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Portland cement

Portland cement is the most common type of cement in general usage, as it is a basic ingredient of concrete and mortar. It consists of a mixture of oxides of calcium, silicon and aluminum. Portland cement and similar materials are made by heating limestone (as source of calcium) with clay or sand (as source of silicon) and grinding the product. The resulting powder, when mixed with water, will become a hydrated solid over time.

Manufacture of portland cement requires the burning of large quantities of fuel, typically coal, which along with impurities contained in the limestone can result in significant emissions of pollutants regulated in many countries, including greenhouse gases such as nitrogen oxides (NOx) and sulfur dioxide, as well as particulates (PM 10, PM 2.5), volatile organic compounds (VOCs), and carbon monoxide. Cement plants are known to emit substantial quantities of arsenic, lead and mercury.

Portland cement was first manufactured in Britain in the early part of the 19th century, and its name is derived from its similarity to Portland Stone, a type of building stone that was quarried on the Isle of Portland in Dorset, England. The patent for Portland cement was issued to Joseph Aspdin, a British bricklayer, in 1824.

Production

There are three fundamental stages in the production of Portland cement:

  1. Preparation of the raw mixture
  2. Production of the clinker
  3. Preparation of the cement

The chemistry of cement is very complex, so cement chemist notation was invented to simplify the formula of common molecules found in cement.

The raw materials for Portland cement production are a slurry of calcium oxide (44%), silicon oxide (14.5%), aluminum oxide (3.5%), ferric oxide (3%), and magnesium oxide (1.6%). The raw materials are usually quarried from local rock, which in some places is already practically the desired composition and in other places requires the addition of clay and limestone, as well as iron ore, bauxite or recycled materials.

The raw mixture is heated in a kiln, a gigantic slowly rotating and sloped cylinder, with temperatures increasing over the length of the cyclinder up to ~1480C. The temperature is regulated so that the product contains sintered but not fused lumps; too low a temperature causes insufficient sintering, but too high a temperature results in a molten mass or glass. In the lower temperature part of the kiln, calcium carbonate (limestone) turns into calcium oxide (lime) and carbon dioxide. In the high temperature part, calcium oxides and silicates react to form dicalcium and tricalcium silicates (C2S C3S). Small amounts of tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF)are also formed. The resulting material is clinker, and can be stored for a number of years before use, prolonged exposure to water decreases the reactivity of cement produced from weathered clinker.

The energy required to produce clinker is ~1700 J/g, however because of heat loss during production actual values can be much higher. The high energy requirements and the release of significant amounts of carbon dioxide makes cement production a concern for global warming.

In order to achieve the desired setting qualities in the finished product, about 2% gypsum is added to the clinker and the mixture is pulverized very finely. This powder is now ready for use, and will react with the addition of water.

The finished cement has approximately the following composition: calcium oxide 64%, aluminum oxide 5.5%, silicon oxide 21%, ferric oxide 4.5%, magnesium oxide 2.4%, sulfate 1.6%, with a loss of ignition about 1% (mostly water).

Use

The most common use for portland cement is the production of concrete. Concrete is a composite material consisting of aggregate, cement, and water. As a construction material, concrete can be cast in almost any shape desired, and once hardened, can become a structural (load bearing) element.

When water is mixed with Portland cement, the product sets in a few hours and hardens over a period of weeks. The initial setting is caused by a reaction between the water, gypsum, and tricalcium aluminate (C3Al), forming the crystalline hydration products calcium-alumino-hydrate (CAH), ettringite (Aft), and monosulfate (Afm). The later hardening and the development of cohesive strength is due to the reaction of water and tricalcium silicate (C3S), forming an amorphous hydrated product called calcium-silicate-hydrate(CSH gel). In each case the hydration products surround and cement together the individual grains. The hydration of dicalcium silicate (C2S) proceeds more slowly than that of the above compounds slowly increasing later-age strength. The ultimate cementing agent is probably gelatinous silica (SiO2). All three reactions mentioned above release heat.

Plastic cement is a type of Portland cement with the addition of a plasticizing material (limestone or hydrated lime), as well as other materials to reduce setting time and facilitate workability. Plastic cement is used primarily for spreading onto walls to make exterior stucco, as Portland cement (used primarily for concrete) would have poor spreadability. In this usage, the term "plastic" does not refer to the addition of a polymer. Rather, it refers to the addition of a substance to increase workability into the desired texture.

Safety and health

When cement is mixed with water a highly alkaline solution (pH ~13) is produced by the dissolution of calcium, sodium and potassium hydroxides. Gloves, goggles and a filter mask should be used for protection. Hands should be washed after contact. Once the cement hydrates, the hardened mass can be safely touched without gloves.

In Scandinavia, the level of chrome VI, which is toxic and a major skin irritant, may not exceed 2 ppm (mg/kg). The total chrome level may be higher.

Living near a portland cement plant can have serious health consequences due to the large quantity of pollutants released by the manufacturing process, as well as fugitive emissions from related on-site activities (such as haul roads, blasting, and truck traffic). The pollutants released by portland cement industry are known to contribute to lung and circulatory problems, including asthma, bronchitis, and heart attacks, as well as cancers. Hovever, many residents in these communities consider these risks to be acceptable in view of the economic benefits. For instance, polls show the residents of Ste. Genevieve County, Missouri to be overwhelmingly in favor of the cement plant being built there beginning in 2005. This plant will be the largest cement plant in the United States.

Portland cement business

As of 2004, the largest producers of portland cement internationally are Lafarge (France), Holcim (Switzerland) and Cemex (Mexico), among others. Cement manufacturers have been subject to numerous fines for environmental and anti-competitive (price-fixing) violations, mainly in the U.S., Canada and Europe. As a rule, the industry is less closely regulated in other parts of the world.

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