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Accelerators increase the initial rate of chemical reaction between the cement and the water so that the concrete stiffens, hardens, and develops strength more quickly. They have a negligible effect on consistence, and 28-day strengths are seldom affected. Accelerating admixtures have been used mainly during cold weather when the slowing down of the chemical reaction between cement and water due to low temperature could be offset by the increased speed of reaction resulting from the use of the accelerator. The most widely used accelerator for some time was calcium chloride but, because the presence of chlorides, even in small accounts, increases the risk of corrosion, the use of admixtures containing chlorides is now prohibited in all concrete containing embedded metal.
Accelerators are sometimes marketed under other names such as hardeners, anti-freezes on frost-proofers, but not accelerator is a true anti-freeze and the use of an accelerator does not avoid the need to protect the concrete from the cold by keeping it warm (with insulation) after it has been placed.
It is to be noted that accelerators are in-effective in mortars because the thickness of mortar, either in a joint or on a rendering is such that any heat generated by the faster reaction is quickly dissipated. 