The word “Calcium” derives from Latin calx “lime”, which was obtained from heating limestone. Calcium is an alkaline earth metal that reacts forming a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to its heavier homologues Strontium and Barium. It is the fifth most abundant element in Earth’s crust and the third most abundant metal, after Iron and Aluminium. The most common Calcium compound on Earth is calcium carbonate, found in limestone and the fossilised remnants of early sea life; gypsum, anhydrite, fluorite, and apatite are also sources of calcium. Calcium is also the most abundant metal and the fifth-most abundant element in the human body. As electrolytes, Calcium ions play a vital role in the physiological and biochemical processes of organisms and cells: in signal transduction pathways where they act as a second messenger; in neurotransmitter release from neurons; in contraction of all muscle cell types; as cofactors in many enzymes; and in fertilisation.
The largest use of metallic Calcium is in steelmaking to improve castability, cleanliness and general mechanical properties. Calcium is also used in maintenance-free automotive batteries, in which the use of 0.1% Calcium–Lead alloys instead of the usual Antimony–Lead alloys leads to lower water loss and lower self-discharging. Due to the risk of expansion and cracking, Aluminium is sometimes also incorporated into these alloys, which are also used in casting, replacing Lead–Antimony alloys. Calcium is also used to strengthen Aluminium alloys used for bearings, for the control of graphitic Carbon in Cast Iron, and to remove Bismuth impurities from Lead. Calcium is also used as a reducing agent in the production of Chromium, Zirconium, Thorium, and Uranium.