If salt water cannot be used in concrete, why is it permissible to add calcium chloride (a salt) as an accelerator?
Salt water is banned not because it contains a salt but because it contains a particular salt or salts in concentrations that are harmful. Actually the banning of salt water usually means the banning of seawater or water from a salt lake. The salt that is present in greatest quantity in these waters is sodium chloride, and it is mainly because of these concentrations of sodium chloride that using salt water is often prohibited (though the use of even such water is permitted under some circumstances). On the other hand calcium chloride, a salt, is useful as an accelerator and is valuable because it furnishes calcium ions (distinct from the sodium ions of seawater) and these calcium ions in combination with chloride ions somehow accelerate the setting and hardening reactions. Even calcium chloride must, however, be added in moderation as indicated by the two percent limit, based on the weight of cement, that is normally placed on the calcium chloride admixture. Calcium chloride also contributes to corrosion of reinforcing steel under some circumstances and for this reason it is not always wise to use it.