Autoclaved aerated concrete

"Aerated concrete" redirects here. For cellular concrete (foamed concrete), see Types of concrete § Cellular concrete. For lightweight blocks, see Expanded clay aggregate.

Autoclaved Aerated Concrete (AAC), also known as autoclaved cellular concrete or autoclaved concrete, is a lightweight, prefabricated concrete building material. Developed in the mid-1920s, AAC offers a modern alternative to traditional concrete blocks and clay bricks.^[1] Unlike cellular concrete, which is mixed and poured on-site, AAC products are prefabricated in a factory.^[2]

Sectional view of AAC.

Palette stacked AAC blocks.

The composition of AAC includes a mixture of quartz sand, gypsum, lime, Portland cement, water, fly ash, and aluminum powder.^[3] Following partial curing in a mold, the AAC mixture undergoes additional curing under heat and pressure in an autoclave.^[4] AAC is versatile and can be manufactured into various forms, including blocks, wall panels, floor and roof panels, cladding panels, and lintels.^[5]

Cutting AAC typically requires standard power tools fitted with carbon steel cutters.^[6][7] Similar to cellular concrete, AAC products can be used in a wide range of construction projects. When used externally, AAC products often require a protective finish to shield against weathering. A polymer-modified stucco or plaster compound can be used for this purpose, as well as a covering of siding materials such as natural or manufactured stone, veneer brick, metal, or vinyl siding.^[8]

History

House construction site using AAC (Ytong) blocks in Ablis, France.

Residential house constructed of AAC (Siporex) blocks in Kuopio, Finland.

AAC was first created in the mid-1920s by the Swedish architect and inventor Dr. Johan Axel Eriksson (1888–1961),^[9][10] along with Professor Henrik Kreüger at the Royal Institute of Technology.^[9][10] The process was patented in 1924.^[11] In 1929, production started in Sweden in the city of Yxhult [sv]. "Yxhults Ånghärdade Gasbetong" later became the first registered building materials brand in the world: Ytong.^[12] Another brand, "Siporex", was established in Sweden in 1939.^{[citation needed]} Following a mid-20th century boom, demand declined in Western Europe, and no new plants were constructed there after the 1990s.^[13] Josef Hebel of Memmingen established another cellular concrete brand, Hebel, which opened their first plant in Germany in 1943.^[14]

Ytong AAC was originally produced in Sweden using alum shale, which contained combustible carbon beneficial to the production process.^{[citation needed]} However, these deposits were found to contain natural uranium that decays over time to radon gas, 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.^{[citation needed][14]}

In 1978, Siporex Sweden opened the Siporex Factory in Saudi Arabia, establishing the Lightweight Construction Company (LCC), supplying Gulf Cooperation Council countries with aerated blocks and panels. Since 1980, there has been a worldwide increase in the use of AAC materials.^[15][16] New production plants are being built in Australia, Bahrain, China, Eastern Europe, India, and the United States. The use of AAC has expanded globally, particularly in regions experiencing rapid urban development.^[17] Currently, LCC has three branches in Saudi Arabia.^[18]

Today, the manufacturing of autoclaved aerated concrete (AAC) is most prevalent in Europe and Asia, with limited production facilities in the Americas and one known plant in Egypt.^{[citation needed]} While growth in the European market has slowed, AAC production in parts of Asia has expanded, particularly in response to urban development demands. As of 2025, China remains the world's largest autoclaved aerated concrete market, hosting over 2,000 manufacturing plants and producing approximately 190 million cubic meters of AAC annually.^[19][20] The most significant AAC production and consumption occurs in China, Central Asia, India, and the Middle East, reflecting the dynamic growth and demand in these regions.^[21] AAC Blocks were introduced in India in the early 1990s. One of the first manufacturers in the country was Biltech Building Elements Limited, which began AAC production in 1993 at its Palwal, Haryana facility.^[22][23][24] In 2017, Biltech acquired the AAC manufacturing units of Siporex India, becoming the exclusive manufacturer and owner of the Siporex brand in the country.^{[citation needed]}

Residential house constructed at the Finnish Seinäjoki Housing Fair in 2016 using AAC blocks.^[25]

AAC blocks on a residential house construction site in Russia.

Uses

AAC is used for both exterior and interior construction.^[26] It has been applied in high-rise construction projects and areas with frequent temperature fluctuation.^[27] Due to its lower density, AAC can reduce the structural load, potentially decreasing the required amounts of steel reinforcement and conventional concrete in certain building applications. 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. Although regular cement mortar is compatible with AAC, many buildings employing its components utilize thin bed mortar, typically around 3.2 millimetres (1⁄8 in) thick, in accordance with specific national building codes.^{[citation needed]}

Manufacturing

Uncured AAC blocks (on the right) ready to be fed into an autoclave to be rapidly cured into a finished product under heat and pressure; AAC production site in China.

