Unlike most other concrete applications, AAC is produced using no aggregate larger than sand. Quartz sand, calcined gypsum, lime (mineral) and/or 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). In some countries, like India and China, fly ash generated from coal fire power plants and having 50-65% silica content is used as an aggregate.
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. At 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 12 hours. During this steam pressure hardening process, when the temperature reaches 190° Celsius (374° Fahrenheit) and the pressure reaches 8 to 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. 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.
In 1978 The First AAC material was opened in the Persian Gulf state in Kingdom Saudi Arabia - LCC SIPOREX- Lightweight Construction Company supplying the GCC countries with the Aerated products of blocks and panels.
Since 1980, there has been a worldwide increase in the use of AAC materials. New production plants are being built in Australia, Bahrain, China, Eastern Europe, India, and the United States. AAC is increasingly used by developers, architects, and home builders worldwide.
AAC has been produced for more than 70 years, and it offers several significant advantages over other cement construction materials, one of the most important being its lower environmental impact.
· Improved thermal efficiency reduces the heating and cooling load in buildings.
· Porous structure allows for superior fire resistance.
· Workability allows accurate cutting, which minimizes the generation of solid waste during use.
· Resource efficiency gives it lower environmental impact in all phases of its life cycle, from the processing of raw materials to the disposal of waste.
· Light weight saves cost & energy in transportation, labor expenses, and increases chances of survival during seismic activity.
· Larger size blocks leads to faster masonry work.
· Reduces the cost of the project.
· Environmentally friendly: When used, it helps to reduce at least 30% of environmental waste as opposed to going with traditional concrete. There is a decrease of 50% of greenhouse gas emissions. When possible, using autoclaved aerated concrete is a better choice for the environment.
· Energy savings: It is an excellent property that makes it an excellent insulator and that means the interior environment is easier to maintain. When it is used, there is usually not a need for any supplementary insulation.
· Fire resistant: Just like with regular concrete, AAC is fire resistant. This material is completely inorganic and not combustible.
· Great ventilation: This material is very airy and allows for the diffusion of water. This will reduce the humidity within the building. AAC will absorb moisture and release humidity; this helps to prevent condensation and other problems that are related to mildew.
· Non-toxic: There are no toxic gases or other toxic substances in autoclaved aerated concrete. It does not attract rodents or other pests nor can it be damaged by such.
· Lightweight: Concrete blocks that are made out of AAC weigh about one-fifth of typical concrete. They are also produced in sizes that are easy to handle for quick construction.
· Accuracy: The panels and blocks made of autoclaved aerated concrete are produced to the exact sizes needed before they even leave the factory. There is less need for on-site trimming. Since the blocks and panels fit so well together, there is a reduced use of finishing materials such as mortar.
· Long lasting: The life of this material is extended because it is not affected by harsh climates or extreme changes in weather conditions. It will not degrade under normal climate changes either.
· Quick assembly: Since it is a lightweight material and easy to work with, the assembly is much quicker and smoother.
AAC has been produced for more than 70 years, however some disadvantages were found when it was introduced in the UK (where cavity wall with clay brick two-skin construction has been the norm).
· Installation during rainy weather: aircrete is known to crack after installation, which can be avoided by reducing the strength of the mortar and ensuring the blocks are dry during and after installation.
· Brittle nature: they need to be handled more carefully than clay bricks to avoid breakages.
· Fixings: the brittle nature of the blocks requires longer thinner screws when fitting cabinets and wall hangings and wood-suitable drill bits or hammering in. Special, large diameter wall plugs (aircrete anchor) are available, though at a higher cost than common wall plugs.
· Insulation requirements in newer building codes of northern European countries would require very thick walls when using AAC alone. Thus many builders choose to return to traditional building methods installing an extra layer of insulation around the building as a whole.