The annual global production of quicklime is about 283 million tons. China is by far the world's largest producer, with a total output of about 170 million tons per year. The United States is the next largest country, with about 20 million tons per year. [9]

Calcium oxide is a key ingredient in the process of making cement.

About 1.8 tons of limestone are needed for every 1.0 tons of quicklime. Quicklime has a high affinity for water and is a more effective desiccant than silica gel. The reaction of quicklime with water is associated with a volume increase of at least 2.5 times. [1

Water Test Cream contains a mixture of calcium oxide and phenolphthalein. If this paste comes into contact with water in a fuel storage tank, the CaO will react with the water to form calcium hydroxide. Calcium hydroxide has a high enough pH to turn phenolphthalein a vivid purple-pink, indicating the presence of water.

Calcium oxide is used to regenerate sodium hydroxide from sodium carbonate in chemical recovery in kraft pulp mills.

In a process known as flue gas desulfurization, solid sprays or slurries of calcium oxide can be used to remove sulfur dioxide from exhaust streams.

Calcium oxide is also a separate mineral species (in CaO) named "lime". [30][31] It has an equiaxed crystal system and can form a solid solution series with brownstone. The crystal is brittle, thermally deteriorated, unstable in humid air, and rapidly transforms into wollastonite (Ca(OH)2).

When it hydrates, an exothermic reaction occurs and the solid expands. Hydrate can be converted back into quicklime by removing the water by heating it to red to reverse the hydration reaction. One liter of water combines with approximately 3.1 kg (6.8 lb) of quicklime to produce calcium hydroxide and 3.54 MJ of energy. The process can be used to provide a convenient portable heat source, such as on-site heating of food in self-heating tanks, cooking and heating water without an open flame. Several companies sell cooking kits that use this heating method.