Frequently asked questions
Short answers to the questions that come up most often, each linked to the page that treats the subject properly.
What is the difference between foam concrete and aerated concrete?
Aerated concrete is an umbrella term for any cementitious material with a deliberately introduced air-void system. Foam concrete is one type, made by mixing a pre-formed foam into a slurry. Autoclaved aerated concrete (AAC) is the other, made by a chemical gas-forming reaction and cured in an autoclave. Because the term aerated concrete covers both, it should not be used on its own in a specification.
How long does foam concrete take to set?
Under ambient conditions a foam concrete element is generally stiff enough to handle and demould after about 24 hours. Low-pressure steam curing at 50 to 70 degrees Celsius accelerates hydration enough to bring that forward to roughly 2 hours. Both figures depend on density, cement type and ambient temperature: high-density mixes are ready sooner, and low-density mixes with blended cements can need 36 to 48 hours.
Why can foam concrete not be vibrated?
Vibration collapses the foam. The air voids are held in place by a thin, stabilised film until the cement paste stiffens around them; vibration breaks those films, the bubbles merge and escape, and the density rises. Foam concrete is self-compacting and self-levelling by design and requires no compaction of any kind.
What is the lowest density that can be produced?
The practical floor is set by the foaming agent, not by the mixer or the cement. Synthetic surfactant agents work reliably down to about 300 to 400 kg per cubic metre. Protein hydrolysate agents reach roughly 150. Protein agents of biotechnological origin are claimed to hold densities down to about 75 kg per cubic metre, though independent published verification of that figure is limited.
Can foam concrete be reinforced with steel?
Only with deliberate additional corrosion protection. Foam concrete has a low alkaline reserve and a low binder content per cubic metre, so it carbonates faster than normal-weight concrete, and its high drying shrinkage puts additional strain on the bond. Reinforced foam concrete elements exist but require corrosion protection to be designed explicitly rather than assumed from concrete cover rules.
How much does foam concrete shrink?
Drying shrinkage of 0.1 to 0.35 percent, or 1 to 3.5 mm per metre, is normal. That is roughly an order of magnitude higher than normal-weight concrete, because there is no coarse aggregate skeleton to restrain the shrinking paste. Shrinkage increases as density falls. Sealed curing for at least seven days, fly ash substitution and polypropylene fibres all reduce it, but movement joints must still be detailed for the real figure.
Does foam concrete absorb a lot of water?
Less than its porosity suggests. The voids created by the foam are mostly closed and are not themselves a transport path; water moves through the capillary porosity of the paste between them. Absorption by volume is not dramatically worse than conventional concrete, though absorption by mass looks much higher simply because the mass is lower. Comparisons should always be made on a volume basis.
Is foam concrete a good sound insulator?
It is a good sound absorber and a poor sound barrier. Airborne sound insulation is governed mainly by mass per unit area, so a lightweight wall performs worse than a heavy one of the same thickness. Foam concrete is effective at reducing reverberation within a space, but it should not be specified to meet a party-wall sound reduction requirement on the strength of absorption figures.
What does a foaming agent cost per cubic metre?
Roughly one litre of concentrate per cubic metre of material is a serviceable planning figure across the common density range, falling as density rises. For a 400 kg per cubic metre mix at 1:30 dilution the calculation gives about 1.1 litres per cubic metre; for a 1000 kg per cubic metre mix, about 0.8 litres. The agent is a small share of material cost and a large share of technical risk.
Can foam concrete be pumped over long distances?
Yes, but every metre of line and every pressure cycle costs density. Progressive cavity and screw pumps are the appropriate choice because they deliver without severe compression; piston pumps compress and re-expand the material on every stroke and drive bubble coalescence. On long runs it is usually better to pump the base slurry and generate the foam at the point of placing.
Why is foam concrete not covered by EN 206?
EN 206 covers normal-weight, heavyweight and lightweight concrete, and its lightweight provisions apply to material with a closed structure and an oven-dry density between 800 and 2000 kg per cubic metre produced with lightweight aggregate. Foam concrete has no coarse aggregate, has a deliberately introduced void system, and mostly sits below that density range. It is covered instead by dedicated guides such as the ACI 523 series and by national approvals for specific applications.
Is foam concrete the same as cellular lightweight concrete?
Yes. Cellular lightweight concrete, or CLC, is the same material under a different name, common in South and South-East Asia and in equipment marketing. Low-density cellular concrete and lightweight cellular concrete are the equivalent North American terms, generally used for cast-in-place fill below about 800 kg per cubic metre.
Where to read further
- Technology — the production route, curing regimes and the common failure modes.
- Density classes — strength, conductivity, shrinkage and absorption by class.
- Foaming agents — the three families, and how to choose between them.
- Mix design — batch calculation with worked examples.
- Standards — what applies, what does not, and how to write a specification that binds.
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