The metal is used as a reducing agent in preparing other metals such as thorium, uranium, zirconium, etc., and is used as a deoxidizer, desulfurizer, or decarburizer for various ferrous and nonferrous alloys. Its natural and prepared compounds are widely used. Mixed with sand it hardens as mortar and plaster by taking up carbon dioxide from the air. Calcium from limestone is an important element in Portland cement. The solubility of the carbonate in water containing carbon dioxide causes the formation of caves with stalactites and stalagmites and is responsible for hardness in water.
Natural calcium contains six isotopes.
Thirteen other radioactive isotopes are known. Ca concentrations were compared to those found in a polluted black snowfall with a trajectory that originated over eastern Europe and to those levels found in seasonal snowpack. The mean concentration of Ca in seasonal snowpack was 2,500 ppm.