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1、35 35 Concrete, made from cement, aggregates, chemical admixtures, mineral admixtures, and water, comprises in quantity the largest of all synthesized materials. The active constituent of concrete is cement paste and the performance of concrete is largely determined by the nature of the cement paste
2、. Admixtures are chemicals that are added to concrete for obtaining some beneficial effects such as better workability, strength, durability, acceleration, retardation, air entrainment, water reduction, plas- ticity, etc. Mineral admixtures, such as blast furnace slag, fly ash, silica fume, and othe
3、rs, are also incorporated into concrete to improve its quality. The performance of concrete depends on the quality of the ingre- dients, their proportions, placement, and exposure conditions. For example, the quality of the raw materials used for the manufacture of clinker, the calcining conditions,
4、 the fineness and particle size of the cement, the relative proportions of the cement phases, and the amount of mixing water influence the physico-chemical behavior of the hardened cement paste in concrete. In addition, the cement type, nature of fine and coarse aggregates, water, temperature of mix
5、ing, admixture, and the environment will deter- mine the physical, chemical, and durability aspects of concrete. Thermal analysis techniques are widely applied to investigate the physico-chemical behaviors of cement compounds, cement, and concrete subjected to various conditions. 2 Introduction to P
6、ortland Cement Concrete 36Chapter 2 - Introduction to Portland Cement Concrete Although added in small amounts, admixtures may influence many of the properties of concrete, from the time water comes into contact with the dry ingredients of concrete, to its long term behavior. Generally, concrete con
7、tains one or more admixtures and their role has been studied extensively by thermal techniques and, hence, a separate chapter is devoted to describe their application. 1.0PRODUCTION OF PORTLAND CEMENT According to ASTM C-150 portland cement is a hydraulic cement produced by pulverizing clinker consi
8、sting essentially of hydraulic calcium silicates, usually containing one or more types of calcium sulfate as an interground addition. The raw materials for the manufacture of portland cement contain, in suitable proportions, silica, aluminum oxide, calcium oxide, and ferric oxide. Since the compound
9、s in cement science are complex, a simplified representation is often used in cement nomenclature: C = CaO; S = SiO2; A = Al2O3; F = Fe2O3; H = H2O; S = SO 3; K = K2O; and N = Na2O. In addition, “W” or “w” represents water, “C” or “c,” cement, and “S” or “s,” solid. Thus, “W/C” or “w/c” is water:cem
10、ent ratio and “W/S” or “w/s” is water:solids ratio. A source of lime is provided by calcareous ingredients such as limestone or chalk and the source of silica and aluminum oxide being shales, clays, or slates. The iron bearing materials are iron and pyrites. Ferric oxide not only serves as a flux bu
11、t also forms compounds with lime and alumina. The raw materials also contain small amounts of other compounds such as magnesia, alkalis, phosphates, fluorine compounds, zinc oxide, and sulfides. The cement clinker is produced by feeding the crushed, ground, and screened raw mix into a rotary kiln an
12、d heated at a temperature of about 13001450C. Approximately 11001400 kcal/g of energy is consumed in the formation of clinker. The sequence of reactions is as follows. At a temperature of about 100C (drying zone), free water is expelled. In the pre-heating zone (100750C), firmly bound water from the
13、 clay is lost. In the calcining zone (7501000C), calcium carbonate is dissociated. 37 In the burning zone (10001450C), partial fusion of the mix occurs, with the formation of C3S, C2S, and clinker. In the cooling zone (14501300C), crystallization of melt occurs with the formation of calcium aluminat
14、e and calcium aluminoferrite. After firing the raw materials for the required period, the resultant clinker is cooled and ground with about 45% gypsum to a specified degree of fineness. Grinding aids, generally polar compounds are added to facilitate grinding. 2.0COMPOSITION The major phases of port
15、land cement are: Tricalcium silicate (3CaOSiO2) Dicalcium silicate (2CaOSiO2) Tricalcium aluminate (3CaOAl2O3) Ferrite phase of average composition (4CaOAl2O3Fe2O3) In a commercial clinker these phases do not exist in a pure form. The 3CaOSiO2 phase is a solid solution containing Mg and Al and is ca
16、lled alite. In the clinker, it consists of monoclinic or trigonal form, whereas synthesized 3CaOSiO2 is triclinic. Alite is the most important constituent of normal portland cement, constituting 5060% and promoting strength development. The 2CaOSiO2 phase occurs in the (belite) forms and contains, i
