Accelerators - Admixtures of concrete

Also see admixtures

Accelerating Admixture - an admixture that causes an increase in the rate of hydration of the hydraulic cement and thus shortens the time of setting, increases the rate of strength development, or both.

Accelerating admixtures are added to concrete either to increase the rate of early strength development or to shorten the time of setting, or both. Chemical compositions of accelerators include some of inorganic compounds such as soluble chlorides, carbonates, silicates, fluosilicates, and some organic compounds such as triethanolamine.

Calcium chloride is a common accelerator, used to accelerate the time of set and the rate of strength gain. It should meet the requirements of ASTM D 98. Excessive amounts of calcium chloride in concrete mix may result in rapid stiffening, increase in drying shrinkage and corrosion of reinforcement. In colder climates, calcium chloride should not be used as an anti-freeze. Large amount of calcium chloride is required to lower the freezing point of the concrete, which may ruin the concrete.

Accelerators counteract the influence of cold weather, which slows down the curing and setting process.

A contractor can use one anytime a curing process needs speed. The admixture may allow a concrete worker to remove forms earlier, get onto a concrete surface earlier for finishing, and sometimes even put loads on it earlier, such as when diverting foot traffic to do patching. The accelerators suitable for reinforced concrete one can find are:

1. sodium thiocyanates and other thiocyanate salts
2. triethanolamine and other alkanolamines
3. products based on sulfates, nitrates and formates