Fly ash bricks vs red clay bricks: an honest comparison

Red clay bricks have been the default walling material in India for generations. Fly ash bricks are now common on building sites too, especially near thermal power plants. Here is how the two compare, for builders choosing a brick and for entrepreneurs deciding what to make.

What each brick is made of

Red clay bricks are moulded from clay, usually dug from fields, then dried and fired in a kiln.

Fly ash bricks are pressed from fly ash, the fine ash left when coal is burned in thermal power plants, mixed with sand or stone dust and a binder such as lime or cement, often with a little gypsum. They are not fired. They gain strength as the binder reacts with the fly ash and water while they cure.

Side by side

Fly ash brickRed clay brick

Main raw material

Fly ash, an industrial by-product

Clay, often topsoil

How it is made

Pressed in a machine, then water-cured

Moulded, dried and fired in a kiln

Firing fuel

None

Coal, wood or other fuel

Size and shape

Uniform, with sharp edges from a steel mould

Varies with moulding and firing

Colour

Grey

Red to brown

Quality depends on

Mix design, compaction and curing

Clay quality and firing temperature

Where fly ash bricks have the edge

• Uniform size. Every brick comes from the same mould under the same pressure, so walls go up straighter, joints are thinner and plaster can be thinner too.
• No kiln. There is no firing fuel and no kiln smoke. Curing needs only water and time.
• Uses a waste product. Fly ash would otherwise sit in ash ponds, and using it saves the fertile topsoil that clay bricks consume.
• Policy support. Government notifications on fly ash use, first issued in 1999 and revised several times since, push construction near coal and lignite power plants towards fly ash-based products.