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 and cured for 24 hours in a 66 °C steam bath, then toughened with an air entrainment agent.

The clay fly ash bricks shall conform to IS 13757. The bricks shall be sound, compact and uniform in shape and colour. Bricks shall have smooth rectangular faces with sharp and square corners. The bricks shall be free from visible cracks, flaws, warpage, nodules of free lime and organic matter, the bricks shall be hand or machine moulded. The bricks shall have frog of 100 mm in length 40 mm width and 10 to 20 mm deep on one of its flat sides. If made by extrusion process may not be provided with frogs.

Fly Ash shall conform to grade I or grade II of IS 3812.

The raw materials for Fly Ash Bricks are:

1. It reduces dead load on structures due to light weight (2.6 kg, dimension: 230 mm X 110 mm X 70 mm.

2. Same number of bricks will cover more area than clay bricks. .

3. High fire Insulation. .

4. Due to high strength, practically no breakage during transport and use. .

5. Due to uniform size of bricks mortar required for joints and plaster reduces almost by 50%]..

6. Due to lower water penetration seepage of water through bricks is considerably reduced. .

7. Gypsum plaster can be directly applied on these bricks without a backing coat of lime plaster. .

8. These bricks do not require soaking in water for 24 hours. Sprinkling of water before use is enough.

1. Mechanical strength is low, but this can be rectified by adding marble waste or mortar between blocks.

2. Limitation of size. Only modular size can be produced. Large size will have more breakages.

3. It is only good for the places like subtropical area or area where climate is warm because it doesn't absorb heat. But during cold it is not helpful.