What is the primary function of calcium chloride when used as an accelerating admixture?

The primary function of calcium chloride (CaCl2) when used as an accelerating admixture is to accelerate the hydration of cement, leading to faster setting and early strength development of concrete. Calcium chloride increases the rate at which the cement compounds react with water, speeding up the formation of calcium silicate hydrate (C-S-H) gel, which is responsible for concrete's strength. This acceleration is particularly beneficial in cold weather, where low temperatures slow down the hydration process. By accelerating the setting time, calcium chloride reduces the risk of the concrete freezing before it gains sufficient strength to resist damage. It also allows for earlier removal of forms and faster construction progress. However, it's crucial to note that calcium chloride can increase the risk of corrosion of reinforcing steel, especially in chloride-rich environments. Therefore, its use is often restricted or prohibited in reinforced concrete structures, particularly those exposed to chlorides from deicing salts or marine environments. Non-chloride accelerators are often preferred in these situations. For example, in precast concrete operations where rapid strength gain is desired to increase production turnover, calcium chloride can be used, provided the potential for corrosion is carefully considered and mitigated where possible.