Foam concrete and cellular lightweight concrete
Foam concrete is a cement-based material whose volume is deliberately filled with a stable, evenly distributed system of air voids. Adjusting the air content alone moves the same base material from a 75 kg/m3 insulating fill to a 1600 kg/m3 semi-structural element. This site explains how that is done, what properties result, and what equipment and standards apply.
What foam concrete is
Foam concrete is made by combining a cementitious base slurry with a separately generated, pre-formed aqueous foam. The foam is not an admixture that reacts chemically; it is a physical volume of stabilised air bubbles that is folded into the slurry and remains there while the cement hydrates around it. When hydration is complete, the bubbles have become a permanent, mostly closed system of spherical voids typically 0.1 to 1.0 mm across.
Because the air is introduced as a measured volume rather than produced by a chemical reaction, the density of the finished material is a dial rather than an outcome. A plant can produce 150 kg/m3 insulation fill in the morning and 1200 kg/m3 block material in the afternoon from the same silo of cement, the same sand and the same foaming agent, changing only the ratio of foam to slurry.
The consequences follow directly from the void structure:
- Low self-weight. Typical working densities are 300 to 1600 kg/m3 against roughly 2400 kg/m3 for normal-weight concrete.
- Low thermal conductivity. Roughly 0.08 to 0.70 W/(m·K) across the usable density range, against 1.4 to 2.0 W/(m·K) for normal-weight concrete.
- Self-levelling, self-compacting flow. The material is placed by pouring or pumping and must never be vibrated: vibration collapses the foam and the density with it.
- Modest strength. 28-day compressive strength runs from below 0.3 MPa at the lightest densities to around 25 MPa at 1600 kg/m3.
- High drying shrinkage. Without coarse aggregate to restrain the paste, shrinkage of 0.1 to 0.35 % is normal, an order of magnitude above normal-weight concrete.
Terminology, and why it matters
The same material is sold under at least half a dozen names, and the names are not interchangeable when a specification is being written.
| Term | Usual meaning |
|---|---|
| Foam concrete, foamed concrete | Cementitious mix aerated with pre-formed foam. The general term used throughout this site. |
| Cellular lightweight concrete (CLC) | Same material. Common in South and South-East Asia and in equipment marketing. |
| Low-density cellular concrete (LDCC), lightweight cellular concrete (LCC) | North American usage, generally for cast-in-place fill below about 800 kg/m3. |
| Aerated concrete | Umbrella term covering both foam concrete and autoclaved aerated concrete. Ambiguous on its own. |
| Autoclaved aerated concrete (AAC) | A different product: gas-generated, autoclave-cured, factory-only. See the comparison. |
| Foamed mortar | Technically the most accurate label for most of it, because there is no coarse aggregate. |
The pedantic point about mortar is worth keeping. Codes and design equations written for concrete assume a coarse-aggregate skeleton that carries load and restrains shrinkage. Foam concrete has neither. Strength, stiffness, creep and shrinkage all behave differently, which is why foam concrete falls outside the scope of general concrete codes such as EN 206 and is handled instead by dedicated guidance and national approvals.
How it is produced
Two production routes exist. In pre-foaming, which dominates industrial practice, the base slurry is batched to its own recipe and a foam generator produces foam of a known and checked density, typically 40 to 80 g/L. A measured volume of that foam is then blended in at low shear until the wet density of the combined material hits target. In mix-foaming, the foaming agent goes straight into the mixer and the air is entrained by intense agitation. Mix foaming needs less equipment but gives coarser, less uniform voids and much looser density control, so it is largely confined to small site work.
Quality control on a foam concrete line is unusually simple and unusually unforgiving: fill a container of known volume, weigh it, and compare against the target wet density. A wet density 5 % off target means the pour is out of specification, and there is no way to correct it after placing. See mix design and calculation for how the targets are derived.
Setting, demoulding and curing times
Foam concrete gains strength through ordinary Portland cement hydration; the air voids neither help nor hinder the chemistry, but they do reduce the amount of load-bearing paste per unit volume, so early strength is low in absolute terms.
Under ambient conditions, a foam concrete element is generally stiff enough to handle and demould after about 24 hours. Low-pressure steam curing at an elevated temperature — commonly in the range of 50 to 70 °C — accelerates hydration enough to bring that forward to roughly 2 hours, which is what makes multi-cycle block production economic.
