As possibly the most extensively used building material, concrete attracts criticism from environmentalists on account of its carbon intensive production techniques and its use of a once-only natural resource, limestone. Cement is formed by heating clay and lime in a rotary kiln to a temperature of about 1450°C which produces some 3000 kg/tonne of carbon dioxide. In addition, the heating process produces a chemical reaction through the conversion of calcium carbonate into calcium oxide that releases about 2200 kg of carbon dioxide. Add to this the carbon miles in transportation, the impacts caused by mining, etc. and concrete gains few points on the sustainability scale.