17、n addition to Al and Mg, some K2O. Four forms, , , , and , of C2S are known although in clinker only the form with a monoclinic unit cell exists. It reacts slowly with water and contributes little to strength development in the first 28 days. At one year, pure alite and belite yield the same strengt
18、hs. The aluminate phase C3A constitutes 412% in most portland cements and is substantially modified by ionic substitution. In some clinkers small amounts of calcium aluminate of formula NC8A3 may also form. The ferrite phase, designated C4AF, is a solid solution of variable composition from C2F to C
19、6A2F. Potential components of this compound are C2F, C6AF2, C4AF, and C6A2F. The MgO content in cement is usually limited to 45% because in the form of crystalline periclase it may cause slow expansion. Free lime behaves similarly. Excessive SO3 can also lead to expansion. Alkalis such as K2O and Na
20、2O in excess of 0.6% equivalent Section 2.0 - Composition 38Chapter 2 - Introduction to Portland Cement Concrete Na2O are not permitted as they promote expansion with certain types of aggregates. ASTM C-150 describes five major types of portland cement. They are: NormalType Iis used for most purpose
21、s and when special properties specified for any other type are not required Moderate Sulfate Resistant or Moderate Heat of Hydra- tionType II High Early StrengthType III Low HeatType IV Sulfate ResistingType V The general composition, fineness, and compressive strength characteris- tics of these cem
22、ents are shown in Table 1.131 Portland cement may be blended with other ingredients to form blended hydraulic cements. ASTM C-595 covers five kinds of blended hydraulic cements. The portland blast furnace slag cement consists of an intimately ground mixture of portland cement clinker and granulated
23、blast furnace slag or an intimate and uniform blend of portland cement and fine granulated blast furnace slag in which the slag constituent is within specified limits. The portland-pozzolan cement consists of an intimate and uniform blend of portland cement or portland blast furnace slag cement and
24、fine pozzolan. The slag cement consists mostly of granulated blast furnace slag and hydrated lime. The others include pozzolan-modified portland cement (pozzolan C3S C2S C3A. At 14 days, the relative values were in the order: C3S C4AF C2S C3A. The Bogue-Lerch strength values both at 10 and 14 days w
25、ere: C3S C2S C3A C4AF. At one year, the corresponding values were: C3S C2S C4AF C3A (Beaudoin-Ramachandran) and C3S C2S C3A C4AF (Bogue-Lerch). Comparison of strengths as a function of the degree of hydration revealed that at a hydration degree of 70100%, the strength was in the decreasing order: C3
26、S C4AF C3A. Figure 4. Compressive strength of hydrated cement compounds. (Reprinted from Beaudoin, J. J., and Ramachandran, V. S., Cement and Concrete Res., 22:689694, 1992, with kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kiddlington OX51GB.) Section 4.0 - Behaviors of
27、Individual Cement Minerals 48Chapter 2 - Introduction to Portland Cement Concrete 5.0HYDRATION OF PORTLAND CEMENT Although hydration studies of the pure cement compounds are very useful in following the hydration processes of portland cement itself, they cannot be directly applied to cements because
28、 of complex interactions. In portland cement, the compounds do not exist in a pure form but are solid solutions containing Al, Mg, Na, etc. The rate of hydration of alites containing different amounts of Al, Mg, or Fe has shown that at the same degree of hydration Fe-alite shows the greatest strengt
29、h. There is evidence the C-S-H formed in different alites is not the same in composition.12 The hydration process of C3A, C4AF, and C2S in cement is affected because of changes in the amounts of Ca2+ and OH- in the hydrating solution. The reactivity of C4AF can be influenced by the amount of SO42- i
30、ons consumed by C3A. Some SO42- ions may be depleted by being absorbed by the C-S-H phase. Gypsum is also known to affect the rate of hydration of calcium silicates. Significant amounts of Al and Fe are incorporated into the C-S-H structure. The presence of alkalis in portland cement also has an inf
31、luence on the hydration of the individual phases. It is generally believed that the rate of hydration in the first few days of cement compounds in cements proceeds in the order of C3A C3S C4AF C2S. The rate of hydration of the compounds depends on the crystal size, imperfections, particle size, part
32、icle size distribution, the rate of cooling, surface area, the presence of admixtures, the temperature, etc. After ninety days, little or no alite or aluminate phase is detectable. Quantitative x-ray diffraction has been used to determine the degree of reaction of individual cement compounds present
33、 in cement. Some errors in these estimations are recognized. Figure 5 shows the fractional amounts of alite, belite, aluminate, and ferrite phases that hydrate in cement when hydrated for different times.3 These rates are not the same when the individual compounds are hydrated. In a mature hydrated
