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Anything added in concrete other then coarse aggregate ,fine aggregate,binding material and water is  referred as admixture.Admixtures are added in order to modify the properties of the concrete .
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A> Plasticizers
These are organic or inorganic substances that allows the reduction in water cement ratio at the same workability or offer higher workability at same water cement ratio.In either of the cases either strong or workable concrete is obtained.This plasticizers acts as a deflocculating reagent hence gets adsorbed over the cement particles these by increasing the force of repulsion in them.Making the entrapped water free which modifies the properties of concrete.There normal dose varies in the range of .1 to .4 % by the weight of cement at which they permit the reduction in the water content by 5 to 15% or increase the slump value by 30 to 150 mm.Commonly use of plasticizers  are cabohydrate,polyglycol esters,Lignusulphates ,Hydroxylated ,carboxylic acid.

B>  Superplasticizers
superplasticizer are  same as that of plasticizer in the terms of the reaction but are chemically different from them effectiveness of superplasticizer is comparatively than that of the plasticizer as they can permit the reduction in the water content up to 30% commonly used superplasticizer includes modified lignosulfonate sulphonated mline formaldehyde, sulphonated naphthalene, formaldehyde.

C> Retarders
these are the type of admixture which slow down the chemical process of hydration so that concrete can remain plastic and workable  for more duration.these admixture are used to overcome the accelerating  effect of temperature over the concrete where normal dose in the range of .05 to.2% by the weight  of cement at which they can delay the hydration process up to 72 hour admixtures find their application in the concreting of oil well. commonly used retarder includes calcium sulphate, turmeric acid ,sugar, starch, cellulose

D> Accelerator
These are  the type of admixtures which increase the rate of gain of  strength in concrete.
 These admixture find their application in:
1> cold weather concreting 
2>prefabricated construction
3>emergency repair
The normal dose very strong in the range of .1 to.2 %. Commonly used accelerator includes calcium chloride silicate fluorosilicate and methanol amine 

>Air entraining admixtures
These are the type of  admixture which entrops millions of the air bubbles in the voids of the mixture. Hence  modify the properties of concrete with respect to  workability, durability, freezing, thawing segregation and bleeding .

Use of the admixtures result in
1>Higher resistance against freezing and thawing 
2>Higher workability
3> Higher durability 
4> Higher water tightness
5>Higher resistance against the action of acid.
Commonly used Air entraining admixture includes plant and animal fatty acid natural wood resin, stearyl acid ,aluminium powder and hydrogen peroxide.

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