Q.: I've been having an argument. My friend says calcium chloride is added to concrete as an antifreeze. I say it's added to help the concrete gain strength faster so it can have a better chance to survive if it freezes the first night. Am I right?
A.: You're closer to the truth than your friend. It's true that calcium chloride depresses the freezing point of water, but the amount of calcium chloride that can be used to accelerate concrete will not depress the freezing point enough to notice. Hence, that's not the reason for adding calcium chloride.
It's often said that the purpose of adding calcium chloride in cold weather is to help concrete achieve a strength of 500 psi before it freezes, and this is a good rule of thumb. However, 500 psi compressive strength represents a very small tensile strength, one that would be inadequate to resist the large internal forces generated when water freezes. Hence, adequate strength is related to the reason for adding calcium chloride but it's not the real reason.
When portland cement hydrates, some of the mixing water gets used up by combining chemically with portland cement. By the time the concrete has achieved a strength of around 500 psi, enough mixing water has combined with the portland cement so that the mixing water fills only 90 percent or less of the pores. That amount leaves enough space available for the water to expand in freezing without damaging the concrete. Hence, the calcium chloride accelerates the reduction in water in the cement paste to a level where an early freeze won't completely disrupt the concrete. The same general principle applies to other accelerators used for cold weather concreting.