Cement is a binder. It hardens when mixed with water and holds materials together. It is not concrete. Concrete is cement plus aggregate plus water. Cement is the powder that makes concrete work. The most common cement is Portland cement, made by heating limestone and clay in a kiln to about 1,450 degrees Celsius. The resulting clinker is ground with gypsum to make the powder. When water is added, the cement undergoes a chemical reaction called hydration. It forms crystals that interlock and harden.

Cement has several types. Type I is general-purpose. Type II is moderate sulfate resistance. Type III is high early strength, used when concrete must gain strength quickly. Type IV is low heat of hydration, used in mass concrete. Type V is sulfate resistant, used in marine environments. Blended cements combine Portland cement with fly ash, slag, or silica fume to improve performance and reduce carbon emissions.

Cement production is a major source of carbon dioxide. The kiln reaction releases CO2 from the limestone, and the fuel used to heat the kiln releases more. Cement accounts for about 8 percent of global CO2 emissions. The industry is working on lower-carbon alternatives, including carbon capture, alternative fuels, and new chemistries. Cement is essential to modern construction. It is also a climate problem. The two facts are in tension.