Manufactured commercially in at least 120 countries, it is mixed with sand and gravel to make concrete. Concrete is used in the construction of buildings, roads, and other structures, as well as in other products and applications. Its use as a residential building material is particularly important in countries where wood is not traditionally used for building or is in short supply. Annual CO2 emissions from cement production in nine major regions of the world are shown in Figure A below. Large quantities of CO2 are emitted during the production of lime, the key ingredient in cement.
Calcium carbonate is derived from limestone, chalk, and other calcium-rich materials.
Like the CO2 emitted during the combustion of coal, oil, and gas, the carbon dioxide released during cement production is of fossil origin. Liberation of this store of carbon is normally very slow, but it has been accelerated many times over by the use of carbonate minerals in cement manufacturing. The lime content of cement does not vary much. There are specialty cements that contain less lime, but they are typically used in small quantities. While research is underway into suitable cement mixtures that have less lime than does Portland cement, options for significantly reducing CO2 emissions from cement are currently limited.