Foam concrete technology
Producing foam concrete is a sequence of four controlled operations — batch a slurry, generate foam of known density, combine the two to a target wet density, and cure the result without disturbing the void structure. Each step has a small number of ways to go wrong, and they are all avoidable.
The base slurry
The base slurry is an ordinary cementitious mix, deliberately kept simple. Its job is to be fluid enough to accept foam without shearing it, cohesive enough not to separate, and rich enough in binder that the thin films between voids actually gain strength.
Binder
CEM I is the usual choice, at 52,5 R where fast turnover matters and 42,5 N where it does not. Binder content runs from about 250 kg/m3 of finished material at low densities to 500 kg/m3 and above for high-strength mixes. Blended cements work but slow the dormant period, which puts more demand on foam stability; CEM III mixes at low density can need 36 to 48 hours before demoulding.
Fillers
Below roughly 600 kg/m3 most mixes are neat cement paste plus foam — there is simply no room for aggregate. Above that, fine sand is added, and the grading matters more than the quantity: material finer than 2 mm, and preferably below 1 mm, with no coarse fraction. Anything coarser settles through a low-viscosity aerated mix and lands on the bottom of the mould.
Fly ash is the most useful supplementary material here. Substituting a large fraction of cement with fly ash reduces cost and heat of hydration, improves flow at constant water content because of the spherical particle shape, and raises long-term strength. Kearsley and Wainwright's work on high fly ash content in foamed concrete is the standard reference, and reports the trade-off directly: lower early strength, higher strength at 90 days and beyond. GGBS behaves similarly and slows things further.
Water
Water/binder ratios of 0.45 to 0.60 are typical, higher than a structural concrete because there is no superplasticiser to lean on and the slurry must stay fluid. Two corrections matter:
- The foam itself carries water. At 50 g/L foam density, 700 litres of foam brings roughly 35 kg of water into a cubic metre of material. That water counts toward the effective w/b ratio and must be subtracted from the mixing water.
- Too little water gives a stiff slurry that shears the foam during mixing. Too much gives a slurry so thin that bubbles rise through it. The workable window is narrower than for conventional concrete.
Foam generation
A foam generator combines a diluted foaming-agent solution with compressed air and forces the mixture through a restriction — a packed lance, a mesh cartridge or a series of baffles — to break it into small, uniform bubbles.
| Wet foam | Dry foam | |
|---|---|---|
| Method | Solution sprayed through a fine mesh | Solution and air forced through a packed lance under pressure |
| Bubble size | Around 2–5 mm, irregular | Typically below 1 mm, uniform |
| Foam density | Higher, roughly 80–100 g/L | Controlled, usually 40–80 g/L |
| Stability | Poor; drains quickly | Good |
| Use | Small site work, high-density fill | All production work |
A generator is calibrated by mass, not by dial position: run it into a container of known volume, strike it level, weigh it, and adjust the air and solution flows until the reading matches target. Generator output drifts as the lance packing wears and as ambient temperature changes, so the check belongs at the start of every shift.
Pre-foaming and mix-foaming
Pre-foaming generates the foam separately and adds a measured quantity to a finished slurry. Density is controlled directly by the ratio of two independently known quantities, so it is repeatable and traceable. Nearly all industrial production works this way.
Mix-foaming puts the foaming agent into the mixer with everything else and relies on intense agitation to entrain air. It needs no generator and no compressor, but the air content depends on mixer geometry, speed, batch size and mixing time, all of which drift. Void structure is coarser and less uniform, and the achievable density range is narrower. It has a place in small-scale and remote work; it does not belong in a plant producing to a specification.
Mixing and density control
Foam is folded in at low shear. A high-speed pan mixer that produces excellent concrete will destroy foam. Paddle and ribbon mixers running slowly, or in-line static mixers on a continuous plant, are the appropriate tools. Mixing continues only until the foam is uniformly distributed — usually 1 to 3 minutes — because every additional revolution costs air.
Fill a container of known volume with fresh material, strike it level, weigh it, divide. Wet density within ±3 % of target means the batch is good. Outside that, discharge it: there is no corrective action available after the material is placed.
Wet density is not dry density. A fresh mix carries mixing water that will eventually leave, so wet density runs roughly 100 to 200 kg/m3 above the oven-dry density the specification refers to, with the gap widening at higher binder content. Every plant should establish its own wet-to-dry relationship for its own mixes rather than borrowing a rule of thumb.
