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Admixtures are materials added to concrete, mortar, or grout other than conventional constituents of cement, water, and aggregates. Their primary purpose is to modify fresh and hardened properties of the composite to meet specific performance requirements, such as workability, setting behavior, early or ultimate strength, durability, or permeability.

Admixtures are broadly categorized into chemical and mineral types. Chemical admixtures are added in small percentages and include water-reducing or plasticizing agents (superplasticizers) that improve flow without increasing water content; air-entraining agents that improve freeze–thaw resistance; set-retarding and accelerating agents that control curing time; viscosity-modifying agents that stabilize highly flowable mixes; corrosion inhibitors that protect reinforcement. Mineral admixtures include fly ash, ground granulated blast-furnace slag, silica fume, natural pozzolans, which refine the pore structure and can enhance durability and long-term strength.

Applications include enabling pumping and placing of concrete, reducing temperature rise, controlling early strength, improving durability in aggressive environments, and enhancing bond with existing substrates or repairs. Dosage and selection depend on cement type, aggregate, exposure conditions, and performance targets, and must be tested through standard workability and durability tests.

Standards and guidance come from organizations such as ASTM International (for example ASTM C494/C1017), European EN 934-2, and other national standards. Admixtures should be compatible with cement and other mix components and should not contain chlorides in reinforced concrete unless explicitly approved. Safety data sheets provide handling guidance, and manufacturers provide compatibility charts.