Aerated concrete is a lightweight building material filled with millions of tiny air bubbles, giving it roughly one-fifth the weight of standard concrete while providing built-in insulation, fire resistance, and soundproofing. Invented in Sweden in 1924, it’s now used worldwide for walls, floors, and roof panels in both residential and commercial construction. The most common form, autoclaved aerated concrete (AAC), is factory-cured under high-pressure steam to produce precise, uniform blocks and panels.
How Aerated Concrete Is Made
The raw materials are simple: quartz sand (or fly ash), cement, lime, water, and a small amount of aluminum powder or paste. These are mixed into a slurry and poured into large molds. When the aluminum reacts with the alkaline mixture, it releases hydrogen gas, which creates thousands of evenly distributed air pockets throughout the material. The slurry rises like bread dough, sometimes doubling in volume within minutes.
Once the mix partially sets, it’s cut into blocks or panels with precise wire-cutting tools. The cut units then enter an autoclave, a large pressurized chamber filled with steam at roughly 180°C. This curing process, lasting about 10 to 12 hours, triggers a chemical reaction that forms calcium silicate hydrate crystals, giving the blocks their final strength and stability. The result is a solid yet porous block that can be handled easily on a job site.
Weight and Density
Standard concrete typically weighs around 2,300 kg/m³ (about 143 lb/ft³). Aerated concrete ranges from 300 to 1,800 kg/m³ depending on how it’s formulated, but the grades most commonly used in construction fall between 400 and 800 kg/m³ (25 to 50 lb/ft³). That means a typical AAC block weighs roughly one-quarter to one-third what an equivalent conventional concrete block would weigh.
This low density has practical consequences. Workers can carry blocks by hand, reducing the need for heavy lifting equipment. Walls built with AAC place less load on foundations, which can mean smaller footings and lower structural costs. Shipping is cheaper per unit of wall area, and cutting or shaping on site requires only a hand saw rather than a masonry blade.
Structural Strength
Aerated concrete is not as strong as conventional concrete in pure compression, but it doesn’t need to be for most applications. Compressive strength ranges from about 2 to 6 MPa (290 to 870 psi), organized into recognized strength classes. The lightest class, rated at 2 MPa with a density around 400 kg/m³, works for non-load-bearing partition walls and infill panels. Higher classes at 4 and 6 MPa, with densities up to 800 kg/m³, are rated for load-bearing construction in multi-story buildings, typically up to six stories.
For taller or more heavily loaded structures, AAC walls can be reinforced with steel bars embedded during manufacturing, or combined with a reinforced concrete frame where the AAC serves as the wall infill.
Thermal Insulation
The trapped air pockets that make aerated concrete light also make it an effective insulator. Thermal conductivity for typical building-grade AAC (400 to 700 kg/m³) ranges from 0.13 to 0.22 W/(m·K). The lightest self-insulating blocks can reach as low as 0.11 W/(m·K). For comparison, standard concrete has a thermal conductivity around 1.0 to 1.8 W/(m·K), meaning it conducts heat roughly 8 to 15 times more readily.
In practical terms, a single-layer AAC wall often meets energy code requirements without added foam or fiberglass insulation, especially in moderate climates. In colder regions, an AAC wall may still need supplemental insulation, but less of it than a conventional masonry wall would require. The material also has good thermal mass, absorbing heat during the day and releasing it slowly at night, which helps stabilize indoor temperatures.
Fire Resistance
Aerated concrete is entirely mineral, contains no organic material, and does not burn. Its fire performance is one of its standout properties. A 100 mm thick non-load-bearing AAC wall panel achieves a fire resistance rating of over 3 hours. A 200 mm block wall exceeds 6 hours of direct fire exposure without structural failure. The thickest standard blocks (250 to 300 mm) carry ratings of 8 hours.
A useful rule of thumb for non-load-bearing AAC: roughly 1 hour of fire resistance for every 25 mm of thickness. Load-bearing walls perform even better relative to their thickness because of the additional mass in block construction. During fire testing, AAC walls do not produce toxic fumes or smoke, which is a meaningful safety advantage over some other lightweight building systems.
Working With AAC on Site
One of the features that originally drove adoption of aerated concrete in Europe was its dimensional precision. Because blocks are wire-cut in the factory, they’re accurate enough to be laid with thin-bed mortar rather than conventional cement mortar. Where traditional blockwork uses mortar joints 10 to 12 mm thick, AAC joints are only 3 to 5 mm, applied with a notched trowel. Thinner joints mean less mortar, faster laying, and fewer thermal bridges (spots where heat escapes through the mortar rather than the insulating block).
AAC can be cut, drilled, chased, and shaped with simple hand tools or standard woodworking saws. Running electrical conduit or plumbing through an AAC wall is straightforward: you can carve channels with a hand router and patch them with the same thin-bed mortar. This workability speeds up construction and reduces labor costs compared to working with dense concrete or clay masonry.
The material does have a key limitation: it absorbs water. Exterior AAC walls need a protective finish, whether that’s render (stucco), cladding, or a water-repellent coating. Without protection, moisture absorption raises thermal conductivity and can eventually lead to surface deterioration in freeze-thaw climates.
Environmental Profile
Aerated concrete production uses significantly less energy than conventional concrete manufacturing, with some manufacturers reporting roughly 50% lower energy consumption per unit of wall area. The lightweight nature of the blocks also reduces transportation emissions. Because AAC provides built-in insulation, buildings made from it consume less heating and cooling energy over their lifespan, which compounds the initial manufacturing savings.
In green building certification systems, AAC construction can contribute meaningfully. Projects using AAC have qualified for LEED Silver, Gold, and Platinum certifications, with the material potentially contributing across categories including energy performance, materials efficiency, and indoor environmental quality. AAC also meets the requirements of several other standards, including Passive House and Net Zero Energy frameworks, when combined with appropriate design strategies.
Standards and Availability
In the United States, AAC is governed by ASTM C1693, which defines strength classes, density limits, and testing requirements. This standard ensures that any AAC product sold in the U.S. meets minimum performance thresholds for compressive strength, density consistency, and dimensional tolerance. Internationally, AAC has been covered by European, Australian, and Asian standards for decades, reflecting its longer history of use in those markets.
AAC was perfected by the Swedish architect and inventor Johan Axel Eriksson, who patented the process in 1924. Commercial production began in 1929 at a factory in Yxhult, Sweden. The material spread rapidly across Europe after World War II, when the demand for fast, affordable reconstruction was enormous. Today it’s a mainstream building material in Europe, the Middle East, China, and India. Adoption in North America has been slower but has grown steadily as energy codes tighten and builders look for wall systems that combine structure and insulation in a single layer.