Autoclaved aerated concrete (AAC) is a lightweight, precast building material made from a mix of cement, lime, sand, water, and a small amount of aluminum powder. The aluminum reacts with the alkaline slurry to produce millions of tiny air bubbles, giving AAC its signature cellular structure. The result is a block that weighs roughly one-fifth as much as standard concrete while offering built-in thermal insulation and fire resistance.
What AAC Is Made Of
The base recipe is straightforward: portland cement, quartz sand (ground to a fine powder), lime, gypsum, and water. Fly ash can replace the sand in some formulations, which puts an industrial byproduct to use. The key ingredient that separates AAC from ordinary concrete is fine aluminum powder, added in small quantities. When the aluminum contacts the highly alkaline liquid, it produces hydrogen gas. Those gas bubbles expand through the mixture and then get locked in place as the material stiffens, creating a uniform honeycomb of air pockets throughout the block.
How AAC Is Manufactured
Production starts by mixing the raw materials into a slurry and pouring it into large molds, sometimes the size of small railway wagons. Over the next several hours, two things happen at once: the cement begins to hydrate and stiffen (forming what’s called a “green cake”), and the hydrogen gas from the aluminum reaction causes the mixture to rise like bread dough.
Once the cake has risen to the target height, it’s firm enough to handle but still soft enough to cut. Workers or machines pass it through a series of taut cutting wires, slicing it into blocks, panels, or lintels of precise dimensions. The cut pieces then move into a large pressure vessel called an autoclave, where they’re exposed to steam at roughly 180°C and 800 kPa of pressure. The autoclave holds these conditions for 8 to 10 hours, sometimes longer for denser products. This high-pressure steam curing is what gives AAC its final strength and dimensional stability, and it’s where the “autoclaved” in the name comes from.
Weight and Density
Because up to 80% of an AAC block’s volume is air, its density typically falls between 400 and 700 kg/m³ for the grades used in residential and commercial construction. Standard concrete, by comparison, sits around 2,300 kg/m³. That dramatic weight reduction means smaller foundations, easier handling on the job site (most blocks can be lifted by one person), and lower transportation costs. Higher-density AAC exists for structural applications, ranging up to 1,700 kg/m³, but it sacrifices some insulating performance in exchange for greater load-bearing capacity.
Thermal Insulation Properties
All those trapped air pockets make AAC a surprisingly good insulator for a masonry product. Thermal conductivity for the common building grades (400 to 700 kg/m³) ranges from 0.10 to 0.22 W/(m·K). For context, solid concrete conducts heat at roughly 1.0 to 1.8 W/(m·K), so AAC blocks can insulate five to ten times better than a solid concrete wall of the same thickness. Newer “self-insulating” AAC blocks have pushed conductivity down to about 0.11 W/(m·K), eliminating the need for a separate insulation layer in mild and moderate climates. In colder regions, AAC walls still benefit from added insulation, but they start from a much better baseline than conventional masonry.
Fire Resistance
AAC is entirely mineral and contains no combustible materials. A standard 6-inch (150 mm) AAC wall carries a 4-hour fire resistance rating for both load-bearing and non-load-bearing applications. That rating meets or exceeds the requirements for most residential and commercial building codes without any additional fireproofing. The air cells also slow heat transfer during a fire, which helps protect structural elements on the opposite side of the wall.
How AAC Is Installed
Working with AAC feels more like woodworking than traditional masonry. The blocks can be cut with a standard hand saw or band saw, drilled, routed, and shaped on site. They’re laid with a thin-bed mortar only 3 to 5 mm thick, applied with a notched trowel. That’s a fraction of the 10 to 12 mm mortar joints typical in conventional block construction. The thinner joints reduce mortar use, speed up installation, and create fewer thermal bridges (spots where heat escapes through the mortar rather than through the insulating block).
A rubber mallet and spirit level are the other essential tools. Because AAC blocks are manufactured to tight dimensional tolerances, walls go up plumb and flat with less fuss than traditional block work. The lighter weight also means less fatigue for masons over a full day of laying.
Strengths and Limitations
AAC works well for exterior walls, interior partitions, floor panels, and roof panels. Its combination of light weight, insulation, fire resistance, and ease of cutting makes it popular in Europe, the Middle East, and parts of Asia, where it has been a standard building material for decades. It’s gaining traction in North America, though it remains less common there.
The main limitation is compressive strength. A typical AAC block used in low-rise construction has a compressive strength around 3 to 5 MPa, while standard concrete blocks can exceed 15 MPa. That makes AAC suitable for load-bearing walls in low-rise buildings (generally up to about five stories with proper engineering) but not for high-rise structural applications without a supporting frame. AAC also absorbs moisture readily, so exterior walls need a weather-resistant finish such as stucco, cladding, or a specialized coating to prevent water infiltration over time.
Environmental Impact and Recycling
AAC’s lighter weight means less raw material per unit of wall area compared to solid concrete or clay brick, and its insulating properties reduce energy consumption over the life of a building. The material is also fully recyclable. Post-demolition AAC can be crushed and fed back into the production of new AAC blocks, light mortar, or lightweight aggregate concrete.
A life cycle study focused on Germany found that recycling demolished AAC instead of sending it to landfill could save up to 280,000 tonnes of CO₂ equivalent per year across the country. For each kilogram of demolished AAC recycled back into new AAC production, the savings reach about 0.5 kg of CO₂ equivalent compared to landfilling. Despite that potential, most demolished AAC currently ends up in landfills, though recycling infrastructure is expanding as demolition volumes grow.