Accelerating admixtures can be used to increase either the rate of stiffening or setting of the concrete or the rate of hardening and early strength gain to allow earlier formwork striking and demoulding. Most accelerators achieve one rather than both of these functions.
Hardening accelerators are most effective at low temperature. Accelerators can be used with superplasticisers where early age strength is required, especially at lower temperatures.
In the UK the prime use of set accelerators is to control the setting time of floor slabs in cold weather as extended setting time delays finishing and power-trowelling. They are also used to reduce the risk of damage to concrete by freezing when concreting in cold weather and to allow the earlier removal of formwork.
Concrete containing an accelerator will set faster than the equivalent plain concrete. The reduction in setting time is typically 1–2 hours but is affected by accelerator type, dose and temperature.
Accelerators containing calcium chloride are restricted to unreinforced concrete due to the increaed risk of reinforcement corrosion. Accelerators based on calcium formate have been used in the UK with no reported harmful effects since the mid-1960s and thiocyanates since the 1970s.