The lime industry is a significant carbon dioxide emitter. The manufacture of one tonne of calcium oxide involves decomposing calcium carbonate, with the formation of 785 kg of CO in some applications, such as when used as mortar; this CO is later re-absorbed as the mortar goes off.
If the heat supplied to form the lime (3.75 MJ/kg in an efficient kiln) is obtained by burning fossil fuel it will release CO: in the case of coal fuel 295 kg/t; in the case of natural gas fuel 206 kg/t. The electric power consumption of an efficient plant is around 20 kWh per tonne of lime. This additional input is the equivalent of around 20 kg CO per ton if the electricity is coal-generated. Thus, total emission may be around 1 tonne of CO for every tonne of lime even in efficient industrial plants, but is typically 1.3 t/t. However, if the source of heat energy used in its manufacture is a fully renewable power source, such as solar, wind, hydro or even nuclear; there may be no net emission of CO from the calcination process. Less energy is required in production per weight than portland cement, primarily because a lower temperature is required.