Several factors have impeded fly ash utilization in India, while it is being extensively used globally. Coal-based thermal power stations have been operational for more than 50 years but the concept of developing environment-friendly solutions for fly ash utilization is only about 15 years old. Overall fly ash utilization in India stands at a fairly low level of about 15 per cent of the quantity generated. Various possibilities for its use are under research.
Among numerous factors that account for the low level of utilization, the chief factors are:
Poor understanding of the chemistry of fly ash and its derivatives for proper end applications
Absence of standards and specifications for fly ash products
Lack of reliable quality assurance for fly ash products Poor public awareness about the products and their performance
Non-availability of dry fly ash collection facilities Easy availability of land with top soil at cheap rates for manufacturing conventional bricks
Lack of proper coordination between thermal plants and ash users.
Fly ash utilization in the country is gaining momentum owing to the stringent regulations that the MoEF has stipulated, as also to increased awareness about the benefits of using fly ash for various products.
Fly ash from coal-fired thermal power stations is an excellent potential raw material for the manufacture of
construction material like blended cement, fly ash bricks, mosaic tiles and hollow blocks. It also has other, high volume applications and can be used for paving roads, building embankments, and mine fills.
Fly ash products have several advantages over conventional products. The use of cement in the manufacture of construction products can be reduced by substitution with fly ash. While the use of cement cannot be completely avoided, for certain products like tiles, the substitution can go up to 50 per cent. These products are known to be stronger and more cost-effective because of substantial savings on raw material.
fly ash bricks
Fly ash products are also environment-friendly. A case in point is fly ash bricks. The manufacture of conventional clay bricks involves the consumption of large amounts of clay. This depletes topsoil and degradation of agricultural land. Fly ash bricks do not require clay and serve two purposes; preservation of topsoil and constructive utilization of fly ash.
Agricultural uses of fly ash
Research on agricultural uses of fly ash has been going on in universities and research institutes (see box) across the country for several years.
The same fly ash that causes harm when it settles on leaves, can prove beneficial when applied scientifically to agricultural fields. It can be a soil modifier and enhance its moisture retaining capacity and fertility. It improves the plant's water and nutrient uptake, helps in the development of roots and soil-binding, stores carbohydrates and oils for use when needed, protects the plants from soil-borne diseases, and detoxifies contaminated soils.
Yields are also
known to increase, as experiments on groundnut, sunflower, linseed and other oilseeds have shown.
Fly ash as fill material
Large scale use of ash as a fill material can be applied where
· Fly ash replaces another material and is therefore in direct competition with that material.
· Fly ash itself is used by the power generating company producing the fly ash to improve the economics of the overall disposal of surplus fly ash.
Full utilisation of generated fly ash in India, will provide employment potential for 3000 people.
Higher ultimate strength Increased durability Improved workability Reduced bleeding Increased resistance to sulfate attack
Increased resistance to alkali-silica reactivity Reduced shrinkage.
Fly ash utilization, especially in concrete, has significant benefits including:
(1) increasing the life of concrete roads and structures by improving concrete durability,
(2) net reduction in energy use and greenhouse gas and other adverse air emissions when fly ash is used to replace or displace manufactured cement,
(3) reduction in amount of coal combustion products that must
¨ Fly ash disposal is combined with the rehabilitation and reclamation of land areas desecrated by other operations.
Fills can be constructed as structural fills where the fly ash is placed in thin lifts and compacted. Structural fly ash fills are relatively incompressible and are suitable for the support of buildings and other structures. Non-structural fly ash fill can be used for the development of parks, parking lots, playgrounds and other similar lightly loaded facilities. One of the most significant characteristics of fly ash in its use as a fill material is its strength. Well-compacted fly ash has strength comparable to or greater than soils normally used in earth fill operations. In addition, fly ash possesses self-hardening properties which can result in the development of shear strengths. The addition of illite or cement can induce hardening in bituminous fly ash which may not self-harden alone. Significant increases in shear strength can be realized in relatively short periods of time and it can be very useful in the design of embankments.
Fly ash in portland cement concrete
Fly ash can be used in portland cement concrete to enhance the performance of the concrete. Portland cement is manufactured with calcium oxide (CaO), some of which is released in a free state during hydration. As much as 20 pounds of free lime is released during hydration of 100 pounds of cement. This liberated lime forms the necessary ingredient for reaction with fly ash silicates to form strong and durable cementing compounds, thus improves many of the properties of the concrete. Some of the resulting benefits are:
be disposed in landfills, and (4) conservation of other natural resources and materials. Typically, 15 to 30 per cent of the portland cement is replaced with fly ash.
Fly ash for roads
Fly ash can be used for construction of road and embankment. This utilization has many advantages over conventional methods.
O Saves top soil which otherwise is conventionally used
O Avoids creation of low lying areas (by excavation of
soil to be used for construction of embankments)
O Avoids recurring expenditure on excavation of soil from one place for construction and filling up of low lying areas thus created.
O Does not deprive the nation of the agricultural
produce that would be grown on the top soil which otherwise would have been used for embankment construction.
O Reduces the demand of land for disposal/deposition of fly ash that otherwise would not have been used for construction of embankment.
O Controls the source of pollution.
Manufacturing process of a typical fly ash product
Mosaic tile manufacture involves preparing the mix for two layers: the wearing layer and the base layer. The wearing layer consists of a plastic mix of mosaic chips, cement, and fly ash and dolomite powder. The base layer consists of a semi-dry mix of fly ash, cement and quarry dust. The tiles are pressed in the tile-making machine and air-dried for 12 hours or more. They then undergo curingi n water tanks for 15 days. The tiles are then polished and stacked for supply.
Fly ash products manufactured at CASHUTEC, Raichur
While the procedure may be similar to the conventional method, the substitution of cement with fly ash by up to 30 per cent serves as value addition. Fly ash is used in both layers of a mosaic tile. Several benefits accrue from this. Fly ash turns from a problem ridden byproduct into a component of a utility product comparable to conventional products in strength and aesthetics. Fly ash tiles, for example, can be used for heavy-duty floors too. The table below suggests that the properties of fly ash mosaic tiles are far superior to those of conventional tiles.
FAL-G (fly ash-lime-gypsum)
Fal-G bricks and blocks are manufactured without using thermal energy, in contrast to the sintering involved in the production of clay bricks.
How do these bricks get strength, if they are not baked?
Fal-G bricks are made of a mixture of fly ash-limegypsum or fly ash-cement-gypsum. In either combination, Fal-G is a hydraulic cement, which means it sets and hardens in the presence of moisture, on the lines of ordinary portland cement, gaining strength progressively over ageing. Nearly 200 tonnes of coal is used to sinter one million clay bricks, a process that generates over 350 tonnes of carbon dioxide (CO2). The production process of
World Bank has offered to buy 800,000 tonnes of CO2 reductions from utilisation of Fly ash.
Fal-G bricks eliminates harmful emissions of this scale. This would also be the amount of carbon credit earned
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