Unit 18 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 1450°C. Partial fusion occurs, and nodules of clinker are produced. The clinker is mixed with a few percent of calcium sulfate and finely ground, to make the cement. The calcium sulfate controls the rate of set and influences the rate of strength development. It is commonly described as gypsum, but this may be partly or wholly replaced by other forms of calcium sulfate. Some specifications allow the addition of other materials at the grinding stage. The clinker typically has a composition in the region of 67% CaO, 22% SiO2, 5% Al2O3, 3% Fe2O3 and 3% other components, and normally contains four major phases, called alite, belite, aluminate and ferrite. Several other phases, such as alkali sulfates and calcium oxide are normally present in minor amounts. Hardening results from reactions between the major phases and water.
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 ionic substitutions. It reacts relatively quickly with water, and in normal Portland cements 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 clinkers. It is dicalcium silicate (Ca2SiO4) modified by ionic substitutions and normally present wholly or largely as the β 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 strengths obtainable from pure alite and pure belite are about the same under comparable conditions.
Aluminate constitutes 5%~10% of most normal Portland cement clinkers. It is tricalcium aluminate (Ca3Al2O6), substantially modified in composition and sometimes also in structure by ionic substitutions. It reacts rapidly with water, and cause undesirably rapid setting unless a setcontrolling agent, usually gypsum, is added.
Ferrite makes up 5%~15% of normal Portland cement clinkers. It is tetracalcium aluminoferrite (Ca2AlFeO5), substantially modified in composition by variation in Al/Fe ratio and ionic substitutions. The rate at which it reacts with water appears to be somewhat variable, pherhaps due to differences in composition or other characteristics, but in general is high initially and low or very low at later ages.
Types of Portland Cement
The great majority of Portland cements made throughout the world are designed for general constructional use. The standard 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 Class 42.5 Portland cement in current European and British standards (42.5 is the minimum 28-days compressive strength in MPa), Type I and II Portland cement in the ASTM (American Society for Testing and Materials) specifications used in the USA, or Ordinary Portland Cement (OPC) in former British standards. Throughout this book, the term 'ordinary' Portland cements is used to distinguish such general-purpose cements from other types of Portland cement, which are made in smaller quantities for special purposes.
Standard 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 MgO is usually limited to 4%~5%, because quantities of this component in excess of about 2% can 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. Excessive contents of SO3 can also cause expansion, and upper limits, typically 3.5% for ordinary Portland cements, are usually imposed. Alkalies (K2O and Na2O) can undergo expansive reactions with certain aggregates, and some specifications limit the content, e.g. to 0.6% equivalent Na2O (Na2O+0.66K2O). Other upper limits of composition widely used in specifications relate to matter insoluble in dilute acid, and loss ignition. Many other minor components are limited in content by their effects on the manufacturing process, or the properties, or both, and in some cases the limits are defined in specifications.
Rapid hardening Portland cements have been produced in various ways, such as varying the compositon to increase the alite content, finer grinding of the clinker, and improvements in the manufacturing process, e.g. finer grinding or better mixing of the raw materias. The alite contents of Portland cements have increased steadily over the one and a half centuries during which the latter have been produced, and many cements that would be considered ordinary today would have been described as rapid hardening only a few decades ago. In ASTM specifications, rapid hardening Portland cements are called high early strength or Type III cements. For both ordinary and rapid hardening cements, both lower and upper limits may be imposed on strength at 28 days, upper limits being a safeguard against poor durability resulting from the use of inadequate cement contents in concrete.
Destructive expansion from reaction with sulfates can occur not only if the latter are present in excessive proportion in the cement, but also from attact on concrete by sulfate solutions. The reaction involves the Al2O3 containing phases in the hardened cement, and in sulfate resisiting Portland cements its effects are reduced by decreasing the proportion of the aluminate phase, sometimes to zero. This is achieved by decreasing the ratio of Al2O3 to Fe2O3 in the raw materials. In the USA, sulfate resisting Portland cements are called 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 colour of Portland cement is due to the ferrite, formation of which in a white cement must thus be avoided. It is impracticable to employ raw materials that are completely free from Fe2O3 and other components are therefore usually minimized by producing the clinker under slightly reducing conditions and by rapid quenching. In addition to alite, belite and aluminate, some glass may be formed.
The reaction of Portland cement with water is exothermic, and while this can be an advantage under some conditions because it accelerates hardening, it is a disadvantage under others, such as in the construction of large dams or in the lining of oil wells, when a cement slurry has to be pumped over a large distance under pressure and sometimes at a high temperature. Slower heat evolution can be achieved by coarser grinding, and decreased total heat evolution by lowering the contents of alite and aluminate. The ASTM specifications include definitions of a Type II or 'moderate heat of hardening' cement, and a more extreme Type IV or 'low heat' cement. The Type II cement is also suitable for conditions exposed to moderate sulfate attack, and is widely used in general construction work. Heating evolution can also be decreased by partially replacing the cement by flyash (pulverized fuel ash; pfa) or other materials, and this is today a widely used solution. The specialized requirements of oil well cements are discussed in Section 11.8.
Selected from "Cement Chemistry" H. F. W. Talor, 2nd Edition. Thomas Talford Publ. 1997
Words and Expressions
Portland Cement 硅酸盐水泥，波特兰水泥
clinker 熟料
calcium sulfate 硫酸钙
gypsum 石膏
alite 硅酸三钙石
belite 二钙硅酸盐
ferrite 铁铝酸四钙
tricalcium silicate 硅酸三钙
standard specification 标准规范
comply 遵照
specific surface area 比表面积
setting time 凝结时间
compressive strength 抗压强度
periclase 方镁石
destructive expansion 破坏性膨胀
concrete 混凝土
aggregate 骨料
exothermic 放热的
