Cement is made by combining clinker, a mixture of limestone and other raw materials that have been pyroprocessed in the cement kiln, with gypsum and other cementitious additives. Clinker production typically occurs in kilns heated to about 1450 degrees C. Clinker production is the most energy-intensive process in cement manufacturing.
Coal is the primary fuel burned in cement kilns, but petroleum coke, natural gas, and oil are also consumed. Waste fuels, such as hazardous wastes from industrial or commercial painting operations (spent solvents, paint solids), metal cleaning fluids (solvent based mixtures, metal working and machining lubricants, coolants, cutting fluids), electronic industry solvents, as well as tires, are often used as fuels in cement kilns as a replacement for more traditional fossil fuels.
China is the world's largest producer of cement, manufacturing 1,330 million metric tons (Mt) in 2007. The next highest producers are India and the U.S., producing 160 Mt and 96 Mt, respectively. China's cement production has grown about 10 percent per year over the past two decades.