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Stacked blocks of AAC (wrapped in foil)
Autoclaved Aerated concrete (AAC), or otherwise known as Autoclave Cellular Concrete (ACC) was invented in Sweden in the mid-1920s by Dr. Axel Eriksson. It is a lightweight, precast building material and provides structure, insulation, fire and mold resistance in a single material. AAC products include blocks, wall panels, floor and roof panels, and lintels.

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 steel 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.

Produced for more than 70 years, AAC offers considerable advantages over other construction materials, one of the most important being its very low environmental impact.

AAC’s high resource efficiency gives it low environmental impact in all phases of its life cycle, from processing of raw materials to the disposal of AAC waste.

AAC’s light weight also saves energy in transportation. The fact that AAC is up to five times lighter than concrete leads to significant reductions in CO2 emissions during transport. To reduce the need for transportation, AAC manufacturers apply the principle of producing as near to their consumer market as possible.

AAC’s excellent thermal efficiency makes a major contribution to environmental protection by sharply reducing the need for space heating and cooling in buildings.

In addition, AAC’s easy workability allows accurate cutting that minimizes the generation of solid waste during use. Unlike other building materials AAC can eliminate the need to be used in combination with insulation products, which increase the environmental impact and cost of construction.

Raw materials

Unlike most concrete applications, no aggregate larger than sand is used. Quartz sand, lime and/or cement is used as a binding agent. Aluminum powder is used 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 (20% the weight of concrete) and 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 (190° Celsius) and the pressure reaches 8 to 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 only about 10% of the compressive strength of regular concrete.

Since 1980, there has been a worldwide increase in the use of AAC materials and new production plants are being built in the USAmarker, Eastern Europe, Israelmarker, Chinamarker, Bahrainmarker, Indiamarker 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, Hebel (Aus), Aircrete and Thermalite (UK), BCA (Romania).

History

The material was perfected in the mid-1920s by Dr. Axel Eriksson, an architect working with Professor Henrik Kreüger at the Royal Institute of Technologymarker. It went into Swedishmarker production in 1929 in a factory in Hällabrottetmarker, and became very popular. In the 1940s the trade mark Ytong was introduced, but often referred to as "blue concrete" in Sweden, due to its blueish tinge. This version of Ytong was produced from alum slate, which due to its combustible carbon content was beneficial to use in the production process. The competing concrete brand Siporex used other raw materials. However, the slate deposits used for Ytong also contains uranium, which makes the material give off small amounts of radioactive radon gas to the surrounding air. In 1972, the Swedish Agency of Radiation Protection pointed out the unsuitability of a radon-emitting construction material, and production ceased in 1975. Ytong produced after 1975 has used other raw materials, without the uranium content.

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