Calcium oxide

Calcium oxide is a white, crystalline solid that is also known as quicklime or burnt lime.

It is an alkaline and caustic compound, which means it can react strongly with other substances. Because it is relatively inexpensive to produce, it is considered an important commodity chemical. This means it is traded and used in massive quantities across many different global industries. Scientists also recognize calcium oxide as a distinct mineral species named "Lime."

This mineral belongs to an isometric crystal system and can form a solid solution series with a mineral called monteponite. However, quicklime is chemically unstable in moist air. It will quickly convert into portlandite, which is another form of calcium hydroxide.

To create quicklime, humans use a process called calcination or lime-burning.

This process begins with materials like limestone or seashells, which contain calcium carbonate. These materials are placed inside a lime kiln and heated to temperatures above 825 °C (1,517 °F). This intense heat causes thermal decomposition, a chemical change where the material breaks apart. During this reaction, a molecule of carbon dioxide gas is liberated and escapes. What remains in the kiln is the solid calcium oxide. This is one of the few chemical reactions that humans have known since prehistoric times.

Quicklime is highly reactive, especially when it meets water. When calcium oxide reacts with water, it undergoes an exothermic reaction, which means it releases thermal energy.

This specific reaction produces a chemical called calcium hydroxide. As the solid hydrates, it actually puffs up in size. The amount of energy released is quite significant; one litre of water combining with approximately 3.1 kilograms of quicklime produces 3.54 MJ of energy. This heat can be used for practical tasks, such as self-heating food cans or cooking without an open flame. However, this reaction is so vigorous that it can cause severe irritation or burns if it touches moist skin, eyes, or the respiratory tract.

Because of its unique properties, quicklime has many different industrial uses. It is a primary ingredient in the production of cement. In the petroleum industry, it is used in water detection pastes. These pastes mix calcium oxide with phenolphthalein, a chemical indicator. If water enters a fuel tank, the quicklime reacts to form calcium hydroxide. This raises the pH level enough to turn the phenolphthalein a vivid purplish-pink color, signaling the leak. Additionally, quicklime is used in chemical pulping to help regenerate sodium hydroxide. It is also used in flue-gas desulfurization to remove sulfur dioxide from exhaust streams.

History shows that quicklime has been vital to human construction for millennia. Archeological evidence shows that Pre-Pottery Neolithic B humans used limestone-based plaster for flooring. These lime-ash floors were used for many centuries until the late nineteenth century. Ancient Roman concrete is also famous for its durability, which is partly due to quicklime. When mixed using a "hot mixing" method, the quicklime creates small structures called lime clasts. If the concrete cracks, water enters and reacts with these clasts to create a calcium-saturated solution. This solution can recrystallize as calcium carbonate, effectively filling the cracks and repairing the structure.

Quicklime has even been used for light and warfare. Before the invention of electric lighting, people used "limelight" for theatrical productions. This was achieved by heating quicklime to 2,400 °C (4,350 °F), which caused it to emit an intense glow. In historical warfare, quicklime was sometimes used as a weapon. It is thought to have been a component of Greek fire, where it could raise temperatures above 150 °C (302 °F) to ignite fuel. Historian David Hume recorded that the English Navy used quicklime to blind an invading French fleet during the reign of Henry III.

Today, the scale of quicklime production is massive. The annual worldwide production is approximately 283 million tonnes. China is the largest producer in the world, creating about 170 million tonnes every year. The United States is the second largest producer, with roughly 20 million tonnes per year. While quicklime is powerful, there are several substitutes available depending on the job. Limestone is a much cheaper but less reactive alternative used in agriculture and sulfur removal. Other industries might use magnesium hydroxide for pH control or calcined gypsum for making plasters.