Autoclaved aerated concrete

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Sectional view of AAC.

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Palette stacked AAC blocks.

Autoclaved aerated concrete (AAC) also referred to as autoclaved cellular concrete (ACC) or simply autoclaved concrete is a lightweight, prefabricated concrete building material. Developed in the mid-1920s, it is an alternative to traditional concrete blocks and clay bricks. AAC products are factory-prefabricated, in contrast to cellular concrete, which is typically mixed and poured on-site.

AAC is a combination of quartz sand, gypsum, lime, Portland cement, water, fly ash, and aluminium powder. After the aerated mixture is partially cured in a mould, the material is further cured under heat and pressure in an autoclave. AAC can be made into blocks, wall panels, floor and roof panels, cladding (façade) panels, and lintels.

AAC materials are typically cut using standard power tools equipped with carbon steel cutters. AAC products may be used in various construction projects similarly to cellular concrete. For any exterior use, they may require an applied finish to guard 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.

History

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

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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), along with Professor Henrik Kreüger at the Royal Institute of Technology. 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: Ytong. Another brand, "Siporex", was established in Sweden in 1939. Following World War II, demand for the product declined, leading to a significant reduction in production, with no new plants constructed after the 1990s. Josef Hebel of Memmingen established another cellular concrete brand, Hebel, which opened their first plant in Germany in 1943.

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 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.

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. 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. Currently, LCC has three branches in Saudi Arabia.

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. 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. 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.

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Residential house constructed at the Finnish Seinäjoki Housing Fair in 2016 using AAC blocks.

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AAC blocks on a residential house construction site in Russia.

Uses

AAC is used for both exterior and interior construction. It has been applied in high-rise construction projects and areas with frequent temperature fluctuation. 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 ACC, many buildings employing AAC components utilize thin bed mortar, typically around thick, in accordance with specific national building codes.

Manufacturing

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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 is usually smaller than sand. 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.

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 in diameter. 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.

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. During this steam pressure hardening process, when the temperature reaches and the pressure reaches , 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. AAC's low density also accounts for its low structural compression strength. It can carry loads of up to , approximately 50% of the compressive strength of regular concrete.

Reinforced autoclaved aerated concrete

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.

RAAC was used in roof, floor, and wall construction due to its lighter weight and lower cost compared to traditional concrete, and its fire resistance properties; it does not require plastering to achieve fire resistance and fire does not cause spalls. RAAC was used in construction in Europe, in buildings constructed after the mid-1950s. 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. The material is liable to fail without visible deterioration or warning. 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.

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, and was subsequently reinforced in 2022 when the Government Property Agency declared the material to be life-expired, and in 2023 when, following the partial or total closure of 174 schools at risk of a roofing collapse, other buildings were found to have issues with their RAAC construction, with some of these only being discovered to have been made from RAAC during the crisis. 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.

On June 21, 2024, the Ontario Science Centre, a major museum in Toronto, Canada, permanently closed its original site due to 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.

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. 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. 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.

Advantages

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Closeup of structure

Autoclaved aerated concrete (AAC) has been in production for over 70 years and is noted for several advantages over traditional cement-based construction materials.

Additional characteristics of AAC include:

• Thermal efficiency: The material's insulating properties can help lower heating and cooling energy requirements in buildings.
• Fire resistance: The porous structure of AAC provides enhanced resistance to fire.
• Workability: AAC can be cut with basic tools, allowing for precise adjustments and reducing solid waste during construction.
• Environmental impact: AAC generally has a lower environmental footprint than conventional concrete and can contribute to meeting criteria under green building standards such as LEED.
• Lightweight: AAC blocks weigh less than conventional concrete, which can simplify handling, lower transportation costs, and reduce labor requirements. Their lower mass may also offer structural advantages in seismic zones.
• Construction efficiency: The availability of larger block sizes may accelerate masonry work and has the potential to reduce project timelines and costs.Template:How
• Ease of handling: The relatively light weight of AAC can simplify installation processes and improve construction efficiency compared to heavier masonry materials.
• Ventilation: AAC is permeable to water vapor, which can assist in regulating indoor humidity and may help mitigate moisture-related issues such as condensation and mold growth.
• Dimensional accuracy: AAC blocks and panels are manufactured to precise dimensions, reducing the need for extensive on-site trimming and limiting the quantity of finishing materials like mortar.
• Durability: AAC is resistant to a range of environmental conditions and maintains its properties under typical climate variations, supporting a long service life.

Disadvantages

Autoclaved aerated concrete (AAC) also presents several disadvantages, particularly in regions where building norms differ, such as the United Kingdom, where double-leaf masonry or cavity walls construction is common.

• Specialized training: The installation of AAC requires specific techniques, and builders may need specialized training to work with the material effectively.
• Shrinkage cracks: Non-structural shrinkage cracks can appear after installation, particularly in humid conditions or following rainfall. This issue is more common in low-quality blocks that have not been adequately steam-cured.
• Brittleness: AAC is more brittle than traditional clay bricks, requiring careful handling during transportation and construction to avoid breakage.
• Fixings and fasteners: Due to its brittle composition, AAC requires specific types of fasteners for securing items such as cabinets or fixtures. Long, thin screws and specialized wall anchors designed for AAC, gypsum board, or plaster tiles are recommended. These fasteners are typically more expensive than standard wall plugs. High-load applications may require safety-rated anchors. It is generally advised to drill fixing holes with high-speed steel (HSS) drill bits at a steady speed without using hammer action. Standard masonry bits and conventional wall plugs are unsuitable for AAC installations.
• Insulation limitations: At lower densities, such as 400 kg/m³ (European standard B2.5), AAC blocks alone would require wall thicknesses of 500 mm or more to meet insulation requirements in colder climates, such as those in Northern Europe.
• Water sensitivity: One of the largest disadvantages of AAC is the fact that since the aeration process creates tiny holes, water can seep into the concrete then expand when it freezes; causing damage to the structure.

External links

• Template:Sister-inline
• History of Autoclaved Aerated Concrete
• Using Autoclaved Aerated Concrete Correctly - Masonry Magazine, June 2008
• Aircrete Products Association
• Autoclaved Aerated Concrete - Portland Cement Association