depends on the mineral structure and pozzolanic property. 

Fly ash is broadly an aluminium-silicate type of mineral rich in alumina and silica. The conversion of these oxides of fly ash to hydrous silico-aluminate  structures by chemical activation has been made under alkaline condition with lime. The effect of chemical transformation of ash in the development of binding property has been observed to find their suitability in manufacturing of fibre cement sheets. Further, it has been attempted to use high volume fly ash (35 to 40 % in formulation) in development of corrugated and flat fibre cement sheets with asbestos and non-asbestos fibres.