Calcium, like most reactive metals, doesn't occur in its pure form in nature. It's extracted from compounds in which it occurs as 2+ ion. It's the fifth most abundant element in the earth's crust and is found mostly in limestone, chalk and dolomite which are all forms of calcium carbonate. It's also found in gypsum, which is calcium sulfate.
Calcium is removed from calcium carbonate by reacting limestone with aluminum in a hot, near-vacuum environment. Under these conditions aluminum replaces calcium according to this equation:
`2 Al + 3 CaCO_3 -> 3 Ca + Al_2(CO_3)_3`
The pure metal is used in alloys and in the processing of uranium. Calcium is more widely used in some of its compound forms. Calcium oxide, which is used in cement, is produced by heating calcium carbonate. Calcium carbonate stone such as limestone and marble is used in building, as is the gypsum form of calcium sulfate.