What is the function of calcium chloride in hot weather concreting?

thanks for the invitation Several chemicals are known to act as accelerators for concrete — calcium formate, aluminum chloride, potassium carbonate, sodium chloride and calcium chloride (among others) — but calcium chloride is most widely used. Its popularity is due to its ready availability, low cost, predictable performance characteristics, and successful application over several decades.

Regardless of the temperature or cement type, concrete mixes containing  Calcium chloride  will always have a faster cure rate than plain concrete.

 

Calcium chloride offers many advantages that make it popular as a concrete accelerator. It causes a substantial increase in early strengths and it speeds rate of set. It is readily available. It dissolves easily in water. Its behavior with most ingredients of concrete and under a wide variety of conditions has been studied and reported over a long span of years, and its effects have been generally understood though in recent years increasing evidence has accumulated that calcium chloride or other source of chloride ions can cause serious problems in concrete.  It is advisable to use Calcium Chloride in Cold Weather Concreting but not in Hot Weather Concreting.Many properties of concrete are affected to various degrees by calcium chloride: the heat of hydration at 1 day increases 30 percent; tensile strength at 28 days is slightly decreased; flexural strength at 7 days is increased by 10 percent; flexural strength at 28 days is a 0 to 15 decrease; and volume change shows a 0 to 15 increase. We now know that chloride greatly increases the likelihood of corrosion. We also know that, under conditions where moisture penetrates to the steel, only the chemical basicity of portland cement pasts prevents corrosion. Unless large amount of chloride are present, the basicity factor usually wins out, at least during the relatively short haul of 20 to 30 years. However, concrete slowly carbonates upon exposure to air and this carbonation destroy the basicity of concrete. The prognosis is bleak when that basicity is lowered and water and oxygen penetrate to the level of the steel. The results can be disastrous if chlorides are present. One factor that has not been widely publicized but merits major emphasis is that the chloride concentration often varies in different portions of a reinforced concrete structure. An example of such an affect is the cast-in-place reinforced columns of a multistory building. After three years of service cracking over and in line with reinforcing steel was permanent in only the chloride-containing portions of the columns. Cover over the steel in several instances was 1 inch and in other instances 3 and one-half inches. The reason was differential chloride content.

Calcium chloride significantly reduces both IST and FST. It's particularly useful for concreting operations carried out at low temperatures. It should not be used in hot weather conditions as setting of concrete may occur so rapidly, that placing and finishing may become very difficult !!