Admixtures may be defined as the materials other than the basic ingredients of concrete i.e., cement, aggregates and water added to the concrete mix immediately before or during the mixing process to modify one or more specific properties of concrete in fresh or hardened state. Whereas an additive is a material which is added at the time of grinding cements clinker in the cement factory.

The use of admixture should offer improvement in the properties of concrete by adjusting the propor­tions of cement and aggregates. However it should not affect adversely any property of concrete. An admixture should be used only after assessing its effects on the concrete to be used under an intended situa­tion. It should be kept in mind that admixtures are no substitute for good workmanship i.e. the effects of bad workmanship cannot be improved by the use of admixtures.

History of Admixtures:

The history of admixtures goes with the history of cement concrete, i.e., it is as old as that of concrete. But a few types of admixtures called as plasticizers and super plasticizer have been developed recently. They are developed in Japan and Germany around 1970. Later on they found use in U.S.A., Europe, Middle East and Far East etc. However in India they became popular around 1985. 

Functions of Admixtures:

Admixtures are used for the following purposes:


1. To accelerate the initial setting and hardening of concrete.

2. To retard the initial setting of concrete.

3. To increase the strength of concrete.

4. To improve the workability of fresh concrete.


5. To improve the durability of concrete.

6. To reduce the heat of evaluation.

7. To control the alkali aggregate expansion.

8. To promote pozzolanic properties in concrete.


9. To aid in curing of concrete.

10. To improve wear resistance to concrete.

11. To reduce shrinkage during setting of concrete.

12. To reduce bleeding of concrete.


13. To impart colour to concrete.

14. To aerate concrete or mortar to produce light weight concrete.

15. To reduce segregation in grout mixes.

16. To increase the bond between old and new concrete surfaces and also between concrete and rein­forcement.


17. To produce fungal, insects etc. resistant concrete.

18. To produce nonskid surface of concrete.

19. To increase the resistance to chemical attack.

20. To decrease the permeability of concrete.

21. To check the corrosion of concrete reinforcement.

Many admixtures have been found useful and effective in accomplishing their intended use but some of them are not found as useful as advocated.

While taking a decision regarding the use of a substance as admixture following points should be kept in mind:

1. The possibility of achieving the desired results by a small modification in the basic mix.

2. Justification of additional cost of admixture against benefits obtained by the modified basic mix.

3. Possible adverse effects on properties other than that which the admixture is intended to improve.

Some admixtures serve more than one purpose. Calcium chloride is one of them. Due to its hygro­scopic properties calcium chloride under some conditions improves the workability, acts as an anti-freezing agent by accelerating setting and hardening of concrete and also tends to aid in curing. Whereas some admixtures, specially finely powdered substances, though improve the workability of concrete, but on the other hand increase drying shrinkage.

Thus an admixture should be judged by its overall effects upon the serviceability of concrete. Further the number of admixtures proposed for use in concrete is quite large and the popularity of a particular admixture changes with time. Therefore the selection of a particular admixture should be made after due consideration.