Autoclaved aerated concrete (AAC) is a lightweight precast building material made from sand or fly ash, lime, cement, water and a small amount of aluminium powder, then cured in a high pressure steam chamber called an autoclave. Roughly 80 percent of a finished block is trapped air, which gives AAC strong thermal insulation and low weight.
Builders reach for AAC when they want masonry that insulates like a foam panel but still carries load, resists fire and cuts with a hand saw. It sits in an unusual spot between structural concrete and insulation board, and that mix of traits is why it has spread across Europe, Asia and parts of North America since the 1920s. The sections below cover how it is made, what it does well, where it falls short, and how it stacks up against brick and ordinary concrete.
What is AAC made of and how is it produced?
The recipe is short. A slurry of fine silica (quartz sand or fly ash), lime, Portland cement, gypsum and water is mixed in a batching plant, then a tiny dose of aluminium powder is added. That aluminium reacts with the lime and water and releases hydrogen gas, which froths through the wet mix and roughly doubles its volume. The result is a cake full of millions of evenly spaced air pockets.
Once the cake has set enough to hold its shape, wires slice it into blocks, lintels or reinforced panels with tight dimensional accuracy. The cut pieces then enter the autoclave, where pressurised steam finishes the chemistry. According to the European standard documented for AAC and confirmed by ASTM, curing runs about 12 hours at temperatures near 190 degrees Celsius and pressures between 800 and 1,200 kilopascals. This steam curing converts the binder into stable calcium silicate hydrates, the same crystalline phase that gives the material its strength and dimensional stability.
🏗️ Real-World Example
Hebel by CSR (Australia, since 1990): CSR has produced AAC under the Hebel brand for more than three decades and reports that reinforced AAC panels and blocks achieve fire resistance levels from 60 to 240 minutes while meeting the Australian Standard AS 5146 series for reinforced AAC.
Who invented autoclaved aerated concrete?
AAC was developed in the mid 1920s by Swedish architect and inventor Johan Axel Eriksson, working with Henrik Kreuger at the Royal Institute of Technology in Stockholm. Eriksson patented the process in 1924, and commercial production started in 1929. The goal was a building material that combined the workability of timber with the durability and fire safety of stone, without the heavy logging that Sweden was trying to reduce.
📌 Did You Know?
One cubic metre of raw mix can produce roughly five cubic metres of finished AAC, because the aluminium reaction expands the material so much. That single fact explains why AAC blocks float in water yet still support multi storey walls.
What properties make AAC different?
The defining feature of AAC is its closed and open cellular structure. Air does the heavy lifting for insulation, while the solid calcium silicate skeleton carries load and resists fire. A peer reviewed study in the journal Materials (2021), "Effect of Pore Structure on Thermal Conductivity and Mechanical Properties of Autoclaved Aerated Concrete," found that as apparent density drops and porosity rises, thermal conductivity falls in a near linear relationship, which is why manufacturers tune the aluminium dose to hit a target density class.
Density typically ranges from about 300 to 700 kilograms per cubic metre, compared with roughly 2,400 for normal weight concrete. Thermal conductivity sits in the 0.08 to 0.16 watts per metre kelvin band for most structural grades, low enough that a single AAC wall can sometimes replace a separate insulation layer. Because the material is non combustible and classified as Euroclass A1 in Europe, it does not feed a fire or release toxic smoke.
🎓 Expert Insight
"Low density is attained by the inclusion of an agent resulting in macroscopic voids, with the material being subjected to high pressure steam curing." (ASTM C1693, Standard Specification for Autoclaved Aerated Concrete)
That single sentence from the governing US standard captures the whole idea: the voids create the performance, and the autoclave locks it in. Specifiers lean on this document to confirm strength class and dimensional tolerances before approving AAC for a project.
Strength, sound and moisture behaviour
AAC trades raw compressive strength for weight savings. It carries loads up to about 8,000 kilopascals, near half the compressive strength of standard concrete, which is plenty for low and mid rise walls but means it needs reinforced lintels and bond beams over openings. Its porous body also dampens airborne sound well, a reason it shows up in party walls and hotel partitions.
Moisture is the trait that catches new users off guard. AAC absorbs water readily through its open pores, so it must stay dry during storage and gets a render, plaster or cladding to shed rain in service. It does not rot or corrode, but saturated blocks lose insulation value and gain weight until they dry out.
💡 Pro Tip
When specifying AAC, use thin bed mortar applied with a notched trowel rather than standard thick mortar joints. The blocks are dimensionally accurate enough that a 2 to 3 millimetre joint keeps the wall true and removes the cold bridging that thick mortar beds create across an otherwise insulating wall.
Which standards govern AAC?
