Clinker phase composition

In this section the departure from equihbrium state hnked with too low fineness of raw mix components or with insufficient homogenization, thus with lack of homogeneity of this mixture will be not discussed. They will be discussed in Sect. 2.3. This section will be limited to the lack of equihbrium in the crystallization process on clinker phase composition. 

Great changes in clinker phase composition are caused by the dropping downwards of coal ash on clinker in the kiln. Using of fuel rich in ash provides a worsening of... 

Clinker phase composition has decisive influence on several properties of cement paste, thus also on concrete. The most important are the following ... 

In the last two decades the unquestionable superiority in clinker phase composition has gained XRD. The inner standard is usually used, frequently mtile or comn-dnm. The last one has advantage that it does not coincidence with clinker phases peaks in the range of 20 till 60°. Taylor [218] states that this method is not useful for belite determination, because the coincidence of its peaks with alite, which cause that the weak ones must be used. 

Numerous experimental works concerned very different clinker phase compositions, which are presented in Table 9.4. 

Slota, R. J., and Lewandowska-Kanas, A. (1986) Modification of clinker phase composition and strength properties of a high-alkali cement clinker by gypsum additions, in Proceedings 8th ICCC, Rio de Janeiro, Vol. 2, pp. 205—210. 

ASTM C845 Type E-I (K) expansive cement manufactured ia the United States usually depends on aluminate and sulfate phases that result ia more ettriagite formation duriag hydration than ia normal Portland cements. Type K contains an anhydrous calcium sulfoaluminate, C A SI. This cement can be made either by iategraHy burning to produce the desired phase composition, or by intergrinding a special component with ordinary Portland cement clinkers and calcium sulfate. 

Portland cement clinker potential phase composition is presented in Table 4. It could be seen that the C3A content in the clinker was 9.46% which is important for the cement hydration rate and cement sulfate resistance. Common Portland cement is not resistant to the sulfate influence because of the significant C3A content, whose hydrates react with sulfate ions resulting in expansive compounds. Portland cement with the higher resistance to sulfates must have low C3A content. Moderate to high content of mineral alite - C3S (54.72%) is usual for the Serbian cement plants and enables the addition of higher quantities of mineral admixtures without influencing the quality of final cement. 

Table 4 Potential phase composition of Portland cement clinker...

In many clinkers, the ferrite phase is closely mixed with aluminate due to a similarity in cell parameters, oriented intergrowth can occur (MIS). The close admixture often renders X-ray microanalysis difficult or unreliable. For ordinary Portland cement clinkers, the compositions found in dilferent laboratories are nevertheless remarkably consistent. Table 1.2 includes an average value based on the results of investigations using X-ray microanalysis (H8,K1,B2,U1,H3,B4) or chemical analysis of separated material (Yl). Table 1.3 includes suggested site occupancies corresponding to these data. 

A knowledge of the relevant high-temperature phase equilibria is necessary for understanding the factors that govern acceptable bulk compositions for Portland cement clinker, the conditions under which the latter can be manufactured, and the phase composition and microstructure of the resulting material. This chapter deals with these equilibria and with the phases to which they relate, with the exception of the major clinker phases, which were described in Chapter I. Some anhydrous phases primarily of interest in relation to other types of cement are also considered here. Principles underlying the preparation of anhydrous silicate, aluminate and other high-temperature phases are outlined. 

Swayze assumed that, for the bulk compositions that he studied, all the MgO was in the liquid phase at equilibrium, apart from the small proportions present as periclase. As noted in the preceding section, all four of the major clinker phases take up significant proportions of MgO. The contents of the latter component in the liquid phase were therefore probably substantially lower than 5%. 

Fig. 3.1 shows these changes for a typical clinker. No attempt has been made to show a detailed sequence of phases below 1300°C, as sufficient data do not exist, and minor phases, including sulphates, are omitted. Quantitative phase compositions at various stages vary considerably with starting materials and other factors. 

The potential uses of XRD powder diffraction in the study of clinker or anhydrous cement include the qualitative and quantitative (QXDA) determination of phase composition, and the determination of polymorphic modification, state of crystallinity and other features of individual phases. In principle, information on compositions of phases is obtainable through cell parameters, but, due to the lack of adequate reference data, XRD is generally less satisfactory for the clinker phases than X-ray microanalysis. Table 4.2 gives the pattern of a typical Portland cement, with indications of the assignments of peaks to phases. 

