Aerated autoclaved concrete

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Autoclaved Aerated concrete (AAC), or otherwise known as Autoclave Cellular Concrete (ACC), is a lightweight, precast building material. AAC provides structure, insulation and fire resistance in a single material. AAC products include blocks, wall panels, floor and roof panels, and lintels.

AAC was first developed by Max Ginsberg between the years 1920 and 1932. It has since been refined into a high thermally insulating concrete-based material used for construction both internally and externally. Besides insulating capability, one of AAC's advantages in construction is its quick and easy installation since the material can be routed, sanded and cut to size on site using standard carbon tip band saws, hand saws and drills.

Even though regular cement mortar can be used, 98% of the buildings erected with AAC materials uses thin bed mortar, which comes to deployment in a thickness of 1/8 inch. This varies on national building codes and creates solid and compact building members. AAC material can be coated with a stucco compound or plaster against the elements. Siding materials such as brick or vinyl siding can also be used to cover the outside of AAC materials.

Aerated autoclaved concrete is an emission-intensive material to produce, carrying a carbon footprint of 480 kilograms of carbon dioxide per cubic metre.

Raw materials

Quartz sand, lime and/or cement as binding agent. Aluminum powder in 5%–8% by volume (depending on the pre-specified density) and water. When mixed and cast in forms, several chemical reactions take place that give AAC its light weight and superior thermal properties. Aluminium powder reacts with calcium hydroxide and water to form hydrogen. The hydrogen gas foams the raw mix to double the volume (with gas bubbles up to 1/8 inch in diameter). At the end of the foaming process the hydrogen escapes to 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 12 hours. During this steam pressure hardening process, when the temperature reaches 374° Fahrenheit and the pressure reaches 12 bars, quartz sand reacts with calcium hydroxide to form calcium silica hydrate, which accounts for the material's high strength and other unique properties. 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 the mass is air. Density also accounts for the low structural compression strength of AAC material, which can carry loads up to 1,200 PSI, approximately 1/6th of stronger concrete.

Since 1980, there has been a worldwide increase in the use of AAC materials and new production plants are being built in the USA, Eastern Europe, Israel, China, Bahrain, India and Australia. AAC is increasingly used by developers, architects and home builders. The Material is also known as: Autoclaved Concrete, Autoclaved Aerated Concrete, Cellular Concrete, Porous concrete, Aircrete and Thermalite (UK). As of August 2007 there are only 2 functioning AAC factories in the USA, Xella Aircrete North America in Georgia, and Aercon in Florida.

See also

• Concrete masonry unit
• LECA

References