Fly ash bricks and comparison with clay bricks: composition and manufacture

Fly ash bricks represent an advancement in masonry construction, offering superior quality over traditional clay bricks while also providing environmental benefits through resource conservation.

Composition of fly ash bricks

Fly ash bricks are made from four primary components.

• Fly ash

• Cement

• Sand

• Water

Source of fly ash

India's power plants are largely coal based, about 72 percent of them, and these plants together produce around 40 million tonnes of fly ash annually. This byproduct is linked to carbon dioxide emissions from thermal power plants and coal burning industries. Power plants use cyclone converters to separate the fly ash, which then serves as raw material for brick manufacturing.

Environmental impact

Fly ash presents significant pollution challenges affecting air, water and land quality. Repurposing it into building materials helps address these environmental concerns.

Manufacturing process

Production involves mixing hydrated lime powder and gypsum in a slow setting pozzolanic cement process. Manufacturers press the mixture using hydraulic machines at approximately 350 kg per square inch, with controlled pressure and duration used to achieve maximum strength.

Key advantages

The manufacturing process produces zero emissions and qualifies under the Clean Development Mechanism (CDM). By comparison, conventional brick production requires around 200 tons of coal and emits around 270 tons of CO2 per million bricks.

Clay bricks compared with fly ash bricks

• Compressive strength: clay bricks range from 30 to 35 kg/cm², fly ash bricks range from 90 to 100 kg/cm², giving fly ash bricks superior load capacity.

• Water absorption: clay bricks absorb 15 to 25 percent, fly ash bricks absorb 10 to 14 percent, reducing moisture related damage.

• Dimensional consistency: clay bricks have low tolerance, fly ash bricks have high tolerance, which can save up to 25 percent on mortar.

• Transit wastage: clay bricks can see wastage of up to 10 percent, fly ash bricks less than 2 percent, reducing costs by up to 8 percent.

• Plaster requirements: clay brick surfaces are uneven, fly ash brick surfaces are even, saving around 15 percent on plaster.