Concrete Admixture

Concrete Admixture
Admixtures are materials other than cement, aggregate and water that are added to concrete either before or during its mixing to alter its properties, such as workability, curing temperature range, set time or colour.

Some admixtures have been in use for a very long time, such as calcium chloride to provide a cold-weather setting concrete.
Others are more recent and represent an area of expanding possibilities for increased performance.

The chemistry of concrete admixtures is a complex topic requiring in-depth knowledge and experience. Based on their functions, admixtures can be classified into the following five major categories:

• Retarding admixtures
• Accelerating admixtures
• Super plasticizers
• Water reducing admixtures
• Air-entraining admixtures
Retarding admixtures:

- Retarding admixtures slow down the hydration of cement, lengthening set time. Retarders are beneficially used in hot weather conditions in order to overcome accelerating effects of higher temperatures and large masses of concrete on concrete setting time. Because most retarders also act as water reducers, they are frequently called water-reducing retarders

Accelerating admixtures:

- Accelerators shorten the set time of concrete, allowing a cold-weather pour, early removal of forms, early surface finishing, and in some cases, early load application. Proper care must be taken while choosing the type and proportion of accelerators, as under most conditions, commonly used accelerators cause an increase in the drying shrinkage of concrete.

Super plasticizers:

- Super plasticizers, also known as plasticizers, include water-reducing admixtures. Compared to what is commonly referred to as a "water reducer" or "mid-range water reducer", super plasticizers are "high-range water reducers". High range water reducers are admixtures that allow large water reduction or greater flowability (as defined by the manufacturers, concrete suppliers and industry standards) without substantially slowing set time or increasing air entrainment

Water reducing admixtures:

- Water reducing admixtures require less water to make a concrete of equal slump, or increase the slump of concrete at the same water content. They can have the side effect of changing initial set time. Water reducers are mostly used for hot weather concrete placing and to aid pumping. A water-reducer plasticizer, however, is a hygroscopic powder, which can entrain air into the concrete mix via its effect on water's surface tension, thereby also, obtaining some of the benefits of air-entrainment (see below).


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