Because performance hinges on density class and curing, AAC is tied to published specifications rather than rule of thumb. In the United States, ASTM C1693 sets the requirements for plain AAC units and ASTM C1452 covers reinforced AAC elements, defining strength classes, dimensional tolerances and the test methods a producer must meet. The ASTM C1693 specification is the document a specifier cites when calling out a grade on drawings.
Elsewhere the harmonised European standard EN 771-4 governs AAC masonry units, while Australia and New Zealand work to the AS 5146 series for reinforced AAC. These standards group products by strength and density, so a wall rated for a given load and fire period can be specified with confidence. They matter because two blocks that look identical can belong to different classes, with real differences in how much load they carry and how well they insulate.
Reinforced panels add another layer of control. Steel bars inside floor, roof and wall panels are given a factory applied anti corrosion coating, since the alkaline protection that ordinary reinforced concrete provides is weaker in the more porous AAC matrix. The standards spell out the cover and coating needed to keep that steel sound over the life of the building.
Is AAC a sustainable building material?
AAC carries a mixed environmental record that depends heavily on how you measure it. The raw material efficiency is striking, since the aluminium reaction stretches one volume of mix into roughly five volumes of finished product, so far less material leaves the plant per wall built. Many producers also fold fly ash, a coal combustion byproduct, into the silica fraction, diverting waste from landfill.
The autoclave is the catch. Heating large steam chambers to roughly 190 degrees Celsius for hours uses real energy, and the cement and lime binders carry the embodied carbon common to all cementitious products. The offsetting benefit comes during the building's life. A wall that insulates well enough to cut heating and cooling demand can repay its production energy many times over across decades of use, which is the calculation that wins AAC a place in energy efficient and passive design briefs.
How does AAC compare to brick and standard concrete?
The clearest way to place AAC is next to the two materials it usually replaces in a wall: fired clay brick and conventional concrete block. Each wins on different metrics, and the right choice depends on whether weight, insulation, strength or speed matters most for the job.
AAC vs clay brick vs standard concrete
The table below summarises the practical trade offs. Figures are typical ranges and shift by grade and supplier.
| Property | AAC | Clay Brick | Standard Concrete |
|---|---|---|---|
| Density (kg/m3) | 300 to 700 | 1,600 to 1,900 | around 2,400 |
| Thermal conductivity (W/mK) | 0.08 to 0.16 | 0.6 to 1.0 | 1.4 to 1.7 |
| Compressive strength | Low to moderate | Moderate to high | High |
| On site workability | Cut and shaped with hand tools | Cut with wet saw, harder | Needs power cutting |
| Fire resistance | Non combustible, up to 240 min | Non combustible | Non combustible |
| Water absorption | High, needs finish coat | Moderate | Low |
The pattern is consistent. AAC beats both rivals on weight and insulation by a wide margin, holds its own on fire, and gives up some compressive strength and moisture resistance in return. For a load bearing foundation wall, concrete still wins. For an insulated above grade wall that needs to go up fast, AAC often comes out ahead on total cost once you account for reduced framing, lighter foundations and the insulation it builds in.
Where is AAC used in construction?
AAC shows up as blocks for load bearing and infill walls, as reinforced floor and roof panels, as lintels over doors and windows, and as cladding panels on steel or concrete frames. Its light weight makes it a favourite for upper floors and renovations where adding mass to an existing structure is a problem. In seismic regions, the low dead load reduces the inertial forces a building must resist, which is one reason it is common across Turkey, Japan and Italy.
The material also suits energy focused projects. Because a properly detailed AAC wall can meet thermal targets without a separate insulation layer, designers use it to simplify wall assemblies and cut the number of trades on site. If you are weighing material choices for a project, the resources on our Architecture and Design Blog cover related detailing and specification topics in more depth.
What are the limits of AAC?
AAC is not a universal answer. Its lower compressive strength rules it out for heavily loaded structural cores without engineered reinforcement. The open pore structure means every wall needs a protective finish, and fixings require special anchors because ordinary expansion plugs can crush the soft material. Transport can also be a factor, since a regional plant is needed to keep the lightweight blocks cost competitive against local brick.
⚠️ Common Mistake to Avoid
Treating AAC like dense concrete when hanging heavy items is a frequent error. Standard screws and expansion plugs pull straight out of the soft cellular body. Use purpose made AAC anchors or chemical fixings rated for low density masonry, and spread heavy loads across a larger area to avoid local crushing.
Technical specifications and load values should be verified by a licensed professional for your specific project, since strength class, code requirements and detailing vary by region.
The Bigger Picture
AAC has lasted a century because it answers a question that keeps coming back: how do you build a wall that is light, warm, quiet and fireproof at once? It will never replace structural concrete where raw strength rules, and it demands careful detailing around moisture and fixings. For architects working on energy efficient, low rise and seismic projects, though, the case for putting air to work inside the wall keeps getting stronger as insulation standards tighten.
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