Cement and concrete
Background (Encyclopedia of Global Resources)
Cement is an important construction material because of the ready availability of its raw materials, its capacity to be shaped prior to setting, and its durability after hardening. When combined with an aggregate (such as sand, gravel, or crushed rock), cement becomes concrete—a durable, load-bearing construction material.
Cements with the ability to set and harden underwater are called hydraulic cements. The most common of these is portland cement, consisting of compounds of lime mixed with silica, alumina, and iron oxide. Gypsum is also added to retard the setting time. When water is added, these ingredients react to form hydrated calcium silicates that will set into a hardened product.
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History (Encyclopedia of Global Resources)
Cement has been used for construction purposes for the past six thousand years. The Egyptians are known to have used a simple cement, and the Greeks and Romans advanced the technology by creating hydraulic cements from various volcanic materials and lime. Many examples of their concrete structures remain today—some underwater, where they were used in harbors.
The quality of cementing materials declined greatly during the Middle Ages but began to improve again in the late eighteenth century. In 1756, the famed British engineer John Smeaton was commissioned to rebuild the Eddystone Lighthouse near Cornwall, England. He undertook a search for lime mortars that would resist the action of the water and discovered that the best limestone contains a high proportion of clayey material. For his project he used lime mixed with pozzolana from Italy (the same volcanic material the Romans had used). Smeaton was followed by a number of researchers, including Joseph Aspdin, a Leeds builder, who patented “portland” cement, named for the high-quality building stone quarried at Portland, England.
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Manufacturing Cement (Encyclopedia of Global Resources)
Cement is a manufactured product, made from raw materials that are found relatively easily in nature. Cement manufacturers have a number of sources for lime, but the most common are limestone and chalk. Coral and marine shell deposits are also used as sources of lime, when available. Silica, alumina, and iron oxide are found in clays, shales, slates, and certain muds. Some raw materials contain almost all the ingredients of cement, especially marl (a compact clay), cement rock, and blast-furnace slag. Industrial wastes such as fly ash and calcium carbonate are also used as raw materials for cement, but not on a large scale.
Raw materials in the form of hard rock—such as limestone, slate, and some shales—are usually quarried, but they may also be mined. If the limestone is of low quality, it may need to go through a concentrating process. Softer materials such as chalk, clay, and mud can be dug by various types of machinery, depending on the physical setting and type of material. Once extracted, the raw materials are transported to the cement manufacturing plant by truck, rail, conveyor belt, or pipeline (when in a slurry).
At the plant, the raw materials are ground into a fine powder and then mixed in predetermined ratios. The mixing can be done wet, semidry, or dry. In the wet process the materials are ground wet and mixed into a slurry. In the semidry process they are ground dry, then moistened for adhesion;...
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Uses of Concrete (Encyclopedia of Global Resources)
Concrete is generally used in four common forms: ready-mixed, precast, reinforced, and prestressed. Ready-mixed concrete is transported to a construction site as a cement paste and is then poured into forms to make roadways, foundations, driveways, floor slabs, and much more. Precast concrete—cast at a plant and then transported to the site—is used for everything from lawn ornaments to major structural elements. Reinforced concrete is created by adding steel mesh, reinforcing bars, or any other stiffening member to the concrete before it sets. Prestressed concrete, the most recently developed form, increases the strength of a beam by using reinforcing steel to keep the entire beam under compression. Concrete is much stronger under compression (pushed in on itself) than under tension (pulled apart).
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Further Reading (Encyclopedia of Global Resources)
Gani, M. S. J. Cement and Concrete. New York: Chapman & Hall, 1997.
Lea, F. M. Lea’s Chemistry of Cement and Concrete. 4th ed. Edited by Peter C. Hewlett. New York: J. Wiley, 1998.
Mehta, P. K., and Paulo J. M. Monteiro. Concrete: Microstructure, Properties, and Materials. 3d ed. New York: McGraw-Hill, 2005.
Mindess, Sidney, J. Francis Young, and David Darwin. Concrete. 2d ed. Upper Saddle River, N.J.: Prentice Hall, 2003.
Neville, A. M. Properties of Concrete. 4th ed. Harlow, Essex, England: Longman Group, 1995.
