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