by U.S. Geological Survey Wednesday, June 13, 2018
Hendrik G. van Oss, a mineral commodity specialist for the U.S. Geological Survey, compiled the following information on hydraulic cement, a key material for the construction industry.
Hydraulic cements are the binders in concrete and most mortars and stuccos. Concrete, particularly the reinforced variety, is the most versatile of all construction materials, and most of the hydraulic cement produced worldwide is portland cement or similar cements that have portland cement as a basis, such as blended cements and masonry cements. Cement typically makes up less than 15 percent of the concrete mix; most of the rest is aggregates. Not counting the weight of reinforcing media, 1 ton of cement will typically yield about 8 tons of concrete.
The term “hydraulic” refers to the fact that the cement sets, hardens and develops strength via the hydration of its component compounds or cement minerals. The primary raw material used to make portland cement is limestone, usually accounting for 90 percent of the raw material. If needed for the chemistry, lesser amounts of clays, iron ore and silica sand are included. Various industrial byproducts — chiefly ferrous slags, mill scale and coal combustion ashes — can be substituted for some of the natural raw materials.
To produce hydraulic cement, the proportioned mix of raw materials is fed into a rotary kiln for pyroprocessing into an intermediate product called clinker. The heat energy required is enormous; almost all of this energy is for preheating the raw materials and then for calcination, during which calcite in limestone is decomposed into calcium oxide and carbon dioxide. Coal and petroleum coke are the dominant fuels used to heat the kiln. After cooling, the semifused nodules of clinker are then finely ground with about 5 percent gypsum into a powder that is portland cement; the gypsum serves to control the setting time of the cement (and hence the concrete).
Globally, the cement industry is the leading industrial source of carbon dioxide, contributing 5 to 8 percent of global anthropogenic emissions. To reduce emissions, noncarbonate sources of calcium oxide like slags and coal combustion ashes can replace some of the limestone, and lower-carbon or biogenic fuels can be used. Emissions of mercury (from trace amounts in raw materials and fuels) during clinker manufacturing are also of concern.
Compared to portland cement, new types of hydraulic cement under development are purported to require less energy and less limestone in the kiln and in some cases to actually absorb carbon dioxide, but these products face acceptance and economic hurdles.
For more information on hydraulic cement and other mineral resources, visit: http://minerals.usgs.gov/minerals.
Cement production and consumption
Cement is produced in more than 150 countries, and world output totaled about 3.4 billion metric tons in 2011. Of this total, China accounted for about 2 billion metric tons, India ranked second with about 210 million metric tons, and the United States was third with about 68 million metric tons.
The United States consumed a record 128 million metric tons of cement in 2005; as a result of the economic recession, 2011 consumption was 72 million metric tons.
Although concrete is all consumed locally, cement can be transported over long distances and is traded internationally. The United States imported 32 million metric tons of cement during the peak importation year of 2006 but only 6 million metric tons in 2011.
Annual world production of cement is enough to make 4 tons of concrete per year for every person on the planet. Concrete is the most abundant of all manufactured products.
The “cement” trucks with the rotating drums are, in fact, ready-mixed concrete trucks, not cement trucks.
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