Natural Resources Canada. Canadian Minerals Yearbook, Mineral and Metal Commodity Reviews. http://www.nrcan-rncan.gc.ca/mms-smm/busi-indu/cmy-amc/com-eng.htm
Portland Cement Association. Cement and Concrete Basics. http://www.cement.org/basics
U.S. Geological Survey. Cement: Statistics and Information. http://minerals.usgs.gov/minerals/pubs/commodity/cement/index.html#mcs
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Cement and concrete
Definition (Encyclopedia of Global Warming)
Concrete, at its most basic, is composed of aggregates and a binding material (cement). Concrete aggregates are coarse, greater than 4.75 millimeters; fine aggregates are less than 4.75 millimeters in size. Aggregates—which are free of silt, organics, sugars, and oils—include sand, gravel, crushed stone, and iron blast-furnace slag. By volume, they make up about 75 percent of a concrete-cement mixture. The aggregates’ size plays an important role in achieving maximum particle packing. Optimum packing reduces the amount of cement needed; with less cement, the durability and mechanical properties of the concrete are improved.
Compressive strength, the measured maximum resistance to axial loading, is one of the outstanding properties of cement. Tensile strength, a measure of resistance to stretching, is much lower for concrete, so it is often reinforced with steel bars to provide additional tensile strength. The durability of concrete is high, because it can be designed and manufactured for resistance to freeze-thaw cycles, seawater exposure, chemicals, and corrosion.
Cement, in the broadest sense, binds concrete elements together in the presence of water. Cement is instrumental in determining the quality of concrete. In properly manufactured concrete, every particle of aggregate must be surrounded by cement, and all voids must be filled with cement.
Early cements, known as soft lime cements, were prepared by...
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Significance for Climate Change (Encyclopedia of Global Warming)
The basic makeup of Portland cement is lime (CaO) from limestone and cement rock, silica (SiO2) from clay and fly ash, alumina (A12O3) from aluminum ore refuse, and iron oxide (Fe2O3) from iron ore. The proportions of these crushed elements are closely defined by industry standards. A mix or slurry of limestone and shale or clay is prepared for burning and final cooling in an inclined, rotating kiln. The dry cement mix slowly heats to 1,260° Celsius, and the carbonates (limestone and cement rock) burn and lose carbon dioxide (CO2). Lime, alumina, and iron oxide fuse between 1,427 and 1,482° Celsius to complete the cement. Approximately 60 percent of all CO2 emissions are from the lime-burning process; the remaining 40 percent of CO2 emissions originate from fossil fuels used for combustion.
Originally, oil was used to heat kilns; the use of pulverized coal began in the late 1890’s and has continued. Electricity, used in plant operation, is often generated by coal, and diesel or gasoline is used for quarrying raw materials. All are fossil fuels—all release CO2 into the atmosphere. U.S. CO2 emission data from the Energy Information Agency (EIA) for cement manufacture show that atmospheric CO2 has risen from 33 million metric tons in 1990 to 46.1 metric tons in 2005, an increase of 13.1 percent in fifteen years. Projected data beyond 2005 continue the increasing CO2 emissions trend. It is estimated that 5...
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Further Reading (Encyclopedia of Global Warming)
“Concrete as a CO2 Sink?” Environmental Building News 4, no. 5 (September/October, 1995): 5. Addresses CO2 emission of cement manufacture, explains why the process releases CO2, and details CO2 sequestering during the curing process and after concrete is fully cured.
Energy Information Administration. Emissions of Greenhouse Gases in the U.S. Washington, D.C.: U.S. Department of Energy, 2006. Full set of industry-wide data on CO2 emissions and fossil fuels consumed. Part of a series that is updated yearly and includes future projections.
Kosmatka, S. H., B. Kerkhoff, and W. C. Panares. Design and Control of Concrete Mixtures. Skokie, Ill.: Portland Cement Association, 2002. Complete manual of concrete and cement manufacture. Includes easily understood descriptions with useful graphics and photographs, a large section on concrete applications, a list of regulations, and research resources.
Long, Douglas. Global Warming. New York: Facts On File, 2004. Confronts global warming on all levels, including science community input (addressing skeptics) and international efforts and protocols to understand and mitigate global warming. Provides a large research resource base, including periodicals, books, and Web sites.
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