Foam control agents are processing aids added to cementitious systems to prevent or eliminate unwanted air bubbles in mortar, render and screed. Air entrapment during mixing and pouring reduces material density and strength, compromises surface finish and creates voids in hardened concrete. The function of these additives is to destabilise foam rapidly so that entrapped gas can escape before the material sets.
Terminology: defoamers, antifoaming agents and air-release agents
The industry uses three terms, often interchangeably, but each describes a distinct mechanism. Defoamers are added to reduce or eliminate foam after it has formed. Antifoaming agents prevent the appearance or emergence of foam when added to a formulation before mixing. Air-release agents remove entrained gas from solution and raise it to the surface. Many formulations exhibit overlapping properties, functioning both as preventive antifoaming agents and as active defoamers. Similarly, certain defoamers affect entrained air, while some air-release agents initiate destabilisation of surface foam. For simplicity, the term "defoamer" is generically used across the industry when referencing any processing aid designed to control foam.
How foam forms and stabilises
Foam occurs when gas is introduced into and stabilised within a liquid. In pure liquids such as water, foam is thermodynamically unstable and any bubbles rapidly rise to the surface and burst. Stabilisation is only possible in solutions containing additional surface-active components that create a surface tension gradient at the gas-liquid interface, known as the foam lamella, foam film or bubble wall. This allows for much greater foam stability as air is introduced. Gravity inherently drains liquid from foam, forming a "dry" polyhedral foam beginning from the top down. Gas naturally diffuses from smaller, higher-pressure bubbles to larger bubbles. However, as the foam film stretches, the concentration of surface-active ingredients decreases, causing a local increase in surface tension that impedes coalescence and stabilises the foam, known as the Gibbs-Marangoni effect.
Mode of action: compatibility, penetration and spreading
An effective defoamer must fulfil several conditions. First, it must be insoluble in the medium being defoamed. Second, it needs a positive penetration coefficient so it can penetrate the foam lamella. Third, it requires a positive spreading coefficient to spread at the interface after penetrating the lamella. By spreading at the interface, the defoamer displaces foam-stabilising surfactants and replaces the elastic lamella with a film of lower cohesive forces, causing the bubble to collapse.
A key design requirement is partial solubility—a controlled balance between compatibility and incompatibility. A defoamer that is too dispersible or soluble will not destroy foam and may even contribute to additional foam build-up. If it is too insoluble or insufficiently dispersible, it can cling to equipment, contribute to surface defects or cause cratering. Each defoamer formulation must be intentionally designed to achieve the proper level of compatibility. Too much compatibility causes a foam-stabilising rather than a defoaming effect; too much incompatibility produces turbidity or craters. The optimum level is found when good defoaming is achieved without defects.
Chemistry: carriers, active particles and emulsification
Defoamers are typically formulated around a carrier fluid, which may be a hydrocarbon, silicone oil, vegetable oil or synthetic polymer. Active ingredients—including waxes, metal salts, silicas or other hydrophobic particles—help with spreading and destabilising foam. An emulsification system can be incorporated to optimise spreading and compatibility with the foaming system. Depending on the chemistry, stabilising agents such as cellulosic or acrylic thickeners may be added to improve shelf stability.
Silicone and organo-silicone defoamers are highly efficient in many applications. Beyond silicas, two proprietary particle technologies have been developed: polyurea and polyamide particles. Polyurea particles offer a better defoaming effect and two further benefits. First, polyurea is synthesised from liquid reactants in situ in the carrier oil, which achieves substantially smaller particles and better resistance to settling, improving storage stability. Second, the adsorption capacity for surfactants is greater due to the larger specific surface area, ensuring optimum defoaming even if the coating or construction chemical is stored for an extended period. Polyamide-based technologies are beneficial in applications with food contact and may achieve greater efficiency in certain formulations.
Application in concrete and mortar systems
During mixing and pouring of concrete, air inevitably becomes entrapped, forming bubbles. If these remain within hardened concrete, they create voids that reduce density and strength. Air bubbles also compromise surface aesthetics, particularly in decorative applications requiring a smooth finish. The function of a defoamer is to disrupt the stability of these bubbles, causing them to collapse or escape rapidly, thereby enhancing overall performance.
Defoamers are developed using polyether and other components suitable for cement-based material systems under strong alkaline conditions. At pH 14, they do not break emulsion or float oil, and maintain good defoaming and foam-suppression effects. They eliminate harmful bubbles, stabilise beneficial bubbles and are non-toxic, odourless and will not cause pollution to the environment. Dosage ranges typically fall between 0.02 and 0.3 per cent of the formulation, or 0.10 to 0.30 kg per 100 kg of cement or binder, depending on the product and application. Defoamers are used to manufacture architectural-quality concrete satisfying the requirement for a largely void-free concrete surface. They are particularly suitable for self-compacting concrete, flowing concrete, mass concrete and precast concrete.
Interaction with air-entraining agents
When using air-entraining agents and defoamers together, certain principles must be followed. First, the purpose of use must be clearly defined to ensure that simultaneous application balances their effects. This guarantees adequate air content to enhance freezing resistance and water resistance while eliminating excess air bubbles to improve strength and surface quality. Second, products should be selected appropriately, choosing air-entraining agents and defoamers with good compatibility and considering their performance characteristics. Finally, the appropriate dosage ratio must be determined through experimentation to ensure both function stably and effectively within the concrete system. Air-entraining agent dosage should be moderate to avoid introducing excessive air bubbles, which would increase the burden on the defoamer. Simultaneously, defoamer dosage must be controlled within reasonable limits.
