The Science Behind Self-Heating Japanese Lunch Boxes Explained! Calcium oxide, or quicklime (CaO), is a white, caustic compound produced by heating calcium carbonate (limestone) in a process called calcination, which releases carbon dioxide. When mixed with water, it undergoes a highly exothermic reaction to form calcium hydroxide (slaked lime). Calcium oxide is widely used in agriculture to neutralize acidic soils, in construction for cement and glass production, in steel manufacturing, and in