Admixture and Surfactant

An admixture is defined as “a material other than water, aggregates, cement and fiber reinforcement that is used as an ingredient of a cement mixture to modify its freshly mixed, setting, or hardened properties and that is added to the batch before or during its mixing,” and a chemical admixture is defined as “an admixture in the form of a liquid, suspension, or water-soluble solid”.
Many chemical admixtures, such as air-entraining, water-reducing, and shrinkage-reducing, belong to a class of chemicals called surfactants. 
Surfactants have an amphiphilic molecular structure. An amphiphilic is a chemical compound possessing both hydrophilic (water-loving, polar) and hydrophobic (fat-loving) properties. The surfactant hydrophilic area of the molecule penetrates into the polar liquid and the surfactant hydrophobic part into the gas phase. Consequently, the surfactant lowers the surface tension and stabilize air in the form of foam.
Surfactants are compounds that lower the surface tension (or interfacial tension) between two liquids, between a gas and a liquid, or between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.
Most commonly, surfactants are classified according to polar head group. A non-ionic surfactant has no charged groups in its head. The head of an ionic surfactant carries a net positive, or negative charge. If the charge is negative, the surfactant is more specifically called anionic; if the charge is positive, it is called cationic. If a surfactant contains a head with two oppositely charged groups, it is termed zwitterionic.