Foaming agents for foam concrete

The foaming agent is the smallest component of a foam concrete mix by mass and the one that decides most of its behaviour. It sets the bubble size, the foam stability, the lowest density that can be produced reliably, and — through all three — the strength of the hardened material.

What a foaming agent has to do

A foaming agent is a surface-active substance dissolved in water. Forced through a foam generator with compressed air, it produces a mass of thin-walled bubbles. Those bubbles then have to survive three consecutive assaults:

1. Mixing. Being folded into a dense, alkaline, abrasive cement slurry without coalescing or collapsing.
2. Transport and placing. Being pumped, sometimes over a hundred metres and up several storeys, under pressure that compresses every bubble and then releases it.
3. The dormant period. Holding position for the two to six hours between placing and the point where the cement paste has stiffened enough to support the void structure by itself. This is where most failures happen: the foam drains, the bubbles merge upward, and the cast settles, giving a density gradient from top to bottom and a hardened density well above target.

An agent that produces beautiful foam in a bucket and fails the third test is worthless. Foam stability in contact with cement paste, not foam volume, is the property that matters.

What separates good foam from bad foam

Three measurable characteristics describe foam quality.

Foam density

Measured by filling a vessel of known volume with foam and weighing it, usually expressed in grams per litre. Industrial pre-formed foam is normally produced at 40 to 80 g/L, with 45 to 60 g/L the common working band. Foam below about 35 g/L carries so little water that it starves the surrounding paste and tends to be fragile; foam above about 90 g/L is wet, drains quickly and adds unwanted water to the mix.

Bubble size and distribution

Fine, uniform, closed spherical voids in the 0.1 to 0.5 mm range give the best strength for a given density. Coarse or irregular voids concentrate stress and act as flaws. This is not a marginal effect: void-structure studies consistently find that at constant density, foam concrete with a narrower void-size distribution and fewer merged voids reaches noticeably higher compressive strength. Bubble size is a function of the agent, the generator design and the air pressure, in that order of importance.

Stability, expressed as drainage or half-life

Left standing, foam loses water from its lamellae to gravity. The usual laboratory measure is the time for half the liquid to drain out. Practically, a foam that will survive contact with cement paste, pumping and the dormant period needs a half-life comfortably longer than the interval between generation and stiffening, with margin for delays on site.

Protein hydrolysate agents

The traditional industrial foaming agents are protein hydrolysates: keratin-rich animal by-products — horn, hoof, hair, bristle, feather — or blood and bone meal, broken down by alkaline or acid hydrolysis into a mixture of polypeptides and free amino acids. The resulting solution is strongly surface-active, and the polypeptide chains form a comparatively thick, visco-elastic film at the air-water interface.

Why they perform well

• The interfacial film is mechanically strong, so bubbles resist coalescence under shear and under the pressure cycles of pumping.
• Bubble size is naturally fine and the distribution narrow.
• Drainage is slow, giving long working stability and good performance at low densities.
• For a given dry density, hardened strength is typically higher than with a synthetic agent, because the void structure is finer and more regular.

Why they are awkward to work with

• Odour. Hydrolysed animal protein smells, both in the drum and, faintly, in the fresh material. On enclosed sites and in occupied buildings this is a genuine constraint.
• Batch variability. Hydrolysis of a heterogeneous biological feedstock gives a distribution of molecular weights that shifts between batches. Two drums of nominally the same product can foam differently.
• Shelf life and storage. The solutions are biologically active. They degrade, they are sensitive to heat and to freezing, and typical usable life is months rather than years.
• Yield. Expansion ratios are lower than for synthetics, so more concentrate is consumed per cubic metre of foam.
• Sourcing. Animal-derived inputs raise supply-chain, religious-compliance and procurement questions that some clients will not accept.

Synthetic surfactant agents

The alternative is a formulated blend of synthetic surfactants — typically anionic types such as alkyl sulfates, alkyl ether sulfates and alkyl benzene sulfonates, often with amphoteric co-surfactants such as betaines, plus solvents and stabilising polymers.

Advantages

• Consistency. A defined chemical formulation gives essentially identical behaviour drum to drum and year to year.
• Shelf life. Usually a year or more, with wide temperature tolerance.
• High expansion. More litres of foam per litre of concentrate, so cost per cubic metre is lower.
• No odour, easy handling, and no animal-derived content.

