Fly Ash Bricks (FAB) | Composition | Manufacture

Fly ash bricks (FAB) are non-conventional bricks with fly ash (FA) as the major ingredient. The standard specifications for the use of FAB in masonry construction are provided in ASTM C1790-15, "Standard Specification for Fly Ash Facing Brick". ( American Standard of Testing Materials)

Fly Ash Bricks

Composition of Fly Ash Bricks

Composition of Fly Ash Bricks (FAB)

The above chart gives the materials used for the manufacture of fly ash bricks. 

1. Fly Ash

The fly ash used for the production of fly ash bricks is the by product obtained from the combustion of powdered coal. During the combustion of powdered coal , the fly ash is transported by the flue gases and collected by a electrostatic precipitator.
As per ASTM, Class F and Class C type of fly ash is mainly used for the manufacture of fly ash brick.

Class F Fly Ash

This is fly ash is normally produced by burning anthracite or bituminous coal. It possess calcium oxide (CaO) less than 5%. It own pozzolanic properties only.

Class C Fly Ash

Fly ash produced by burning lignite or sub-bituminous coal. It has CaO content in excess of 10 %. It also posses cementious properties.  The specific gravity of class C fly ash is 2.19.

2. Binder ( Lime/Cement/Gypsum)

Lime used in fly ash brick production act as a binder. It is a natural association of CaO with MgO. The lime reacts with ordinary temperature and forms a compound that finally possess cementitious properties. 

Gypsum is used as a non-hydraulic binder. It is obtained naturally as a soft crystalline rock or sand. It helps to strengthen the material or increase the viscosity. 


3. Aggregate Filler  ( Stone/Stone Dust)

The commonly used aggregate is sand and sand dust. The filler used must be free of clay and silt (less than 5 %). 

Several combinations used for the manufacture of fly ash bricks are listed in the table.

Option 1

Fly Ash

57 to 65%

River Sand or Stone Dust

18 to 27 %

Hydrated Lime

9 to 12%


5  % 

Option 2

Fly Ash

50 to 60%

River Sand or Stone Dust

30 to 40 %

Hydrated Lime

8 to 12 %

Option 3

Fly Ash

55 to 60%

River Sand or Stone Dust

20 to 25 %

Hydrated Lime

15 to 22 %


5  %

The options mentioned above are some of the examples of mix proportion used for FAB construction. 

Manufacture of Fly Ash Bricks (FAB)

Fly ash bricks are compressed at 28 MPa and cured for 24 hours in 66 degrees steam bath. It is later toughened with an air-entrainment agent. This process makes the bricks ready for 100 freeze and thaw cycles. 

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