Aerated autoclave concrete blocks

Autoclaved Aerated Concrete (AAC) blocks are lightweight, precast, cellular blocks made from fly ash (or sand), cement, lime, gypsum and water, with a small amount of aluminium powder as the foaming agent. The aluminium reacts with lime to give hydrogen gas, which forms millions of tiny air cells. The mix is cut into blocks and cured in an autoclave (steam at about 180 °C and 12 bar pressure) for 8–12 hours.

Properties

• Density 500–800 kg/m³ (about one-third of clay brick), so dead load is reduced.
• Compressive strength about 3–4.5 N/mm² (IS 2185 Part 3).
• Good thermal insulation, sound absorption and fire resistance (2–4 hours).
• Accurate sizes (e.g. 600 × 200 × 100–300 mm), so thin joints and less plaster are needed.
• Easily cut, nailed, drilled and chased; quick construction.

Uses and limits

Used for non-load-bearing partitions and infill walls in framed buildings, and in load-bearing low rise buildings. Water absorption is higher than brick, so plastering and waterproofing are needed on outer walls, and special mortar (adhesive) is used.