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Portland cement
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Portland cement is the most common type of cement in general usage,
as it is a basic ingredient of concrete, mortar and plaster. It
consists of a mixture of oxides of calcium, silicon and aluminium.
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 (clinker), with a source of sulfate (most
commonly gypsum). The resulting powder, when mixed with water, will
become a hydrated solid over time. Portland cement was developed from cements (or correctly hydraulic limes) made 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. Joseph Aspdin, a British bricklayer, in 1824 was granted a patent for a process of making a cement which he called Portland cement. |
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However, his cement was not Portland cement but an artificial
hydraulic lime similar to the material known as "Roman Cement" which
was Patented in 1796 by James Parker. Aspdin's process was similar
to that patented in 1822 and used since 1811 by James Frost who
called his cement "British Cement". The name "Portland Cement" is
recorded in a directory published in 1823 being associated with a
William Lockwood and possibly others. Aspdin's son William in 1843
made an improved version of the "Roman Cement" and he initially
called it "Patent Portland Cement" but he had no patent and his
cement still was not Portland Cement. In 1848 William Aspdin further
improved his cement and in 1853 moved to Germany where he was
involved in cement making. (See "The Cement Industry 1796-1914: A
History" by A J Francis 1977) The first true Portland cement was
probably made by the factory of "Portlandzementfabrik Stern" in
Germany about 1867. This works is mentioned by Henry Reid in his
1868 book on Cement Manufacture wherein his states that the cement
is superior to any other by a large margin. The German Government
issued a Standard on Portland Cement in 1878 (possibly the first
standard of any kind). The Stern cement would have complied with
that standard. There are three fundamental stages in the production of Portland cement: Preparation of the raw mixture Production of the clinker 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 mixture (as fine dust in the 'Dry process' or in the form of a slurry in the 'Wet process') of calcium oxide, silicon oxide, aluminium oxide, ferric oxide, and magnesium oxide. 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 cylinder up to ~1480°C. 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. The powder is now ready for use, and will react with the addition of water. The most common use for portland cement is the production of concrete. Concrete is a composite material consisting of aggregate (gravel), 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 (C3A), 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 cementPlastic 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 (see superplasticizer). 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 an organic polymer. Rather, it refers to the addition of a substance to increase the workability of the mixture.In 2002 the world production of hydraulic cement was 1,800 million metric tons. The top three producers were China with 704, India with 100, and the United States with 91 million metric tons for a combined total of about half the world total by the world's three most populous states. "For the past 18 years, China consistently has produced more cement than any other country in the world.China's cement export peaked in 1994 with 11 million tons shipped out and has been in steady decline ever since. Only 5.18 million tons were exported out of China in 2002. Offered at $34 a ton, Chinese cement is pricing itself out of the market as Thailand is asking as little as $20 for the same quality." Jan 7, 2004 "Demand for cement in China is expected to advance 5.4% annually and exceed 1 billion metric tons in 2008, driven by slowing but healthy growth in construction expenditures. Cement consumed in China will amount to 44% of global demand, and China will remain the world's largest national consumer of cement by a large margin. |
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