Unslaked lime — also known as quicklime or burnt lime, is a chemical compound with the formula calcium oxide (CaO). It is a white, caustic, alkaline, and crystalline solid at room temperature. Unslaked lime is produced by heating limestone or other calcium-rich materials in a kiln, a process known as calcination.

Unslaked lime is highly reactive and exothermic when it comes into contact with water. It undergoes a chemical reaction called hydration, where it combines with water to form calcium hydroxide, also known as slaked lime (Ca(OH)2). This reaction releases a significant amount of heat and produces a powdery substance.

Unslaked lime is commonly used in the production of mortar and cement, where it serves as a key ingredient. When mixed with water, unslaked lime reacts with carbon dioxide from the air and undergoes carbonation, transforming into calcium carbonate, which adds strength and stability to the construction materials.