Fly ash brick

Fly ash brick (FAB) is a building material, specifically masonry units, containing class C or class F fly ash and water. Compressed at 28 MPa (272 atm) and cured for 24 hours in a 66 °C steam bath, then toughened with an air entrainment agent, the bricks can last for more than 100 freeze-thaw cycles. Owing to the high concentration of calcium oxide in class C fly ash, the brick is described as "self-cementing". The manufacturing method saves energy, reduces mercury pollution in the environment, and often costs 20% less than traditional clay brick manufacturing.

Fly ash bricks are considered a key low-carbon building material in India. They are manufactured from fly ash, sand, lime, gypsum, and water without kiln firing, using curing or autoclaving processes. According to the Ministry of Environment, Forest and Climate Change (MoEFCC) notification of 2016, all construction projects within 300 kilometres of a coal or lignite thermal power plant are required to use fly-ash-based products in a prescribed proportion. This regulation aims to reduce top-soil consumption from conventional clay-brick kilns and lower greenhouse-gas emissions.

Each fly-ash brick avoids approximately 0.2 to 0.25 kilograms of carbon-dioxide emissions compared with a fired clay brick, largely because it eliminates coal-kiln firing and reuses industrial waste. A typical multi-storey residential building of 7000–8000 square feet using 250000 fly-ash bricks can save 50–60 tonnes of CO₂ and prevent removal of more than 1000 cubic metres of fertile soil. The material also provides 40–50 percent savings in mortar usage due to dimensional accuracy and smoother surfaces.

Adoption in India has been growing but remains uneven. Small-scale brick kilns and informal supply chains still dominate, and quality variation among manufacturers affects confidence among builders. Enforcement of fly-ash utilisation rules and awareness among architects and contractors are cited as key barriers to universal adoption.