The aggregate used in AAC typically consists of finely ground sand or fly ash, depending on regional availability.^[28] Binding agents include quartz sand, lime, calcined gypsum, cement, and water. Aluminium powder constitutes 0.05–0.08% by volume. Some countries (such as India and China) use fly ash from coal-fired power plants (50–65% silica) as the aggregate.^[29]

When AAC is mixed and cast in forms, aluminium powder reacts with calcium hydroxide and water to form hydrogen. The hydrogen gas foams, doubling the volume of the raw mix and creating gas bubbles up to 3 millimetres (1⁄8 in) in diameter.^[30] At the end of the foaming process, the hydrogen escapes into the atmosphere and is replaced by air, resulting in a product weighing approximately 20% of conventional concrete.^{[citation needed]}

Once the forms are removed, the material is solidified but remains soft. It is then cut into either blocks or panels, if necessary, and placed in an autoclave chamber for 12 hours.^{[citation needed]} 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 fired bricks but lightweight concrete masonry units. 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.^{[citation needed]} 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]}

Reinforced autoclaved aerated concrete

See also: 2023 United Kingdom reinforced autoclaved aerated concrete crisis

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 there in 1929. However, 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.^[31][32]

RAAC was used in roof, floor, and wall construction due to its lighter weight and lower cost compared to traditional concrete,^[33] and its fire resistance properties; it does not require plastering to achieve fire resistance and fire does not cause spalls.^[34] RAAC was used in construction in Europe, in buildings constructed after the mid-1950s.^[35][36] RAAC elements have also been used in Japan as walling units, owing to their good behavior in seismic conditions.

RAAC has been shown to have limited structural reinforcement bar (rebar) integrity in 40 to 50 year-old roof panels, which began to be observed in the 1990s.^{[36][37][38][39][40]} The material is liable to fail without visible deterioration or warning.^[36][40] 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.^[41]

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,^[42] and was subsequently reinforced in 2022 when the Government Property Agency declared the material to be life-expired,^[43] and in 2023 when, following the partial or total closure of 174 schools at risk of a roofing collapse,^[44][45] other buildings were found to have issues with their RAAC construction,^[46][47][48] with some of these only being discovered to have been made from RAAC during the crisis.^[49][50][51] 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.^[32]

On June 21, 2024, the Ontario Science Centre, a major museum in Toronto, Canada, permanently closed its original site due to the severely deteriorated roof panels from its 1969 opening. Despite the proposed repair options, the provincial government of Ontario, the center's ultimate owner, had already announced plans to relocate the center and therefore requested immediate closure of the facility instead of funding 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.^[52]

Sustainability

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 cubic meter of product and is five times less than the production of other building materials.^[53] There is little 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.^[54] The curing process also saves energy. Steam curing occurs at relatively low temperatures, and the hot steam generated in the autoclaves is reused for subsequent batches.^[55][56]

Advantages

Closeup of structure

Autoclaved aerated concrete (AAC) has been used in construction since the early 20th century and is valued for its lightweight and insulating properties.

AAC possesses several characteristics that make it suitable for specific construction applications:

• Thermal efficiency: Due to its cellular structure, AAC offers thermal resistance that may help reduce heating and cooling loads in buildings. ^[57]
• Fire resistance: AAC exhibits good fire resistance; its mineral composition and porous structure can provide extended fire ratings under standard testing conditions. ^[58]
• Workability: AAC can be cut and shaped using standard tools, allowing for precise fitting and reducing material waste on construction sites.
• Environmental profile: AAC generally has a lower embodied energy and environmental footprint compared to traditional concrete. In some contexts, it may contribute to achieving points in green building certification systems such as LEED.^[59]
• Low density: The material is significantly lighter than conventional concrete, which can simplify manual handling, reduce transportation demands, and may lower structural loads in seismic design scenarios.^[60]
• Construction efficiency: AAC blocks and panels are often available in larger dimensions than traditional masonry units, potentially reducing installation time and labor. However, the overall impact on construction schedules depends on site-specific logistics and integration with other systems. ^[61]
• Moisture regulation: The vapor-permeable nature of AAC allows for limited water vapor diffusion, which can help moderate indoor humidity and mitigate moisture accumulation or condensation-related issues. ^[62]
• Dimensional accuracy: Factory production techniques yield blocks and panels with tight tolerances, which can reduce the need for on-site trimming and minimize the use of finishing materials such as mortar or plaster.
• Durability: AAC is resistant to pests, mold, and typical climate variations. It maintains its structural and thermal performance over time under normal conditions, although its long-term durability may depend on design detailing and exposure environments. ^[63]

Disadvantages

While autoclaved aerated concrete (AAC) has several advantages, it also presents limitations, particularly in regions with construction practices that differ from monolithic masonry, such as the United Kingdom, where cavity wall systems are common.

• Specialized training: Installation of AAC requires construction techniques that may differ from those used with traditional masonry. As a result, builders may need targeted training to handle, cut, and assemble AAC components effectively^[64]
• Shrinkage cracks: Non-structural shrinkage cracks may develop in AAC installations, particularly under humid conditions or after rainfall. This phenomenon is more frequently observed in low-quality blocks or those that have not undergone adequate steam-curing during production.^[65]
• Brittleness: AAC is more brittle than conventional clay bricks or dense concrete blocks. This characteristic necessitates careful handling during transport and installation to minimize breakage.
• Fixings and fasteners: Due to its porous and brittle composition, AAC requires specialized fasteners for securing fixtures such as cabinets or shelves. Long screws and wall anchors designed for AAC, gypsum board, or lightweight masonry are typically used. Conventional plastic wall plugs and masonry drill bits are generally unsuitable. ^[66] In high-load applications, certified anchors may be required,^[67][68][69] and drilling should be performed using high-speed steel (HSS) drill bits without hammer action to avoid cracking.^[67][68]
• Insulation limitations: AAC blocks manufactured at lower densities (e.g., 400 kg/m³ under European Standard B2.5) may require substantial wall thicknesses—often exceeding 500 mm—to meet thermal insulation standards in colder climates such as Northern Europe. This can offset space and cost benefits in some applications.^[64]
• Water sensitivity: The aerated structure of AAC contains fine air voids, which can absorb moisture. In freeze–thaw environments, absorbed water may expand upon freezing and cause damage to the block structure. Protective finishes and proper detailing are typically recommended in such conditions.^[70]