Definition: a material other than water, aggregates, cementitious materials, and fiber reinforcement, used as an ingredient of a cementitious mixture to modify its freshly mixed, setting, or hardened properties and that is added to the batch before or during its mixing.
Admixtures are materials added to concrete before or during mixing. Most admixtures are chemicals that change some property of the fresh or hardened concrete. Water, aggregates, cementitious materials (pozzolans and hydraulic cements [fly ash, silica fume, slag cement]), and fiber reinforcement are not considered admixtures.
Types of chemical admixtures include: air-entraining admixtures; accelerating admixtures; retarding admixtures; water-reducing admixtures (normal and mid-range); high-range water-reducing admixtures; and special-purpose admixtures such as pigments, corrosion inhibitors, shrinkage reducers, admixtures for producing flowing concrete, cold-weather admixtures, and pumping aids.