1、外文资料译文 Portland cement of its Types and Manufacture of Portland cement Portland 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 clink
2、er 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 typi
3、cally 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 min
4、or 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 w
5、ith 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 foreig
6、n ions 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 pu
7、re belite 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+
8、, Fe3+, 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 compositio
9、n by variation 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 ag
10、es. The 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
11、Cement) 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 co
12、ntent of 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 similarl
13、y, and its 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 natio
14、nal specifications 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
15、process, 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
16、process , 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 describ
17、ed 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 sulfates can occur not only if the latter are present in excessive proportion in the cement, but also for
18、m attack on concrete by sulfate solutions. The reaction involves the Al2O3 containing phases in the hardened cement, and in sulfate-resisting Portland cements, its effects are reduced by decreasing the proportion of the aluminates phase, sometimes to zero. This is achieved by decreasing the ratio of
19、 Al2O3 to Fe2O3 in the materials. In the USA, sulfate-resisting Portland cements are called Type V cements. White Portland cements are made by increasing the ratio of Al2O3 to Fe2O3, and thus represent the opposite extreme in composition to sulfate-resisting Portland cements. The normal, dark color
20、.of Portland cement is due to the ferrite phase, formation of which in white cement must thus be avoided. It is impracticable to employ raw materials that are completely free from Fe2O3 and other components, such as Mn2O3, that contribute to the color. The effects of these components are therefore u
21、sually minimized by producing the clinker under slightly reducing conditions and by rapid quenching. In addition to alite, belite and aluminates phase, some glass may be formed. Portland cement is made from some of the earths most abundant materials .about two-thirds of it is derived from calcium ox
22、ide, whose source is usually some form of lime-stone(calcium carbonate),marls, chalk, or shells(for example, oyster).the other ingredients-silica,SiO2,about20 ；alumina ,Al2O3 ,about 5 ； and iron oxide,Fe2O3,about 3 are derived from sand shale, clays, coal ash, and iron ore metal slag. Because the in
23、dividual ingredients must be fused and sintered to produce new compounds they must de ground to pass a 200 mesh screen in order to react within a reasonable time in the kiln .in addition, the composition of the raw materials must be held within narrow limits of the above oxides to produce a useful p
24、roduct. Other elemental oxides which can be detrimental to the cement must be limited: these include magnesium MgO； potassium oxide, K2O； sodium oxide, and phosphorus oxide, P2O5.after blending to the proper composition, the raw materials are interground in ball mills, rod mills, or roller millers.
25、Depending on the raw materials characteristics, they are ground either dry (dry process) or in water (wet process). The resultant raw feed is introduced into the kiln system, usually a rotary kiln, where the material is heated to about 2700F. The material progressively loses first the water, then th
26、e carbon dioxide CO2, at about 1750F, and at about 2300F, a small amount at liquid phase forms. This liquid is the medium through which the higher-melting phases are formed. The resultant product, called clinker because the whole never truly melts, is cooled and again ground, in ball mills to such a
27、 fineness that about 90 will pass a screen having 325 openings per linear inch. The final product has a texture much like face powder. During grinding, about 5 of calcium sulfate(gypsum or anhydride) is added to control setting time, strength development, and other properties. The major trend in man
28、ufacture of Portland cement has shifted to a greater emphasis on the reduction of the energy consumed for its production and increasing use of coal to replace gas and oil, which were the major fuels for burning the clinker. Energy consumption is generally greater for the wet process； therefore most
29、new plants use the dry process. The characteristics of the final product are not any different for either process. The worlds largest kiln (as of 1957) produced about 7500 tons (6750 metric tons) per day of clinker. An average kiln produces about 1800 tons (1620 metric tons) per day. The latest kiln
30、s utilize some form of preheating system, which fully utilizes the hot exit gases to warm the incoming raw materials； In addition, decarbonation of the limestone can be done on the raw feed prior to its entrance to the rotary kilns by use of auxiliary burners. These techniques enable much shorter ro
31、tary kilns for equal production and save much energy. Because of these developments, the worlds longest kiln (760 ft or 228 m long, 25 ft or 7.5 m in diameter) will probably remain the longest. Another trend is toward a newer type of grinding mill, called a roller mill. This mill can use waste heat
32、for drying, lends itself readily to automatic control, and uses less energy. These mills can grind up to 400 tons (360 metric tons) per hour. Several employees in a control room can operate a whole plant except for the quarry. Control is exercised by means of television monitors, sensors, computers, and automatic continuous chemical analysis. Other types of kilns which have been used or are in the process of being developed are vertical or shaft kilns, fluid-bed furnaces, and swirl calciners.
