The Material
cement and gypsum . The chemical reaction due to the aluminum paste provides AAC its distinct porous structure , lightness , and insulation properties , completely different compared to other lightweight concrete materials .
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 12 hours . During this steam pressure hardening process , when the temperature reaches 190 ° Celsius ( 374 ° Fahrenheit ) and the pressure reaches 8 to 12 bars , 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 . Depending on its density , up to 80 % of the volume of an AAC block is air . AAC ' s low density also accounts for its low structural compression strength . It can carry loads of up to 8 MPa ( 1,160 PSI ), approximately 50 % of the compressive strength of regular concrete \
Calcium silicate hydrate compositions in AAC
1.1 nm tobermorite ( C5S6H5 ) is usually the main hydration
product in AAC where cement , lime and sand are used
C-S-H ( I ) - more crystalline than C-S-H in dense concrete , typically 0.8 < Ca / Si < 1.0
Xonotlite ( C6S6H ) - forms with longer autoclaving times , or higher temperatures
' Normal ' tobermorite shows lattice shrinkage , while nonshrinking tobermorite is called ' anomalous ' tobermorite . Tobermorite in AAC made with cement , lime and sand is usually normal tobermorite . Tobermorite in autoclaved aerated concrete made with cement , lime and PFA is usually anomalous tobermorite . Aluminum and alkali together in solution ( such as will be present in mixes of cement , lime and PFA ) tend to produce anomalous tobermorite , with some aluminum and alkali taken up into the tobermorite crystal structure . The differences between the different forms of autoclaved calcium silicate hydrates are not well-defined ; in an AAC block , intimatelymixed hydrates of different compositions and crystallinity are likely to occur .
Advantages
AAC block walls provide better insulation than regular concrete but still need added
insulation for recommended green R-values . They are highly resistant to water , sun , wind ( withstand 150mph ), fire , sound , termites and pests . They are usually thin-bed mortared . Since they are lighter , fuel usage in transportation is lower and they go up more quickly . They can be sawn with woodworking tools . They use less material than regular blocks , and their use saves forests . They do not emit toxic gases even in fires . They can contain recycled materials like fly ash and rebar for LEED ® credits . They are durable and need less maintenance since they do not rot and deteriorate , and no repainting is needed with colored stucco finish , resulting in less chemical and