Unlike most other concrete applications, AAC is produced using no aggregate larger than sand. Quartz sand, calcined lime, gypsum (mineral) cement and water are used as a binding agent. Aluminum powder is used at a rate of 0.05 % -0.08 % by volume ( depending on the pre - specified density ) . When AAC is mixed and cast in forms, several chemical reactions take place that give AAC its light weight ( 20 % of the weight of concrete ) and thermal properties. Aluminum powder reacts with calcium hydroxide and water to form hydrogen. The hydrogen gas foams and doubles the volume of the raw mix creating gas bubbles up to 3mm ( ⅛ inch ) in diameter A the end of the foaming process, the hydrogen escapes into the atmosphere and is replaced by air. When the forms are removed from the material, it is solid but still soft. It is then cut into either blocks or panels, and placed in an autoclave chamber for 12hours. During this steam pressure hardening process, whenthe temperature reaches 190 Celsius (374° Fahrenheit) and the pressure reaches 8to 12 bar, quartz sand reacts with calcium hydroxide to form calcium silicate hydrate, which gives AAC its high strength and other unique properties. Because of the relatively low temperature used AAC blocks are not considered fired brick but a lightweight concrete masonry unit. After the autoclaving process, the material is ready for immediate use on the construction site.