Admixtures are used to adjust and optimize the properties of concretes and mortars. They influence the cement hydration and the properties of the fresh and hardened concrete and mortar. Their active ingredients are inorganic and organic salts as well as polymers. Commercial products are usually formulations consisting of several components and auxiliary additives. Retarders and accelerators influence the setting and hardening behaviour of the cement. The rheological behaviour of mortars and concretes can be controlled by superplasticizers and stabilizers. Other admixtures are used e. g. to introduce air voids, as a sealant, for internal curing (superabsorbent polymers) or to improve adhesive strength (polymer dispersions).
Superplasticizers are nowadays commonly used in concrete technology to improve the workability of cementitious systems. On one hand, they enable the production of concretes with special flow properties like self-compacting concrete. On the other hand, superplasticizers allow concrete mix designs with very low water/cement ratios. Thus, high performance concretes with very high strength and durability can be achieved. Among the first generation superplasticizers are sulphonated naphthalene-formaldehyde condensates, sulphonated melamin-formaldehyde condensates and lignosulphonates; the new generation products with enhanced properties are based on comb-shaped polycarboxylate-based polymers (PCE).
Shrinkage is one of the main causes of cracking. Drying shrinkage and autogeneous shrinkage may be distinguished. In large sized building elements for instance cracking, originating from drying shrinkage may occur even month after installation. One modern method to limit the shrinkage capacity is the use of special chemical additives which are added upon mixing and reduce the surface tension of the pore solution and hence the microstructural shrinkage tension.
Superabsorbent Polymers (SAP) can be used as internal curing agent for high-performance concrete with low water to cement ratio. In addition, since the SAP form a system of empty pores in the hardened concrete, they are being considered as an alternative to air entrainment in concrete exposed to freeze-thaw damage.