Admixtures are any chemical additive to the concrete mixture that enhances the properties of concrete in the fresh or hardened state. ACI 116R  defines the term admixture as ‘a material other than water, aggregates, hydraulic cement, and fiber reinforcement, used as an ingredient of concrete or mortar, and added to the batch immediately before or during its mixing’.
In normal use, the admixtures make up less than 5% of the cement weight, and are added to the concrete at the time of batching / mixing.
The most common types of admixtures are:
– Accelerators speed up the hydration (“hardening”) of the concrete.
– Retarders slow the hydration of concrete.
– Air-entertainers add and distribute tiny air bubbles in the concrete.
Plasticizers can be used to increase the “workability” of concrete, allowing it be placed more easily, with less compactive effort. Super plasticizers allow a properly designed concrete to flow in place even around congested reinforcing bars. Alternatively, they can be used to reduce the water content of a concrete (water reducers) while maintaining workability. This improves its strength and durability characteristics