Pumping and placing
Foam concrete is self-compacting and self-levelling. It is placed by pouring or pumping and is never vibrated, tamped or poker-compacted. Vibration collapses the foam; the density rises, the strength changes unpredictably and the element loses its thermal performance. This is the single most common error made by crews used to conventional concrete.
Pump selection
Progressive cavity (Moineau) pumps and screw pumps move the material without compressing it severely, and are the standard choice. Piston pumps subject each stroke to a pressure cycle that compresses and re-expands every bubble, driving coalescence; where one has to be used, expect a density rise between hopper and outlet and calibrate for it. Peristaltic pumps are gentle but limited in throughput.
Lift heights
Low-density material cannot support much of its own weight before it stiffens. Casting in lifts of about 0.5 to 1.0 m, allowing each to stiffen, avoids the density gradient that otherwise develops as the lower part of a deep pour is compressed by what is above it. High-density mixes tolerate deeper lifts.
Formwork
Pressure on formwork is proportional to density, so foam concrete exerts a fraction of what conventional concrete does — a real economy in deep pours. Formwork does, however, need to be genuinely watertight: the material is fluid, and it finds every gap.
Curing regimes
Foam concrete is more vulnerable to early moisture loss than conventional concrete. Its surface area per unit volume is enormous, its bleed water is minimal, and its shrinkage potential is high. Uncovered elements crack.
Ambient curing
Cover immediately after placing with sheeting or apply a curing membrane; keep sealed for at least seven days. Demoulding is generally possible after about 24 hours, sooner at high density and later at low density or low temperature.
Low-pressure steam curing
Where production cycles matter, elements are held in a chamber at elevated temperature and high humidity. The regime has four parts: a pre-set delay of two to four hours before heat is applied, a controlled ramp of no more than about 20 °C per hour, a hold at a peak in the 50 to 70 °C band, and a controlled cool-down. Under such a regime an element can be ready for demoulding in roughly two hours of holding time, against 24 hours ambient.
The ceiling around 70 °C is not arbitrary. Higher curing temperatures coarsen the hydrate microstructure and cap long-term strength — the crossover effect well documented in precast practice — and above that band delayed ettringite formation becomes a risk in sulfate-bearing systems. In foam concrete there is an additional constraint: entrained air expands with temperature, so heating before the paste has stiffened distorts the void structure and lifts element surfaces. Falliano and co-authors (2018) measured the effect of curing conditions on foamed concrete strength across cement types, foaming agents and densities, and the sensitivity to the curing regime is substantial.
Not autoclaving
Autoclaved aerated concrete is cured at roughly 180 to 190 °C and 10 to 12 bar of saturated steam, which chemically converts the binder to tobermorite. Foam concrete is never autoclaved. The two curing regimes, and the two products, are not comparable — see the comparison.
Quality control that actually catches problems
| Check | When | Catches |
|---|---|---|
| Foam density at generator outlet | Start of every shift | Generator drift, wrong dilution, compressor problems |
| Wet density of fresh mix | Every batch | Dosing errors, over-mixing, foam collapse in the mixer |
| Spread or flow test | Every batch, or hourly on continuous plant | Water content drift, slurry too stiff or too thin |
| Density gradient through a cast section | Weekly, or on any new mix | Foam instability during the dormant period |
| Oven-dry density and compressive strength, 28 d | Per production run | Everything else, too late to fix but essential for records |
The five common failures
- Hardened density above target. Almost always foam loss: over-mixing, an incompatible admixture, a piston pump, or vibration. Check the wet density at the mixer and again at the point of discharge to find where the air is going.
- Density gradient from top to bottom of a cast. Foam instability during the dormant period. Shorten the dormant period with an accelerator, add a viscosity modifier, cast in shallower lifts, or move to a more stable foaming agent.
- Surface and map cracking within days. Inadequate curing. Foam concrete needs to be sealed immediately and kept sealed; it will not tolerate the exposure a conventional slab shrugs off.
- Strength below expectation at correct density. Coarse or merged voids rather than a binder problem. Look at the foam, the generator and the mixing time before touching the cement content.
- Batch-to-batch scatter. Usually the foaming agent — a drifting hydrolysate batch, degraded stock, or a dilution being made up by eye. Fix the measurement before changing the mix.
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