Fly Ash Brick Calculator: Mix Design, Strength & Eco Impact
What are Fly Ash Bricks?
Fly ash bricks are manufactured from Class C or Class F fly ash (a byproduct of coal-fired power plants) combined with lime, gypsum, and sand. The pozzolanic reaction between fly ash and lime forms calcium silicate hydrate (C-S-H) at room temperature — no kiln firing required.
Standard size: 230×110×75mm (IS 12894:2002). Compressive strength: 7.5–17.5 MPa (Grades 1–3). Density: 1700–1900 kg/m³. 20–30% cheaper than clay bricks in most markets.
Mix Design
Standard mix: 60% fly ash + 10% lime + 5% gypsum + 25% sand (by dry mass). Water: 10–15%. Bricks are hydraulically pressed and steam-cured for 24h or ambient-cured for 28–90 days.
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Environmental Benefits
- No kiln firing → ~78% CO₂ reduction vs clay bricks
- Utilizes industrial waste (fly ash) → diverts from landfill
- No topsoil excavation → conserves agricultural land
- Energy: ~0.12 kWh/brick vs ~0.80 kWh/brick for clay
FAQs
What is the compressive strength of fly ash bricks?
IS 12894 Grade 1: ≥7.5 MPa, Grade 2: ≥10 MPa, Grade 3: ≥17.5 MPa at 28 days. Strength continues to increase up to 90 days due to ongoing pozzolanic reaction.
Are fly ash bricks water resistant?
Water absorption of fly ash bricks: 10–15% by mass (IS 12894 max 20%). Generally better than clay bricks due to denser microstructure. Not recommended for permanently submerged applications without waterproofing.
How long does it take for fly ash bricks to gain full strength?
Initial strength gain in 7 days (~50% of 28-day strength). Full rating strength at 28 days. Continue gaining strength up to 90 days (35% additional). Proper curing (wet or steam) is essential for strength development.
Can fly ash bricks be used for load-bearing walls?
Yes, Grade 2 and Grade 3 fly ash bricks (≥10 MPa) can be used for load-bearing masonry in low-rise construction. Consult structural engineer for specific load calculations.