‘Fly ash’ produced in thermal power plants is an ecofriendly resource for use…

Concept: Fly ash is the fine, glassy residue captured from the flue gas of coal-fired thermal power plants. Two properties turn it from a waste into a resource. First, it is a pozzolana: an amorphous silica-alumina powder that reacts with free lime and water to form cementing compounds. Second, it is a light, fine, porous solid that carries plant-available trace elements such as boron, zinc, molybdenum and iron. Every recognised use of fly ash follows from one of these two properties - the pozzolanic (binding) property or the physical and chemical soil-conditioning property.

Application: apply those two properties to each use listed in the stem.

• (a) Agriculture as a micro-nutrient source - fly ash supplies boron, zinc, molybdenum and iron in plant-available form, while its fine porous grains lower bulk density and raise the water-holding capacity of heavy soils. This draws on the soil-conditioning property.

• (b) Wasteland development - the same soil-conditioning action is applied in bulk to degraded land, mine voids and abandoned quarries, where fly ash corrects texture and bulk density so that a vegetative cover can establish.

• (c) Dam and water-holding structures - here the pozzolanic property governs. Replacing part of the cement with fly ash lowers the heat of hydration, which is critical in the mass concrete of a dam, and it reduces permeability while raising long-term strength, exactly what dam bodies, embankments and canal linings need.

• (d) Brick industry - the pozzolanic property again. Fly ash-lime-gypsum (FaL-G) blocks and fly ash-clay bricks gain strength from the lime reaction, saving both fertile topsoil and kiln fuel.

Cross-check: these same four routes are the ones enumerated in India's regulatory framework for fly-ash utilisation. The MoEFCC Fly Ash Notification (1999, as amended) and the Central Electricity Authority's annual fly-ash utilisation reports both list soil amendment and agriculture, reclamation of low-lying areas and wasteland, dams and embankments and other civil-engineering fill, and bricks, blocks and tiles among the approved utilisation routes. No listed use has to be dropped.

Result: all four uses - (a), (b), (c) and (d) - are recognised utilisation routes for fly ash, so the code that covers (a), (b), (c) and (d) is the correct one.