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1、外文资料译文Portland cement of its Types and Manufacture of Portland cementPortland cement is made by heating a mixture of limestone and clay, or other materials of similar bulk composition and sufficient reactivity, ultimately to a temperature of about 1450C. Partial fusion occurs, and nodules of clinker
2、 are produced. The clinker is mixed with a few percent of gypsum and finely ground to make the cement. The gypsum controls the rate of set and may be partly replaced by other forms of calcium sulfate. Some specifications allow the addition of other materials at the grinding stage. The clinker typica
3、lly has a composition in the region of 67% CaO, 22% SiO2, 5% Al2O3 , 3%Fe2O3 , and 3% of other components,and normally contains four major phases,called alite , belite , aluminate phase and ferrite phase . Several other phases, such as alkali sulfates and calcium oxide, are normally present in minor
4、 amounts.Alite is the most important constituent of all normal Portland cement clinkers,of which it constitutes 50%-70%.It is tricalcium silicate (Ca3SiO5)modified in composition and crystal structure by incorporation of foreign ions, especially Mg2+, Al3+ and Fe3+. It reacts relatively quickly with
5、 water, and in normal Portland cement is the most important of the constituent phases for strength development at ages up to 28 days, it is by far the most important.Belite constitutes 15%-30% of normal Portland cement clinker. It is declaim silicate (Ca2SiO4) modified by incorporation of foreign io
6、ns and normally present wholly or largely as the polymorph. it reacts slowly with water , thus contributing little to the strength during the first 28 days ,but substantially to the further increase in strength that occurs at later ages .By one year, the strength obtainable form pure alit and pure b
7、elite are about the same under comparable conditions.The aluminates phase constitutes 5%-10% of most normal Portland cement clinkers. it is Tricalcium aluminates (Ca3Al2O6), substantially modified in composition and sometimes also in structure by incorporation of foreign ions , especially Si4+ , Fe3
8、+, Na+ and K+. It reacts rapidly with water and can cause undesirably rapid setting unless a set-controlling agent, usually gypsum, is added.The ferrite phase makes up 5%-15% of normal Portland cement clinkers. It is tetra calcium aluminoferrite (Ca4AlFeO7) substantially modified in composition by v
9、ariation in Al/Fe ratio and incorporation of foreign ions. The rate at which it reacts with water appears to be somewhat variable, perhaps due to differences in composition or other characteristics, but in general is high initially and intermediate between those of Alite and Belite at later ages.The
10、 great majority of Portland cements made throughout the world are designed for general constructional use. The specifications with which such cements must comply are similar, but not identical, in all countries and various names are used to define the material, such as OPC (Ordinary Portland Cement)
11、 in the UK, or Type I Portland Cement in the USA.Specifications are, in general based partly on chemical composition or physical properties such as specific surface area, and partly on performance tests, such as setting time or compressive strength developed under standard conditions. The content of
12、 MgO is usually limited to either 4 or 5%, because quantities of this component in excess of about 2% are liable to occur as periclase (magnesium oxide), which through slow reaction with water can cause destructive expansion of hardened concrete. Free lime (calcium oxide) can behave similarly, and i
13、ts potential formation sets a practical upper limit to the Alite content of a clinker. Excessive contents of SO3 can also lead to delayed expansion, and upper limits of 2.5%-4% are usually imposed. Alkalis (K2O and Na2O) can undergo expansive reactions with certain aggregates, and some national spec
14、ifications limit the content, e.g. to 0.6% equivalent Na2O (Na2O+0.66K2O) .other upper limit of composition widely used in specifications relate to matter insoluble in dilute acid, and loss on ignition. Many other minor components are limited in content by their effects on the manufacturing process,
15、 or the properties, or both, and in some cases the limits are defined in specifications.Rapid-hardening Portland cement have been produced in various ways , such as varying the composition to increase the alite content , finer grinding of the clinker , and improvements in the manufacturing process ,
16、 e.g. finer grinding or better mixing of the raw materials . The alite contents of Portland cements have increases steadily over the one and a half centuries during which the latter have been produced, and many presentday cements that would be considered normal today would have been described as rapid hardening only a few decades ago. In USA specifications, rapid-hardening Portland cements are called high early strength or Type III cements.Destructive expansion from reaction with