Limitations

Surfactant films are thin and mobile. They stabilise a bubble effectively while it is being formed but resist coalescence less well over time. In practice this shows up as larger and less uniform voids, faster drainage, and a higher practical density floor. Synthetic agents are entirely reliable in the 600 to 1600 kg/m³ range that covers most block and screed production; below roughly 400 kg/m³ they need careful formulation and process control, and the very low densities are hard to hold consistently. Comparative testing has repeatedly found lower compressive strength at equal density for synthetic agents relative to protein agents, attributable to the coarser void structure rather than to any chemical effect on hydration.

Proteins of biotechnological origin

A third family sits between the two. These agents are protein-based, and so share the mechanically strong interfacial film that makes protein foams stable — but the proteins are produced by biotechnological routes, typically microbial fermentation, rather than obtained by hydrolysing animal by-products. The active constituents are described by their manufacturers as highly active proteins of biotechnological origin, explicitly not the product of protein hydrolysis, and the formulations are often referred to as enzyme-based or enzymatic foaming agents.

The claimed technical consequence of the different origin is a defined, reproducible molecular species instead of a hydrolysis product with a broad molecular-weight distribution. The properties claimed to follow are:

• A very fine and uniform bubble structure, finer than a typical hydrolysate foam.
• High stability and slow drainage, giving a long working window.
• Batch-to-batch consistency comparable to a synthetic agent.
• Neutral odour, since there is no animal by-product feedstock.
• Stable production of very low densities — down to about 75 kg/m³, which is at or below the practical limit of ordinary hydrolysate agents and well below what synthetic agents hold reliably.

The three families compared

Stabilisers and secondary additives

Foam stability can be reinforced from the paste side as well as the foam side. The usual interventions:

• Viscosity modifiers. Cellulose ethers, welan and xanthan gum thicken the pore solution, slowing drainage between bubbles. Effective and widely used, at the cost of a stiffer fresh mix.
• Fine mineral additions. Silica fume, fine fly ash and limestone powder increase paste cohesion and can park solid particles at the bubble interface, physically obstructing coalescence.
• Polymer latex. Improves the tensile behaviour of the thin paste films and reduces shrinkage cracking, at a significant cost premium.
• Accelerators. Shortening the dormant period is the most direct route to foam stability, because it shortens the time the foam has to survive. Calcium formate is common; chloride-based accelerators are ruled out wherever there is embedded steel.
• Fibres. Polypropylene and glass fibres control plastic and drying shrinkage cracking. They do not stabilise foam and, above about 0.5 % by volume, can interfere with the flow the material depends on. The 2020 Materials review by Amran and co-authors is the standard survey of what fibre reinforcement does and does not achieve here.

Dilution, dosage and testing

Foaming agents ship as concentrates and are diluted with water before entering the generator. Typical dilution is 1 part concentrate to 20 to 40 parts water by volume, though the manufacturer's figure always governs. That solution is then expanded with compressed air at roughly 15 to 25 times its volume to reach the target foam density.

Working through it for a concrete example: producing 1 m³ of 400 kg/m³ foam concrete needs on the order of 600 to 700 litres of foam. At 50 g/L that foam contains about 30 to 35 kg of foaming solution, which at 1:30 dilution is about 1 litre of concentrate. Roughly one litre of agent per cubic metre of low-density material is a serviceable first estimate for costing; the figure falls with increasing density. Consumption is tabulated across the density range on mix quantities by density, and the cost consequences under what drives foam concrete cost. The mix designer returns the concentrate figure for any foam density and dilution ratio you enter.

Standard test methods

• ASTM C869 — Standard Specification for Foaming Agents Used in Making Preformed Foam for Cellular Concrete. The acceptance specification an agent is bought against.
• ASTM C796 — Standard Test Method for Foaming Agents for Use in Producing Cellular Concrete Using Preformed Foam. The procedure by which conformity with C869 is demonstrated, covering foam and cellular-concrete properties.
• ASTM C495 — Compressive Strength of Lightweight Insulating Concrete, for the hardened material.

On a production line, the two checks that catch nearly everything are a foam density measurement at the generator outlet at the start of every shift, and a wet density measurement on every batch. Both take under a minute. See mix design and calculation for the arithmetic and standards for the wider framework.

Selecting an agent

A note on this address

This page sits at /en/lithofoam-additives/ because that address was in use on this domain when its previous owner, a foam concrete equipment manufacturer, published product pages here under the LithoFoam and LithoPore names. External references — including encyclopaedic citations — still point at it. Rather than break them, the address has been kept and now carries an independent, vendor-neutral treatment of the same subject: foaming agents for cellular lightweight concrete.

LithoFoam and LithoPore are trademarks of their respective owners and are used here only to identify the products they name. This site is not affiliated with, endorsed by or operated by Luca Industries International GmbH. See the history note for the full background.

Last reviewed: 7 September 2026