Fly Ash (Pulverized Fuel Ash)

Fly ash is a fine particulate by-product of pulverised coal combustion in thermal power plants, captured by electrostatic precipitators or fabric filters. With production volume of ~270 million tonnes per year in India (2022), fly ash is a major industrial waste stream that has been productively diverted into concrete and cement production. As a Supplementary Cementitious Material (SCM), fly ash is the most-widely-used pozzolanic admixture in Indian concrete — used directly in RMC mixes (replacing 15-30% of cement), in PPC cement production (15-35% replacement), and as the principal raw material in fly-ash bricks. The Indian Standard IS 3812 Part 1:2013 specifies fly-ash class and grade requirements.

Fly ash has two main classifications: Class F (low calcium, from anthracite or bituminous coal) and Class C (high calcium, from sub-bituminous or lignite coal). Most Indian thermal plants produce Class F fly ash, with the Class F finer, more spherical, and strictly pozzolanic. Class C is partly cementitious itself and reacts more rapidly. For cement and concrete use: 80% siliceous + aluminous content, < 5% loss-on-ignition (LOI), spherical particles 1-100 micrometre, fineness 280-500 m²/kg (Blaine). Major Indian fly-ash producers: NTPC (largest), Tata Power, Adani Power, Reliance Power.

Using fly ash in concrete: (1) PPC production — IS 1489 governs blending. (2) Direct RMC addition (Type II per IS 3812) — fly ash partially replaces OPC, typically 15-30%, requiring mix design adjustment. (3) High-volume fly ash concrete (HVFAC) — 50-60% replacement for sustainable mass concrete. The benefits: lower cost (fly ash is much cheaper than clinker), reduced embodied CO2 (~25% saving at 30% replacement), better long-term strength, improved durability, lower heat of hydration. Drawbacks: slower early strength (extended curing required), and the need for extended curing in hot weather. Modern Indian construction has effectively normalised 25-30% fly ash replacement in PPC; many plants use 50%+ in mass concrete applications.