Admixtures which accelerate the set by decreasing the time when the mix changes from the plastic to the rigid state and or which accelerate the hardening by increasing the early strength gain.
Accelerators are used to increase the initial rate and speed of the chemical reaction between cement and the mixing water. This results in a faster stiffening of the concrete (set accelerating admixture) or faster hardening and strength development (hardening accelerating admixture) or both.
Due to the increased early age strength development hardening accelerating admixture can be used to reduce the period during which the fresh concrete is sensitive against frost damage. Hardening accelerating admixture are used to reduce demoulding time or to speed access onto concrete floors and pavements and by this speed up the construction process.
Because of the corrosion promoting effect of chlorides to the reinforcement “chloride free” accelerators has been developed for the use in reinforced concrete.