For a calculation of the quantitative phase composition of a clinker from the bulk chemical analysis to give correct results, the following conditions are necessary and sufficient ... 

The calculation of phase composition from bulk analysis for cements presents no special difficulties if the content and composition of the gypsum and any other admixture are known, as the analysis can then readily be corrected to give the composition of the clinker. If they are not known, an assumption must be made as to the amount of SOj in the clinker, so that the contribution of the gypsum to the CaO content of the cement may be estimated. Further assumptions and corrections may have to be made to allow for impurities in the gypsum or other additives (e.g. calcite). 

The experimental considerations applying to calcium silicate pastes (Sections 5.1 and 5.2) are equally relevant to cement pastes. Of the methods so far used in attempts to determine the degrees of reaction of the individual clinker phases as a function of time, QXDA (C39,D12,T34,P28) has proved much the most satisfactory. Procedures are essentially as for the analysis of a clinker or unreacted cement (Section 4.3.2), but it is necessary to take account of overlaps with peaks from the hydration products, and especially, with the C-S-H band at 0.27-0.31 nm. The water content of the sample must be known, so that the results can be referred to the weight of anhydrous material. If a sample of the unhydrated cement is available, and its quantitative phase composition has been determined, it may be used as the reference standard for the individual clinker phases in the paste. 

Water retained after D-drying, known as non-evaporable water, has often been wrongly identified with chemically bound water. It excludes much of the interlayer water in C-S-H, AFm and hydrotalcite-type phases and much of the water contained in the crystal structure of AFt phases. It is often used as a measure of the fraction of the cement that has reacted, but can only be approximate in this respect, because the clinker phases react at different rates and yield products containing different amounts of non-evaporable water. Fully hydrated cement pastes typically contain about 23% of non-evaporable water, referred to the ignited weight. Copeland et al. (C38) determined the non-evaporable water contents of a series of mature cement pastes and carried out regression analyses on the cement composition. For pastes of w/c ratio 0.8 and aged 6.5 years, they obtained the approximate expression ... 

Major influences on the kinetic curve of a cement include the phase composition of the clinker, the particle size distribution of the cement and the RH and temperature regimes during curing. Other influences include the w/c ratio, the content and distribution of admixtures, including gypsum, the reactivities of individual clinker phases and probably others, such as the microstructures of the clinker and of the cement particles. 

Uchikawa (UI7) reviewed the hydration chemistry of pfa and other composite cements. Pfa cements differ from pure Portland cements notably in (i) the hydration rates of the clinker phases, (ii) CH contents, which are lowered both by the dilution of the clinker by pfa and by the pozzolanic reaction, (iii) the compositions of the clinker hydration products and (iv) formation of hydration products from the pfa. The two last aspects cannot be wholly separated. 

Quantitative data on the rate of consumption of pfa are few and somewhat variable. Those based on differences between the CH contents of pure Portland and pfa cements are suspect, because the calculation involves the effects of pfa substitution both on the rate of consumption of the clinker phases and on the compositions of the products, which are not fully understood. Unreacted pfa has been directly determined by dissolution of the other phases with HCl (C43) or with salicylic acid in methanol followed by HCl (T44), chemical separation of the residual pfa followed by QXDA determination of its content of crystalline phases (D12) and a trimethylsily-lation method (U19). A method based on EDTA extraction was found unsatisfactory (L46). 

Parallel with cement production development the significant progress in cement chemistry was achieved. The real revolution we owe to French scientist Le Chat-eher. This great chemist determined the phase composition of Portland cement clinker and the hypothesis of hydration process. Le Chatelier stated that, similarly as in the case of gypsum, the anhydrite cement components dissolve, the solution became oversaturated in relation to hydrates, which causes their crystallization [3],... 

The chemical composition of clinker is complex, however, it is easy to notice that the sum of four components, CaO, Si02, AI2O3 and Fe203 is as a rule higher than 95 %. The processes of Portland cement clinker phases ciystallization in the four components system can thus be presented. It is especially justified that MgO is not forming own compounds in the rich in calcium part of five-components system, but solid solutions with remaining clinker phases, or is present as periclase. However, the four-components system is seldom used, because the use of ternary systems is much more convenient. They are usually presented as the horizontal projection. 

---