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Autoclaved aerated concrete (AAC) is a lightweight, precast, cellular concrete building material, eco-friendly,^[1] suitable for producing concrete-like blocks. It is composed of quartz sand, calcined gypsum, lime, portland cement, water and aluminium powder.^[2][3] AAC products are cured under heat and pressure in an autoclave. Developed in the mid-1920s, AAC provides insulation, fire, and mold-resistance. Forms include blocks, wall panels, floor and roof panels, cladding (façade) panels and lintels.^[4][5] It is also an insulator.^[2][6]

AAC products see use in construction, such as industrial buildings, residential houses, apartment buildings, and townhouses. Their applications include exterior and interior walls, firewalls, wet room walls, diffusion-open thermal insulation boards, intermediate floors, upper floors, stairs, opening crossings, beams and pillars. Exterior uses require an applied finish to guard against weathering, such as a polymer-modified stucco or plaster compound, or a covering of siding materials such as natural or manufactured stone, veneer brick, metal or vinyl siding.^[2] AAC materials can be routed, sanded, or cut to size on-site using a hand saw and standard power tools with carbon steel cutters.^[2][7][8]

Autoclaved aerated concrete is also known by various other names, including autoclaved cellular concrete (ACC), autoclaved concrete, cellular concrete, porous concrete, Aircrete, Thermalite, Hebel, Aercon,^[9] Starken, Gasbeton, Airbeton, Durox, Siporex (silicon pore expansion), Suporex, H+H and Ytong.^[10][11]

AAC was first created in the mid-1920s by the Swedish architect and inventor Dr. Johan Axel Eriksson (1888–1961),^[12][13] along with Professor Henrik Kreüger at the Royal Institute of Technology.^[12][13] The process was patented in 1924. In 1929, production started in Sweden in the city of Yxhult. "Yxhults Ånghärdade Gasbetong" later became the first registered building materials brand in the world^{[citation needed]}: Ytong. Another brand, "Siporex", was established in Sweden in 1939, and presently licenses and owns plants in 35 locations around the world.^{[citation needed]} Josef Hebel of Memmingen established another cellular concrete brand, Hebel, which opened their first plant in Germany in 1943.^{[citation needed]}

Ytong AAC was originally produced in Sweden using alum shale, which contained combustible carbon beneficial to the production process. However, these deposits were found to contain natural uranium, which decays over time to radon, which then accumulates in structures where the AAC was used. This problem was addressed in 1972 by the Swedish Radiation Safety Authority, and by 1975, Ytong abandoned alum shale in favor of a formulation made from quartz sand, calcined gypsum, lime (mineral), cement, water and aluminium powder currently in use by most major brands.^{[citation needed]}

In 1978, Siporex Sweden opened the Siporex Factory in Saudi Arabia, establishing the Lightweight Construction Company - Siporex - LCC SIPOREX, targeting markets in the Middle East, Africa, and Japan. This factory was still in use in 2018.^{[citation needed]}

Today, the production of AAC is widespread, concentrated in Europe and Asia with some facilities located in the Americas. Egypt has the sole manufacturing plant in Africa. Although the European AAC market has seen a reduction in growth, Asia is experiencing a rapid expansion in the industry, driven by an escalating need for residential and commercial spaces. Currently, China has the largest Aircrete market globally, with several hundred manufacturing plants. The most significant AAC production and consumption occur in China, Central Asia, India, and the Middle East, reflecting the dynamic growth and demand in these regions.^[14]

Like other masonry materials, the product Aircrete is sold under many different brand names. Ytong and Hebel are brands of the international operating company Xella, headquartered in Duisburg. Other more internationally renowned brand names in Europe are H+H Celcon (Denmark) and Solbet (Poland).^{[citation needed]}

AAC is a concrete-based material used for both exterior and interior construction.^[16] One of its advantages is quick and easy installation because the material can be routed, sanded, or cut to size on-site using a hand saw and standard power tools with carbon steel cutters.^[2]

AAC is well suited for high-rise buildings and those with high temperature variations.^[17] Due to its lower density, high-rise buildings constructed using AAC require less steel and concrete for structural members. The mortar needed for laying AAC blocks is reduced due to the lower number of joints. Similarly, less material is required for rendering, because AAC can be shaped precisely before installation. Even though regular cement mortar can be used, most buildings that use AAC materials use thin bed mortar in thicknesses around 3.2 millimetres (1⁄8 in), depending on the national building codes.

