Q. What is Admixtures?

These are the substance which are added to the concrete to improve its qualities either its placing or in its subsequent performance. An admixture may be added to the mixing water to the batch before or during mixing. If its presence in concrete effects the properties of the concrete it may be intergrout with the clinker during the manufacture of cement. Gypsum is not an admixture as it is used in the normal manufacture of cement.

Admixtures are not generally recommended they may be used if they do not effect the durability and compressive strength by more than 10%.
Admixtures are added to improve the workability reduction of heat evolution and bleeding increasing setting time and prevention of shrinkage.
Admixtures can be divided into following classes according to the characteristic effects produced by them -

(1) Workability agent.

(2) Retarders.

(3) Accelerators.

(4) Water repelling agents.

(5) Air entraining agents.

(6) Gas forming agents.

(7) Pozzolanic materials.

Fly ash surkhi ground with cement carbohydrate salts, Heat treated raw shales or clay, Fine sand, Hydrated lime , Talc pulverized stone, Calcium chloride, natural cements, Blast furnace slag etc are the commonly used as admixtures.