Admixtures are natural or manufactured chemicals added to the concrete before or during mixing. The most often used chemical admixtures are air- entraining agents, water reducers, water-reducing retarders, and accelerators. Admixtures are used to give special properties to fresh or hardened concrete.

Successful use of admixtures depends on the use of appropriate methods of batching and concreting. Most admixtures are supplied in ready-to-use liquid form and are added to the concrete at the plant or at the jobsite. Certain admixtures, such as pigments, expansive agents, and pumping aids are used only in extremely small amounts and are usually batched by hand from premeasured containers.

The effectiveness of an admixture depends on several factors including: type and amount of cement, water content, mixing time, slump, and temperatures of the concrete and air. Sometimes, effects similar to those achieved through the addition of admixtures can be achieved by altering the concrete mixture-reducing the water-cement ratio, adding additional cement, using a different type of cement, or changing the aggregate and aggregate gradation.

Admixtures are used to modify the properties of concrete or mortar to make them more suitable for the work at hand or for economy or for such other purposes as saving energy.