A defoamer or an anti-foaming agent is a surfactant that suppresses or eliminates foam, with good chemical stability. It should have lower surface tension than the medium to be defoamed, a certain compatibility with the medium to be defoamed, and good dispersion. An effective defoamer can not only quickly break foams, but also can prevent the regeneration of air bubbles over a fairly long time. The function of anti-foaming agent is opposite to that of air-entraining agent. The action mechanism of anti-foaming agent can be divided into the foam-breaking effect and the foam-suppressing effect. The foam-breaking effect means breaking the conditions for stable existence of foam, making the stably existing air bubbles turn into unstable air bubbles, which will then become still larger and precipitated, as well as breaking foams already formed. The foam-suppressing effect means not only breaking foams already formed, but also preventing the formation of foam over a longer time.

Since cellulose ethers, redispersible latex powder and air-entraining agents are added into dry-mixed mortar, a certain amount of air bubbles will be brought in the mortar; in addition, the dry powder material stirred with water will also produce air bubbles. This has affected the compressive, flexural and bond strengths of mortar, reduced the elastic modulus and had a certain effect on the mortar surface. Some dry-mixed mortar products have higher requirements for the exterior appearance, such as self-leveling mortar, which usually requires its surface to be smooth. However, during the construction of self-leveling mortar, the pores formed on the surface will affect the surface quality and aesthetics of final product, so anti-foaming agents should then be used to eliminate the pores on the surface. Taking waterproof mortar as another example, the air bubbles formed will affect the impermeability of mortar, and so on. Therefore, in some dry-mixed mortar, anti-foaming agents can be used to to eliminate the air bubbles brought in the mortar, making the mortar surface smooth, improving the impermeability of mortar and increasing the strength.

There are many types of anti-foaming agents, such as organic silicon, polyethers, fatty acids, phosphoric acids and etc., but each kind of anti-foaming agent has its own adaptability respectively. Dry-mixed mortar is a kind of strong alkaline environment, so the powdery anti-foaming agent suitable for the alkaline medium should be selected.