Calcium Chloride

Concrete finishers like to use calcium as an accelerator when the temperature is cool to speed up setting. Word is that calcium weaken concrete. Is this true and if so by what extent. The use of calcium at times just can not be avoided. I would appreciate an answer. Canada

First, a correction - the material is Calcium Chloride. Calcium alone cannot be found as a mineral admixture. Calcium Chloride does not weaken the concrete when used as an additive to cement in quantities of 1-2% by weight. (1 to 2 pounds for 100 pounds of cement). It actually will result in significantly higher strength in the first 7 days or so. By 28 days, the strength is comparable to the same concrete without the admixture. The concrete's resistance to abrasion and erosion will improve long term, while its tendencies to shrink or creep will increase. The big problem with Calcium Chloride is its effect on steel rebars in concrete. Usually, concrete protects these bars (or cables, bolts, ...) from rusting because of its alkalinity (high pH). The admixture reduces the alkalinity and may accelerate corrosion. It does not cause corrosion - only reduces the protective properties of the concrete. I am not sure about the Canadian Building Code requirements, but in Britain, for example, it is not allowed to use CaCl2 in structural concrete with reinforcement because of the potential corrosion.