Cold weather slows the hydration of cement significantly. Adding a chemical accelerator to mortar can offset this retardation. CALCIUM CHLORIDE SPEEDS SET THE MOST Accelerators used in masonry mortars are made from inorganic salts or organic compounds. Calcium chloride is an inorganic salt that has been used as an accelerator in mortars and concretes since 1873. Calcium chloride mainly speeds the reaction rates of two cement components--tricalcium silicate and dicalcium silicate. Calcium chloride also increases early age flexural strengths, but not as well as it increases compressive strengths. But using calcium chloride has two major setbacks. The soluble chlorides it brings to mortar contribute to corrosion of metals. Calcium chloride also increases the potential for efflorescence. NONCHLORIDES DON'T CAUSE CORROSION Calcium nitrite and calcium nitrate, both inorganic compounds, commonly are used as nonchloride accelerators in mortars. These nonchloride accelerators usually cost more and must be used in higher dosages than calcium chloride-based accelerators. But they are noncorrosive to metals when used at recommended dosage rates, and they don't contribute to efflorescence.