Key points
- AAC is made with aluminium paste in an alkaline mix that generates gas bubbles, then steam-cured under pressure (autoclave).
- Its porosity is generally 70–80% with pores 0.5–2 mm across — the secret of its light weight and insulation.
- It is used for infill walls and load-bearing walls depending on the design.
- It is normally covered with lightweight render outside and plaster inside, using mortar and render specified by the manufacturer.
Autoclaved aerated concrete (AAC) is a lightweight building material sold as large white blocks. Brand names vary, so it's best to ask about density, compressive strength and thermal conductivity in the data sheet.
1) How is it made?
Aluminium paste is added to an alkaline mix, generating gas bubbles that form the pores; the product is then steam-cured under pressure in an autoclave1. The result is a porous material with porosity generally 70–80% and large pores 0.5–2 mm across1.
2) Why does it insulate?
The higher the porosity, the lower the thermal conductivity, so AAC is described as an energy-saving material combining light weight and thermal insulation, used in buildings as infill walls or load-bearing walls1. Its mass also gives an additional thermal benefit, validated in Oak Ridge National Laboratory tests on AAC walls2.
3) Construction and finishing
In the practice described by Oak Ridge, an AAC wall is normally covered with lightweight stucco outside and plaster inside2.
Common practical notes; the binding reference is the manufacturer's instructions.
- Usually laid with a thin-bed adhesive mortar specified by the manufacturer rather than thick conventional mortar.
- External render must be compatible with it (suitably flexible and breathable) to reduce cracking.
- Heavy fixings need plugs designed for lightweight materials. See drilling walls.
- Protect blocks from rain and moisture on site before laying.
4) Pros and cons
Pros
- Much better thermal insulation than typical concrete block of the same thickness.
- Light, reducing dead loads and foundation loads.
- Easy to cut and shape; large sizes speed up construction.
Cons and cautions
- Lower compressive strength than concrete block; load-bearing use only per an engineer's design.
- Porous; needs a suitable external finish and must not be left exposed.
- Needs specific mortar, render and fixings, and trades who know it.
For comparison with concrete block see blocks, and with other alternatives the comparison of alternatives.
Sources
The number next to a statement points to its source in this list. Sources back the general information; anything marked “practical advice” is editorial experience, not a quoted source. Anything that affects safety, structural design or electrical installations needs a licensed professional, and in Saudi Arabia the binding reference is the Saudi Building Code.
- 1Effect of Pore Structure on Thermal Conductivity and Mechanical Properties of Autoclaved Aerated Concrete — Materials (MDPI)
- 2Whole Wall Performance Analysis of Autoclaved Aerated Concrete — ACEEE Summer Study — Oak Ridge National Laboratory