The following is taken from Environmental Buidling News.
For a more complete list of the constituents of Portland cement, visit ASU cement site. Cement and Concrete Production
Cement is the key ingredient in concrete products. Comprising roughly 12% of the average residential-grade ready mix concrete, cement is the binding agent that holds sand and other aggregates together in a hard, stone-like mass. Portland cement accounts for about 95% of the cement produced in North America. It was patented in England by Joseph Aspdin in 1824 and named after a quarried stone it resembled from the Isle of Portland.
Cement production requires a source of calcium (usually limestone) and a source of silicon (such as clay or sand). Small amounts of bauxite and iron ore are added to provide specific properties. These raw materials are finely ground and mixed, then fed into a rotary cement kiln, which is the largest piece of moving industrial equipment in the world. The kiln is a long, sloping cylinder with zones that get progressively hotter up to about 2700°F (1480°C). The kiln rotates slowly to mix the contents moving through it. In the kiln, the raw materials undergo complex chemical and physical changes required to make them able to react together through hydration. The most common type of cement kiln today (accounting for 70% of plants in the U.S.) is a dry process kiln, in which the ingredients are mixed dry. Many older kilns use the wet process.
The first important reaction to occur is the calcining of limestone (calcium carbonate) into lime (calcium oxide) and carbon dioxide, which occurs in the lower-temperature portions of the kiln--up to about 1650°F (900°C). The second reaction is the bonding of calcium oxide and silicates to form dicalcium and tricalcium silicates. Small amounts of tricalcium aluminate and tetracalcium aluminoferrite are also formed. The relative proportions of these four principal compounds determine the key properties of the resultant portland cement and the type classification (Type I, Type II, etc.). These reactions occur at very high temperatures with the ingredients in molten form. As the new compounds cool, they solidify into solid pellet form called clinker. The clinker is then ground to a fine powder, a small amount of gypsum is added, and the finished cement is bagged or shipped bulk to ready mix concrete plants.
Concrete is produced by mixing cement with fine aggregate (sand), coarse aggregate (gravel or crushed stone), water, and--often--small amounts of various chemicals called admixtures that control such properties as setting time and plasticity. The process of hardening or setting is actually a chemical reaction called hydration. When water is added to the cement, it forms a slurry or gel that coats the surfaces of the aggregate and fills the voids to form the solid concrete. The properties of concrete are determined by the type of cement used, the additives, and the overall proportions of cement, aggregate, and water.
Raw Material Use
The raw materials used in cement production are widely available in great quantities. Limestone, marl, and chalk are the most common sources of calcium in cement (converted into lime through calcination). Common sources of silicon include clay, sand, and shale. Certain waste products, such as fly ash, can also be used as a silicon source. The iron and aluminum can be provided as iron ore and bauxite, but recycled metals can also be used. Finally, about 5% of cement by weight is gypsum, a common calcium- and sulfur-based mineral. It takes 3,200 to 3,500 pounds of raw materials to produce one ton (2,000 lbs.) of finished cement, according to the Environmental Research Group at the University of British Colombia (UBC).
The water, sand, and gravel or crushed stone used in concrete production in addition to cement are also abundant (typical proportions of a residential concrete mix are shown in Table 1). With all of these raw materials, the distance and quality of the sources have a big impact on transportation energy use, water use for washing, and dust generation. Some aggregates that have been used in concrete production have turned out to be sources of radon gas. The worst problems were when uranium mine tailings were used as concrete aggregate, but some natural stone also emits radon. If concerned, you might want to have the aggregate tested for radon.
Table 1: Typical Concrete Mix
| Component | Percent by weight | Portland cement | 12% | Sand | 34% | Crushed stone | 48% | Water | 6% |
Source: Based on figures provided by the Ready Mix Concrete Association, personal communication.
The use of fly ash from coal-fired power plants is beneficial in two ways: it can help with our solid waste problems, and it reduces overall energy use. While fly ash is sometimes used as a source of silica in cement production, a more common use is in concrete mixture as a substitute for some of the cement. Fly ash, or pozzolan, can readily be substituted for 15% to 35% of the cement in concrete mixes, according to the U.S. EPA. For some applications fly ash content can be up to 70%. Of the 51 million tons of fly ash produced in 1991, 7.7 million tons were used in cement and concrete products, according to figures from the American Coal Ash Association. Thus, fly ash today accounts for about 9% of the cement mix in concrete.
Fly ash reacts with any free lime left after the hydration to form calcium silicate hydrate, which is similar to the tricalcium and dicalcium silicates formed in cement curing. Through this process, fly ash increases concrete strength, improves sulfate resistance, decreases permeability, reduces the water ratio required, and improves the pumpability and workability of the concrete. Western coal-fired power plants produce better fly ash for concrete than eastern plants, because of lower sulfur and lower carbon content in the ash. (Ash from incinerators cannot be used.)
There are at least a dozen companies providing fly ash to concrete producers. Talk to your concrete supplier and find out if they are willing to add fly ash to the mix. (If your local plant doesn't know where to get the fly ash, a list of companies is available from EBN.) Portland cement with fly ash added is sometimes identified with the letter P after the type number (Type IP). The EPA requires fly ash content in concrete used in buildings that receive federal funding (for information call the EPA Procurement Guidelines Hotline at 703/941-4452). Fly ash is widely used in Europe as a major ingredient in autoclaved cellular concrete (ACC); in the U.S., North American Cellular Concrete is developing this technology (see EBN, Volume 1, No. 2 -- September/October 1992).
Other industrial waste products, including blast furnace slag, cinders, and mill scale are sometimes substituted for some of the aggregate in concrete mixes. Even recycled concrete can be crushed into aggregate that can be reused in the concrete mix--though the irregular surface of aggregate so produced is less effective than sand or crushed stone because it takes more cement slurry to fill all the nooks and crannies. In fact, using crushed concrete as an aggregate might be counterproductive by requiring extra cement--by far the most energy-intensive component of concrete.
The importance of chemistry in the curing of concrete is periodically brought to light when catastrophic failures occur. The reasons can be a simple as too much or too little water, aggregate or cement. Or the reasons may be more subtle, involving the use of materials whose composition is not completely known. Check out the speeway collapse which occurred recently at the Lowe's Motor Speedway in Charlotte, North Carolina.
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