Autoclaved aerated concrete (AAC)
A lightweight autoclaved concrete with a closed pore structure that lays fast and true, insulates well for a masonry unit, and absorbs water readily.
What is autoclaved aerated concrete and what do its grades mean?
Autoclaved aerated concrete, universally shortened to AAC, is a lightweight concrete. A mix of lime, cement, finely ground silica sand and water is expanded with aluminium powder, which creates millions of small pores, and the material is then cured in steam under pressure. It is the air in those pores that gives the block its thermal performance: the declared thermal conductivity sits at roughly 0.08 to 0.13 W/(m·K) depending on the grade. Blocks therefore split into lighter thermal grades and denser load-bearing grades.
More pores mean a better conductivity but less material to carry load, and no product can escape that trade. In practice it means designing the build-up as a whole: either a thicker wall in a thermal grade where the structure allows it, or a thinner load-bearing grade with external insulation. The measure is STN 73 0540, whose Part 2 gives a recommended value of 0.22 W/(m²·K) for the external wall of a new dwelling and a target value of 0.15 W/(m²·K) applying to new buildings from 1 January 2021.
Why is building with aerated concrete so fast?
The block is cut with a hand saw, weighs a fraction of fired clay of comparable volume, and comes in precise ground formats laid in thin-bed mortar or adhesive. One mason covers a large area in a day and the wall comes out flat, so the plaster can be thinner. Services are just as easy: chases for cables and pipework are milled or scraped out without breaking tools, and openings can be adjusted on site with a saw rather than a grinder.
The same softness cuts the other way. An ordinary expansion plug will not hold, so fixings need anchors made for the material and heavier loads need a defined detail. In fire terms the material is reaction to fire class A1 to EN 13501-1, non-combustible and contributing nothing to a fire, which is a practical advantage in separating walls and in open-plan interiors where a wall may have to be fire rated.
How does aerated concrete behave with moisture?
Moisture is the subject that decides whether an AAC house is trouble-free. The material is strongly capillary-active and absorbs water readily, which is a characteristic to be worked with rather than a defect to be argued about.
- Keep pallets covered on site and protect part-built masonry from rain. A saturated block is heavy and dries slowly.
- A fresh wall carries a large amount of moisture from manufacture and from the site, and that moisture has to leave before final surfaces go on.
- The finish must allow the drying. Lime plaster and lightweight plasters suit the material; coatings with a high vapour resistance do not.
- The plinth and the junction with the foundation need thorough damp-proofing so the wall cannot wick water from below.
Houses that are closed up and heated too quickly show damp corners and peeling paint in their first winter. That is a fault of the programme, not of the material, and it is avoided by planning the drying period into the schedule and ventilating through it.
Where does aerated concrete reach its limits?
The first limit is thermal mass. AAC is light, so it stores less heat than fired clay of the same thickness and damps the summer temperature peak less. In houses with large glazed areas that has to be answered with external shading and night ventilation, and the summer criterion is itself set in STN 73 0540 Part 2. The second limit is acoustics: lower areal mass means worse airborne sound insulation at the same thickness, so a partition between a bedroom and a living room has to be designed rather than copied from the external wall. The third is load capacity, where long spans, concentrated loads and tall walls are decided by the strength grade and the structural calculation, not by habit.
| Design question | What AAC gives you | What it asks in return |
|---|---|---|
| Thermal performance of the unit | Roughly 0.08 to 0.13 W/(m·K), good for a masonry unit | The better grades are the weaker ones |
| Speed of the shell | Fast, accurate, flat walls in thin-bed mortar | Setting out has to be right, there is little to grind away later |
| Summer comfort | Little | External shading and night purge ventilation |
| Acoustic separation | Little at ordinary thicknesses | A designed partition build-up, not the external wall recipe |
| Fixings | Easy chasing and cutting | Anchors made for AAC, loads defined in the drawings |
| Moisture | Dries eventually, tolerates one wetting | Covered storage, a drying period, a vapour-open finish |
How does aerated concrete compare with a ceramic block wall?
Both are non-combustible masonry systems laid in thin-bed mortar, and in Slovakia the two together account for most new house walls, so the comparison is the one most clients actually face.
| Property | Aerated concrete | Ceramic block |
|---|---|---|
| Weight of the wall | Light | Heavy |
| Summer damping | Weaker | Stronger |
| Speed of laying | Faster, hand tools | Slower, power tools for cuts |
| Flatness of the finished wall | Very good, thinner plaster | Good |
| Water uptake | High, slow to dry | Moderate, dries more readily |
| Airborne sound at equal thickness | Worse | Better |
Aerated concrete is a good material for a fast, accurate, non-combustible masonry house. It is not a material that forgives everything, and the two places it is least forgiving, summer overheating and moisture during construction, are both decided long before the first block is laid. Where the U-value of a thin wall matters more than mass, the honest alternative to compare against is a timber frame rather than another block.
Frequently asked questions
- What is autoclaved aerated concrete made of?
- Lime, cement, finely ground silica sand and water, expanded with a small dose of aluminium powder and then cured in steam under pressure. The aluminium reaction generates hydrogen, which creates millions of small pores; the autoclave gives the material its final strength. It is the air in those pores, not the solid, that does the insulating.
- Why do aerated concrete blocks come in different grades?
- More pores mean a lower thermal conductivity but less material left to carry load, so a block cannot be the best insulator and the strongest unit at the same time. Manufacturers therefore split the range into lighter thermal grades and denser load-bearing grades. Which one is correct follows from the structural calculation for the specific house.
- Does an aerated concrete wall need external insulation?
- A thick wall in a thermal grade can reach the recommended Slovak wall value on its own. Reaching the target value of 0.15 W/(m2K) that applies to new buildings from 1 January 2021 usually means either a thicker thermal-grade wall or a thinner load-bearing wall with an external insulation system.
- Is it true that aerated concrete gets wet and stays wet?
- It is strongly capillary-active, so it takes up water quickly and gives it back slowly. That is a characteristic to work with, not a defect. Pallets stay covered on site, part-built masonry is protected from rain, the plinth is properly damp-proofed, and the finish has to let the wall dry rather than seal it in.
- Can you hang a wall unit on an aerated concrete wall?
- Yes, but not with an ordinary expansion plug. The material is soft, so fixings have to be anchors designed for aerated concrete, and heavier loads such as wall-hung sanitaryware, large televisions or kitchen units belong in the drawings with a defined fixing detail rather than in the fitter’s judgement on the day.