Cement is a rocky powder that can be mixed with water and molded to any desired shape, after which it hardens to a rock-like consistency. It is used worldwide both as a mortar to join bricks and blocks together and as an ingredient of concrete, which is mixture of crushed rock, sand, and cement that is used to construct buildings, bridges, roads, pipes, dams, and other structures. Cement manufacture releases large amounts of carbon dioxide (CO2), the most important greenhouse gas. The cement manufacturing industry is the third-largest source of anthropogenic (human-caused) carbon dioxide emissions after forest destruction and the burning of fossil fuels for transportation and electricity generation. About 5% of global anthropogenic carbon dioxide emissions are from cement manufacture.
Historical Background and Scientific Foundations
Roman builders used an early form of cement to construct the dome of the Pantheon temple in Rome, which is still standing after almost 1,900 years. Cement was
little used, however, from the fall of the Roman Empire to the beginning of the Industrial Revolution in Europe. Inventors in England and France experimented with fast-setting cements and structural concrete in the late 1700s and early 1800s, and concrete reinforced by internal steel rods was first used for building construction in the 1890s. Today, cement-based concrete is the most abundant manufactured material on Earth.
Three parts of the cement-manufacture process release carbon dioxide:
- Calcination. Most cement is made from a mixture of calcined limestone, clay, and chalk. Limestone is a rock that consists mostly of calcium carbonate (CaCO3). Calcined limestone has been heated in a furnace to break CaCO3 down into CaO and CO2. The CO2 is released to the atmosphere. In producing 2.2 lb (1 kg) of clinker (dry powder), calcination releases about 1.1 lb (0.5 kg) of carbon dioxide.
- Fuel. Large amounts of fuel are burned in special ovens or kilns to provide heat for calcination. How much carbon dioxide is released by fuel-burning during calcination depends on the kind of fuel used. Coal, natural gas, fuel oil, and other fuels are all used by industry, with coal being the most common fuel.
- Electricity. Electricity is used to run the machines that crush and grind limestone before and after calcination. Smaller amounts of electricity are used for conveyor belts, packing, and other miscellaneous aspects of manufacture. Worldwide, most electricity is produced by burning coal, which releases carbon dioxide. Most of the CO2 emitted during cement manufacture is due to calcination and fuel-burning, not electricity usage.
Impacts and Issues
More than 150 countries produce cement or the baked powder called clinker that is the main ingredient of cement. China produces about 33% of the world CO2 output from cement-making, the United States about 6%, and India about 5%.
Like all processes that release greenhouse gases and so contribute to climate change, cement manufacturing is hazardous. However, since concrete usage is entwined with all aspects of economic activity, and strong, cheap substitutes do not exist for most applications, cement production is unlikely to be reduced. On the contrary, its use is growing steadily worldwide. Yet, there are other ways in which CO2 releases during cement manufacture may be reduced.
Much of the heat energy supplied by fuel-burning in conventional kilns during calcination of limestone is wasted: by substituting more efficient machinery and using “dry” rather than “wet” calcination processes, direct fuel usage can be reduced by up to 48%, which would reduce CO2 emissions by 27%. However, some estimates of how much efficiency can be improved without diminishing profits are as low as 11%, which would yield a CO2 reduction of only 5%. Substitution of lower-carbon fuels, such as natural gas, would reduce CO2, as would shifting construction practices to the use of blended cement, which is cement in which some limestone-based clinker is replaced by industrial wastes such as coal fly ash (the ash left over from burning coal in power plants) or volcanic ash.
WORDS TO KNOW
ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.
CALCINATION: An old term used to describe the process of heating metals and other materials in air.
FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.
GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.
INDUSTRIAL REVOLUTION: The period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.
France, Germany, and the Netherlands committed to a voluntary United Nations-led initiative in the mid 1990s to reduce their CO2 emissions per ton of cement. A private-sector effort to reduce cement CO2 intensity, the Cement Sustainability Initiative, was announced in 1999 by the World Business Council for Sustainable Development. As of 2007, the Initiative still consisted primarily of commissioned studies on ways to reduce emissions. Some individual companies have, however, changed actual manufacturing processes. For example, U.S. Concrete claimed to have reduced its CO2 output in 2006 by 328,000 tons by switching largely to blended-cement manufacture using coal fly ash. As of mid-2007, the governments of China, India, and the
United States had not officially committed to reductions in cement CO2 emissions.
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“The Sustainable Cement Initiative.” World Business Council for Sustainable Development, 2002. < http://www.wbcsdcement.org/pdf/cement_initiative_arp.pdf> (accessed August 6, 2007).
“Why Cement-Making Produces Carbon Dioxide.” Climate Change Fact Sheet 30. United Nations Environment Programme. Information Unit on Climate Change. < http://www.cs.ntu.edu.au/homepages/jmitroy/sid101/uncc/fs030.html> (accessed August 5, 2007).