Exceptionally lightweight precast concrete with high thermal qualities and fire resistance, suitable for cutting with ordinary hand tools. The mix design is composed of Portland cement, sand or siliceous material, lime, gypsum, finely powdered aluminum, and water. The finely powdered aluminium reacts with the alkaline components of the cement and lime to produce hydrogen gas, which increases the volume approximately five times producing a uniformly, dispersed cellular structure. Units are cut to required shape and placed in an autoclave, an enclosed pressurised chamber, and steam cured at 350 0 F. Approximately 80% of the ultimate volume consists of air voids.