Per §4.9, the most commonly used gas-forming admixture in concrete is:

1. §4.9: 'Aluminium powder is used most commonly as gas forming agent. Hydrogen gas is produced due to reaction of aluminium with Ca(OH)2.' 2. Dosage is 0.005 to 0.02 percent by weight of cement; larger doses give light-weight concrete (Siporex blocks). _Source: IGNOU ET-522 Block-1 Unit-4 Admixtures, §4.9, p. 52_