Both figures are working numbers, not guarantees. What actually governs them:
- Density. A 1200 kg/m3 mix has roughly four times the paste volume of a 300 kg/m3 mix and reaches handling strength considerably sooner.
- Cement type and content. CEM I 52,5 R behaves very differently from a CEM III blend, which may need 36 to 48 hours before demoulding at low densities.
- Ambient temperature. Below about 10 °C, ambient setting stretches out sharply, and the foam itself becomes less stable in the fresh state.
- Accelerators and fine additions. Calcium formate, silica fume and fine fly ash all shift the curve, though fly ash usually slows early strength while improving 28-day and 90-day values.
Why steam curing is limited to about 70 °C
Accelerated curing of Portland cement systems is a well-established trade-off. Raising the curing temperature speeds early hydration but produces a coarser, less uniform C-S-H microstructure, which caps long-term strength — the "crossover effect" familiar from precast practice. Going above roughly 70 °C also risks delayed ettringite formation in sulfate-bearing systems. In foam concrete there is a second constraint: the entrained air expands with temperature, so an aggressive ramp before the paste has stiffened can distort the void structure and lift the surface of the element. Controlled ramp rates, a pre-set holding period before heat is applied, and a ceiling in the 50 to 70 °C band are therefore standard.
This is different in kind from the autoclaving used for AAC, which runs at roughly 180 to 190 °C and 10 to 12 bar of saturated steam and chemically converts the binder to tobermorite. Foam concrete is never autoclaved; the curing regimes are not comparable and neither are the products.
Properties at a glance
| Dry density | Compressive strength, 28 d | Thermal conductivity | Typical use |
|---|---|---|---|
| 75–150 kg/m3 | < 0.3 MPa | 0.04–0.06 W/(m·K) | Insulating fill, void filling |
| 200–300 kg/m3 | 0.3–1.0 MPa | 0.06–0.09 W/(m·K) | Insulation boards, roof screeds |
| 400–600 kg/m3 | 0.8–3 MPa | 0.09–0.18 W/(m·K) | Non-load-bearing blocks, floor screeds |
| 800–1000 kg/m3 | 2–8 MPa | 0.18–0.33 W/(m·K) | Load-bearing blocks in low-rise work |
| 1200–1600 kg/m3 | 5–25 MPa | 0.32–0.70 W/(m·K) | Semi-structural and precast elements |
The density classification page breaks these bands down further and adds shrinkage, water absorption and fire behaviour.
Equipment and plants
Four equipment classes cover essentially all production:
- Foam generators. The one indispensable machine. Compressed air and a diluted foaming-agent solution are forced through a restriction or a packed lance to produce dry foam of controlled density.
- Mobile all-in-one units. Skid- or trailer-mounted mixer, foam generator and pump for site-cast fill, trench reinstatement and screeds.
- Semi-automatic block plants. Silo storage, weigh batching, mixing and foam dosing, moulding or slab casting with wire cutting, curing and packaging, with manual transfer between stages.
- Fully automatic plants and ready-mix integration. Continuous in-line mixing with automated foam dosing, or foam injected into a ready-mix truck at the batching plant so that conventional RMC logistics can deliver foam concrete.
The equipment page covers sizing, pump selection and the failure modes specific to each class.
Where it is used
Foam concrete is chosen when low weight, self-levelling placement or thermal performance matters more than compressive strength. The recurring applications are trench reinstatement and utility backfill, void and annulus filling, floor and roof screeds, lightweight embankment and abutment fill over soft ground, filling of hollow blocks, precast blocks, panels and insulation boards, and aircraft arrestor beds. Each of these is set out on the applications page, with the density range and the reason the material wins.
About this site
This is an independent technical reference, written for engineers, contractors and students. It sells nothing and represents no manufacturer. Figures are given with their sources, and the research page lists the peer-reviewed literature behind them.
The domain previously belonged to a Berlin foam concrete equipment manufacturer, and a number of external references — including citations in encyclopaedic and academic sources — still point at addresses on it. Those addresses have been kept working and now lead to the corresponding technical topic. The history note explains what was here before and states plainly that this site has no connection to that company.
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