Setting and hardening of high alumina cement are

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Unformatted text preview: p-holes for the molten cement can also be used. Typical analytical compositions for a high-alumina cement are -40% each of A1203 and CaO and about 10% each of Fe203 and Si02; the most important compounds in the cement are CaA1204, Ca2A12Si07 and Ca6A18FeSiOzl. Setting and hardening of high-alumina cement are probably due to the formation of calcium aluminate gels such as Ca0.A1203.10H20, and the more basic 2CaO.Al203.8H20, 3CaO.Al203.6H20 and 4CaO.Al203.13H20, though these empirical formulae give no indication of the structural units involved. The most notable property of high-alumina cement is that it develops very high strength at a very early stage (within 1 day). Long exposure to warm, moist conditions may lead to failure but resistance to corrosion by sea water and sulfate brines, or by weak mineral acids, is outstanding. It has also been much used as a refractory cement to withstand temperatures uP to 1500". Aluminium, Gallium, Indium and Thallium 252 Ch. 7 Portland Cernen@ The name "Portland cement" was first used by J. Aspdin in a patent (1824) because, when mixed with water and sand the powder hardened into a block that resembled the natural limestone quamed in the Isle of Portland, England. The two crucial discoveries which led to the production of strong, durable, hydraulic cement that did not disintegrate in water, were made in the eighteenth and nineteenth centuries. In 1756 John Smeaton, carrying out experiments in connection with building the Eddystone Lighthouse (UK), recognized the importance of using limes which contained admixed clays or shales (i.e. aluminosilicates), and by the early 1800s it was realized that firing must be carried out at sintering temperatures in order to produce a clinker now known to contain calcium silicates and aluminates. The first major engineering work to use Portland cement was in the tunnel constructed beneath the Thames in 1828. The first truly high-temperature cement (1450-1600°C) was made in 1854, and the technology was revolutionized in 1899 by the introduction of rotary kilns. The important compounds in Portland cement are dicalcium silicate (CazSiOJ) 26%. tricalcium silicate (Ca3SiOs) 51%. tricalcium aluminate (Ca3A1206) 11% and the tetracalcium species CaqA12Fe:1010 (1%). The principal constituent of moistened cement paste is a tobermorite gel which can be represented schematically by the following idealized equations: + 4H20 +3Ca0.2Si02.3H20 + Ca(OH)2 2Ca3Si05 + 6H2O +3Ca0.2Si02.3HzO + 3Ca(OH)2 2CazSi04 The adhesion of the tobermorite particles to each other and to the embedded aggregates is responsible for the strength of the cement which is due, ultimately, to the formation of -Si-0-Si-0 bonds. Portland cement is made by heating a mixture of limestone (or chalk, shells, etc.) with aluminosilicates (derived from sand, shales, and clays) in carefully controlled amounts so as to give the approximate composition CaO -70%. Si02 -20%, A1203 -5%, Fe2O3 -3%. The presence of NazO, KzO, MgO and PzO5 are detrimental and must be limited. The raw materials are ground to pass 200-mesh siev...
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This note was uploaded on 10/12/2012 for the course CHEMISTRY CHM1040 taught by Professor Jhon during the Spring '12 term at Florida A&M.

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