Admixture is defined as a material other than the basic ingredients of concrete, that is , cement, water and aggregates , added to the concrete mix immediately before or during mixing to modify one or more properties, which otherwise could not be achieved economically, but at the same time they could not adversely affect any property of concrete.i.e

  1. Rate of hydration
  2. Setting time
  3. Workability
  4. Dispersion
  5. Air Entrainment
  6. Strength
  7. Segregation
  8. Durability

The admixture generally added is reactively in small quantity.A degree of control must be exercised to ensure the proper quantity of the admixture, as an excess quantity may be detrimental to the properties of concrete.

Additive , on the other hand is a material , which is added at the time of grinding cement clinker at the cement factory.

Function of Admixtures

Following are the important purposes for which admixtures are added,

  1. To accelerate the initial set of concrete.
  2. To retard the initial set of concrete.
  3. To increase the strength of concrete.
  4. To improves workability.
  5. To reduce heat evaluation due to hydration of cement.
  6. To increase durability of concrete.
  7. To control alkali aggregate expansion.
  8. To increase impermeability of concrete to liquids and also to decrease the capillary flow of water through concrete.
  9. To improve penetration and pumpatrility of concrete.
  10. To reduce segregation in grouting mixture.
  11. To increase bond between old and new surface.
  12. To inhibit corrosion to concrete.
  13. To increase to resistance chemical attack.
  14. To produce colored concrete.
  15. To produce light weight concrete.