Calcium oxide on a watch glass. Calcium oxide; CaO; Molar mass 56.0774??/mol. Also known as quicklime, burnt lime, unslaked lime, pebble lime, calcia, free lime. It is a chemical compound which is caustic, alkaline, crystalline solid. Widely used, calcination or lime-burning in a lime kiln uses thermal decomposition of calcium carbonate rich materials such as seashells and limestone. Quicklime is unstable and reacts spontaneously with CO2 in the air. Given enough time it reverts to calcium carbonate. Adding or slaking it with water will create lime plaster or lime mortar. China produces more than half of the 280 million tonnes world annual production.