Chemical Admixtures for Concrete

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Chemical Admixtures

Admixtures for concrete, mortar or paste are inorganic or organic materials
These are added to the normal components of a mix not normally exceeding 5% by mass of cement or cementitious materials
Admixtures interact with hydrating cement by physical, chemical or physicochemical actions.
They modify one or more properties of concrete, mortar or paste either in fresh or hardened state.
Admixtures are usually very complex compounds chemically excepting a few.
They are unlikely to be pure compounds but associated with some other minor chemical compounds or they may be mixed formulations.
It is rather difficult for a civil engineer to understand them through their chemical nature.
Their effect on cement concrete, mortar or paste can be assessed by (the usual) simple tests on cement, mortar and concrete.
In most advanced countries, admixtures have become as essential ingredient of concrete as cement, aggregate and water themselves.

If we are pouring a low grade concrete, say M20, If we are not unduly concerned about its water content nor its water-cement ratio,
If the cost of materials and labour are paid entirely by the owner (or some one else) at whatever rates we have quoted, then, Admixtures will make the concrete more expensive. But then our concrete will be an indifferent one,
Its durability, water tightness and consistent strength will be suspect.
If we have a problem later on, there is little we can do to rectify it.
But, in many other situations we cannot get away with such concrete or such an attitude.
We may have to observe strictly a low w/c ratio as per design requirement
We may have to transport concrete over large distances or to great heights
We may need high flow ability either due to reinforcement congestion or narrowness of sections or the inability to use vibrators

All major bridges in India require admixtures for their concrete.
Cost of admixtures is often compensated by savings in labour costs, placing costs and/ or cement costs.

For our country, the most common chemical admixtures are plasticizers and Super-plasticizers. These are also known as water reducers and high-range water reducers, respectively.
Retarders are likely to be needed, especially for hot weather concreting, ready mixed concrete or for special continuous pours

The chemical, physical or physico chemical actions of admixtures in cement concrete are quite complex.
In fact, cement itself is an extremely complex compound with major compounds such as calcium silicates, calcium aluminates, gypsum.
Besides it contains many alkali and other calcium salts.
The action of admixtures can, however, be simplified for the sake of Understanding, as:

(i) adsorption

(ii) De-flocculation or dispersion

(iii)Chemical absorption or interaction

Often, all the three take place. We should know a little about these so that we can choose admixtures for our job more correctly