Admixtures:

Admixtures are the substances, other than the cement, water and aggregate, which are used to incorporate or modify certain properties of concrete for certain particular jobs and are added to the batch immediately before or during the mixing of ingredients of concrete. Concrete is used for various reasons under different conditions. Thus, Ordinary Concrete might not serve the required qualities. In such cases, admixtures are used to modify the properties of Ordinary Concrete so as to make it suitable for any conditions. Admixtures are classified as follows:

Accelerators

Accelerators are the materials which increase the rate of development of strength of concrete and may be used where early strength of concrete is required. Calcium chloride reduces the time of initial and final setting. With2% of CaCl2 in cement, the setting time is reduced one-third of the normal time. 2% mixture results in 170% more strength after one day and 30% more strength after seven days. After 28 days raise in strength is 10%. Beyond 4% it has been found to be harmful.