Unlike most other concrete applications, AAC is produced using no aggregate larger than sand.^[18] Quartz sand (SiO₂), 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-fired power plants, and having 50–65% silica content, is used as an aggregate.^{[citation needed]}

When AAC is mixed and cast in forms, aluminium 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 3 millimetres (1⁄8 in) in diameter—it has been described as having bubbles inside like "a chocolate Aero bar".^[19] At the end of the foaming process, the hydrogen escapes into the atmosphere and is replaced by air, leaving a product as light as 20% of the weight of conventional concrete.^{[citation needed]}

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 °C (374 °F) and the pressure reaches 800 to 1,200 kPa (8.0 to 12.0 bar; 120 to 170 psi), 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 to be a fired brick but a lightweight concrete masonry unit. After the autoclaving process, the material is stored and shipped to construction sites for use. 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 8,000 kPa (1,200 psi), approximately 50% of the compressive strength of regular concrete.^{[citation needed]} In 1978, the first AAC material factory - the LCC Siporex- Lightweight Construction Company - was opened in the Persian Gulf state of Saudi Arabia, supplying Gulf Cooperation Council countries with aerated blocks and panels. Since 1980, there has been a worldwide increase in the use of AAC materials.^[20][21] New production plants are being built in Australia, Bahrain, China, Eastern Europe, India and the United States. AAC is increasingly used worldwide by developers.^{[citation needed]}

Reinforced autoclaved aerated concrete (RAAC) is a reinforced version of autoclaved aerated concrete, commonly used in roofing and wall construction. The first structural reinforced roof and floor panels were manufactured in Sweden, soon after the first autoclaved aerated concrete block plant started up there in 1929, but Belgian and German technologies became market leaders for RAAC elements after the Second World War. In Europe, it gained popularity in the mid-1950s as a cheaper and more lightweight alternative to conventional reinforced concrete, with documented widespread use in a number of European countries as well as Japan and former territories of the British Empire.^[22][23]

RAAC was used in roof, floor and wall construction due to its lighter weight and lower cost compared to traditional concrete,^[24] and has good fire resistance properties; it does not require plastering to achieve good fire resistance and fire does not cause spalls.^[25] RAAC was used in construction in Europe, in buildings constructed after the mid-1950s.^[26][27] RAAC elements have also been used in Japan as walling units owing to their good behaviour in seismic conditions.

RAAC has been shown to have limited structural reinforcement bar (rebar) integrity in 40 to 50 year-old RAAC roof panels, which began to be observed in the 1990s.^{[27][28][29][30][31]} The material is liable to fail without visible deterioration or warning.^[27][31] This is often caused by RAAC's high susceptibility to water infiltration due to its porous nature, which causes corrosion of internal reinforcements in ways that are hard to detect. This places increased tensile stress on the bond between the reinforcement and concrete, lowering the material's service life. Detailed risk analyses are required on a structure-by-structure basis to identify areas in need of maintenance and lower the chance of catastrophic failure.^[32]

Professional engineering concern about the structural performance of RAAC was first publicly raised in the United Kingdom in 1995 following inspections of cracked units in British school roofs,^[33] and was subsequently reinforced in 2022 when the Government Property Agency declared the material to be life-expired,^[34] and in 2023 when, following the partial or total closure of 174 schools at risk of a roofing collapse,^[35][36] other buildings were found to have issues with their RAAC construction,^[37][38][39] with some of these only being discovered to have been made from RAAC during the crisis.^[40][41][42] During the 2023 crisis, it was observed that it was likely for RAAC in other countries to exhibit problems similar to those found in the United Kingdom.^[23]

