Chemical Admixtures for Concrete

Chemical admixtures are used to improve workability and quality of concrete during mixing, transporting, application, placement, curing and setting time. 
Chemical admixtures are the admixtures that are added to concrete in a very small amount for a specific function to concrete. If chemical admixtures are added more than the defined than it has a very wide range of negative effects on the properties of fresh as well as hardened concrete. 

chemical admixtures are the extra ingredients other than water, cement, and aggregates. These are added to the concrete batch plant during batch mixing or at the start when other quantities are added. Admixtures offer very favorable effects to the properties of fresh or hardened concrete only if proper use of admixtures is made possible.

ASTM C494 specifies the requirements for chemical admixture types.

Type A: Water-reducing admixtures
Type B: Retarding admixtures
Type C: Accelerating admixtures
Type D: Water-reducing and retarding  admixtures
Type E: Water-reducing and accelerating admixtures
Type F: Water-reducing, high range admixtures
Type G: Water-reducing, high range, and retarding admixtures
Type S: Specific performance (  corrosion inhibitors, shrinkage control, alkali-silica reactivity inhibitors, and coloring ) admixtures.

The materials used in the concrete mixtures shall include Type I or Type II cement, pozzolan, fine and coarse aggregates, and air-entraining admixture.
Types F and G admixtures may exhibit much higher water reduction in concrete mixtures having higher cement factors.
Mixtures having a high range water reduction generally display a higher rate of slump loss.
Admixtures that contain relatively large amounts of chloride may accelerate corrosion of prestressing steel.