An admixture used to speed up the initial set of concrete is called an accelerator. These admixtures are added to concrete either to increase the rate of hydration of hydraulic cement and thereby early strength development or to shorten the setting time.
An increase in the rate of early strength development may help in: a)Earlier removal of forms. b) Reduction in curing period.
Accelerating admixtures are also used advantageously when the concrete is to be placed at low temperatures, underwater concreting, basement waterproofing operations.
The general action of accelerators is to cause a more rapid dissolution of compounds of cement particularly tricalcium silicate in water and thereby facilitate more rapid hydration of these compounds.
The most widely used accelerator is calcium chloride (CaCI) The other less commonly used accelerators consist of NaCI, Na2SO4, NaOH, KOH etc.
The admixtures slow down the initial rate of hydration of cement or prolong the setting of cement paste in concrete. They are generally used to offset the accelerating and damaging effect of high temperature and to keep the concrete workable during the entire placing period which is sufficiently long. They are also used for grouting oil wells. Retarders arc also added in the concrete which has to be hauled long distances in transit mix trucks to keep the concrete workable. Retarders delay setting of cement either by forming a thin coating on the cement particles and thus slowing down their dissolution in water or by increasing the intra-molecular distance of reacting silicates and aluminates from water molecules some of the retarding admixtures also reduce the water requirement of mixture. Examples of retarders are Sugar. Soluble Starch etc.
Air-entraining admixtures help to incorporate a controlled amount of air in the form of non-coalescing bubbles distributed throughout the body of the concrete. The entrained air -improves certain properties of plastic concrete like workability, increased durability, better resistance to frost -action and reduction in bleeding and segregation. The commonly used air-entraining admixtures are Natural wood resin, Olive oil etc. Read Also: Water Reducing admixture /Plasticizers, Action of Plasticizer