The original site of the Ontario Science Centre in Toronto, Canada, a major museum with similar roof construction, was ordered permanently closed 21 June 2024 because of severely deteriorated roof panels dating from its opening in 1969. While repair options were proposed, the centre's ultimate owner, the provincial government of Ontario, had previously announced plans to relocate the centre and therefore requested the facility be closed immediately rather than paying for repairs. Approximately 400 other public buildings in Ontario are understood to contain the material and are under review, but no other closures were anticipated at the time of the Science Centre closure.^[43]

The high resource efficiency of autoclaved aerated concrete contributes to a lower environmental impact than conventional concrete, from raw material processing to the disposal of aerated concrete waste. Due to continuous improvements in efficiency, the production of aerated concrete blocks requires relatively little raw materials per m³ of product and is five times less than the production of other building materials.^[44] There is no loss of raw materials in the production process, and all production waste is returned to the production cycle. Production of aerated concrete requires less energy than for all other masonry products, thereby reducing the use of fossil fuels and associated carbon dioxide (CO₂) emissions.^[45] The curing process also saves energy, as the steam curing takes place at relatively low temperatures and the hot steam generated in the autoclaves is reused for subsequent batches.^[46][47]

AAC has been produced for more than 70 years and has several 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 gives superior fire resistance.
• Workability allows accurate cutting, which minimizes the generation of solid waste during use.
• Is eco-friendly and does not pollute the environment and contributes to LEED rating green building material.^[1]
• Resource efficiency gives it lower environmental impact than conventional concrete in all phases from the processing of raw materials to the ultimate disposal of waste.^[1]
• Being lighter allows for easier handling of concrete bricks. The lighter weight saves cost and energy in transportation, labour expenses, and increases chances of survival during seismic activity.^[48]
• Larger size blocks leads to faster masonry work.
• Reduces project cost for large constructions.
• Fire-resistant: AAC, like other concretes, is fire-resistant.
• Ease of Handling: AAC Blocks are lightweight, making them Easier to Lift, Carry, and Install, Smoothing out construction and further improving Efficiency compared to Traditional Materials.
• Good ventilation: This material is very airy and allows the diffusion of water, reducing humidity inside the building. AAC absorbs moisture and releases humidity, helping to prevent condensation and other problems 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, and cannot be damaged by them.
• Accuracy: Panels and blocks made of autoclaved aerated concrete are produced to the exact sizes needed before leaving the factory. There is less need for on-site trimming. Since the blocks and panels fit so well together, there is less use of finishing materials such as mortar.
• Long-lasting: The life of this material is longer because it is not affected by harsh climates or extreme weather changes, and will not degrade under normal climate changes.

AAC has been produced for more than 70 years. However, some disadvantages were found when it was introduced in the UK (where double-leaf masonry, also known as cavity walls, are the norm).

• The process of using AAC is somewhat complex, so builders have to undergo special training.^[49]
• Non-structural shrinkage cracks may appear in AAC blocks after installation in rainy weather or humid environments. This is more likely in poor-quality blocks that were not properly steam-cured. However, most AAC block manufacturers are certified and their blocks are tested in certified labs, so poor-quality blocks are rare.^[50]
• Has some brittle nature: requires more care than clay bricks to avoid breakage during handling and transporting.
• Fixings: the brittle nature of the blocks requires longer, thinner screws when fitting cabinets and wall hangings. Special wall fasteners (screw wall plug anchors) designed for autoclaved aerated concrete including gypsum board and plaster tiles are available at a higher cost than standard expandable wall plugs, including special safety-relevant anchors for high load bearing;^[51][52][53] It is recommended that fixing holes be drilled using HSS drill bits at a steady constant speed without hammer action.^[51][52] Masonry drill bits and standard expandable wall plugs are not suitable for use with AAC blocks.^[52]
• Using European standard density (400 kg/m³, B2,5), AAC blocks alone would require very thick — 500mm or thicker — walls to achieve the insulation levels required by newer building codes in Northern Europe.^[49]

• Media related to Autoclaved aerated concrete at Wikimedia Commons
• History of Autoclaved Aerated Concrete
• Using Autoclaved Aerated Concrete Correctly - Masonry Magazine, June 2008
• Aircrete Products Association
• Autoclaved Aerated Concrete - Portland Cement Association