34、portland cement, the products formed are C-S-H gel, Ca(OH)2, ettringite (AFt phase), monosulfate (AFm phase), hydrogarnet phases, and possible amorphous phases high in Al+ and SO4 ions. A small amount of cryptocrystalline CH may be intimately mixed with C-S-H phase. The C-S-H phase in cement paste i
35、s amorphous or semicrystalline calcium silicate hydrate, the hyphens denoting that the gel does not necessarily consist of 1:1 molar CaO:SiO2. The C-S-H phase of cement pastes gives powder patterns very similar to that of C3S pastes. The 49 composition of C-S-H (in terms of C/S ratio) is variable de
36、pending on the time of hydration. At 1 day, the C/S ratio is about 2.0 and 1.41.6 after several years. The C-S-H can take up substantial amounts of Al3+, Fe3+, and SO42- ions. Figure 5. Fraction of cement phases hydrated in cement pastes at different times. Section 5.0 - Hydration of Portland Cement
37、 Recent investigations have shown that in both C3S and portland cement pastes, the monomer that is present in the C3S and C2S compounds (SiO44- tetrahedra) polymerizes to form dimers, and larger silicate ions as hydration progresses. The gas liquid chromatographic analysis of the trimethyl silylatio
38、n derivatives has shown that anions with 3 or 4 Si atoms 50Chapter 2 - Introduction to Portland Cement Concrete are absent. The polymer content with five or more Si atoms increases as the hydration proceeds and the amount of dimer decreases. In C3S pastes the disappearance of monomer results in the
39、formation of polymers. In cement pastes even after the disappearance of all C3S and C2S, some monomer is detected possibly because of the modification of the anion structure of C- S-H through replacement of some Si atoms by Al, Fe, or S. Admixtures can influence the rate at which the polymerization
40、occurs in portland cement and C3S pastes. The minimum water:cement ratio for attaining complete hydration of cement has been variously given from 0.35 to 0.40, although complete hydration has been reported to have been achieved at a water:cement ratio of 0.22.13 In a fully hydrated portland cement C
41、a(OH)2 consti- tutes about 2025% of the solid content. The ettringite group, also called AFt phase in cement paste, stands for Al-Fe-tri (tri = three moles of CS) of the formula C3A3CS H 32 in which Al can be replaced by Fe to some extent. The AFt phase forms in the first few hours (from C3A and C4A
42、F) and plays a role in setting. After a few days of hydration only a little amount of it may remain in cement pastes. It appears as stumpy rods in SEM and the length does not normally exceed a few micrometers. The principle substitutions that exist in the AFt phase are Fe3+ and Si4+ for Al3+ and var
43、ious anions such as OH-, CO32-, and silicates for SO42-. The monosulfate group, also known as the AFm phase, is repre- sented by the formula C4AS H 12 or C3ACS H 12. AFm stands for Al-Fe- mono, in which one mole of CS is present. In portland cement this phase forms after the AFt phase disappears. Th
44、is phase may constitute about 10% of the solid phase in a mature cement paste. In SEM, this phase has a hexagonal morphology resembling that of Ca(OH)2 and the crystals are of sub-micrometer thickness. The principle ionic substitutions in the AFm phase are Fe3+ for Al3+, and OH-, CO32-, Cl-, etc. fo
45、r SO42-. The density of this phase is 2.02 g/ml. The amount of crystalline hydrogarnet present in cement paste is less than 3%.14 It is of the type Ca3Al2(OH)12 in which part of Al3+ is replaced by Fe3+ and 4OH- by SiO44- e.g., C3(A0.5F0.5)SH4. It may be present in small amounts in mature cement pas
46、tes and is also formed at higher temperatures. The crystal structure of this phase is related to C3AS 3 (garnet). Hydrogarnet is decomposed by CO2 with the formation of CaCO3 as a product. It is the opinion of some workers that the lowest sulfate form of calcium sulfohydroxyaluminate hydrate, a crys
47、talline solid solution phase in the system CaO-Al2O3-CaSO4-H2O, is also formed in cement pastes. 51 The mechanisms that have already been described for pure cement compounds form a basis for a study of the hydration mechanism of portland cement. The conduction calorimetric curves of C3S and portland
48、 cement are similar except portland cement may yield a third peak for the formation of monosulfate hydrate (Fig. 1). The detailed influence of C3A and C4AF on the hydration of C3S and C2S in cement is yet to be worked out. The delayed nucleation models and the protective layer models, taking into ac
49、count the possible interactions, have been reviewed.8 Although the initial process is not clear for C3S (in cements), it appears that C3A hydration products form through solution and topochemical processes. 6.0PROPERTIES OF CEMENT PASTE 6.1Setting A mixture of cement and water, mixed in certain proportions, causes setting and hardening, and it is called a cement paste. The stiffening times of cement paste or mortar fraction are deter- mined by setting times. The setting characteristics are assessed by initial