Guide for the manufacture of standard burnt clay building bricks by semi-mechanized process
1991 Edition

For semi-mechanized brick manufacturing, clays should be locally sourced with minimal organic content and impurities. Common additives include fly ash, sandy loam, rice husk ash, and finely ground basalt stone dust. These materials improve the molding, drying, and firing characteristics of bricks, promote recycling of waste, and maintain brick durability. Additives must be finely graded (passing a 1 mm sieve), chemically compatible with the clay, and free from components that might reduce strength or lifespan.

A semi-mechanized brick plant typically includes a double-deck extrusion machine for shaping bricks, a manual cutting table, natural drying sheds with floor or rack drying, and an archless zig-zag kiln with fan-assisted draught. Other equipment involves a coal crusher, manual clay excavation tools, and control instruments like thermocouples and pyrometers. The recommended layout features specific dimensions such as a 1.5 m by 8 m machine shed, 7 m by 4 m pump room, and drying sheds sized 45 m by 10 m. Site selection should prioritize adequate clay reserves, potable water access, transport connectivity, labor and power availability, and a subsoil water table below 3 meters.

Firing should commence from an end chamber of the kiln, constructing a temporary wall with three furnaces ahead of the first brick row using 25 mm mild steel bars as grates. The gap between this wall and the main brick setting must be sealed with corbelled bricks and ash. A consistent firing sequence involves firing two chambers every 24 hours to achieve uniform heat distribution. Bricks must be well dried and fired at temperatures between 950°C and 1000°C. Post-firing quality checks include evaluating cracking, warping, water absorption, crushing strength, and efflorescence in accordance with IS 3495.

The standard recommends initially drying bricks on the floor for 3 to 4 days, followed by stacking them in 'hacks' according to IS 2117:1975 for an additional 7 to 15 days until moisture content reaches approximately 5-7%. For clays sensitive to drying stresses, protection from strong winds using gunny-cloth screens on drying shed sides is advised, especially during hot seasons. Alternatively, bricks can be naturally dried on wooden pallets arranged in multiple tiers. Wire-cut bricks require drying on a well-prepared sanded floor. Proper drying minimizes cracking, shrinkage, and warpage during firing.

IS 11650 permits incorporating fly ash as an admixture to enhance brick properties while promoting waste utilization. The fly ash used should have a fineness of 2000–3000 cm²/gm, less than 0.5% coarse particles (>1 mm), under 15% unburnt carbon, and minimal water-soluble components (<0.1%). Fly ash contains silicate compounds resembling those in brick earth, which contribute to strength development upon firing. The optimal fly ash proportion depends on the clay's physico-chemical and plastic characteristics. Its inclusion improves brick strength, durability, and environmental sustainability.