Clinker phases formation

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. 

Flame formation must be favorable to a dense, stable coating on the brick refractory lining in the burning zone of the kiln as well as to a nodular clinker formation with low dust content and correctly developed clinker phases. 

When comparing the influence of the polymers, the most apparent result is the incomplete consumption of clinker phases in the c/PVAc samples, whereas in the c/PVA samples, the clinker phases are consumed in less then four days. It should be noted that the consumption of the clinker phases is the lowest for c/PVAc in pure water. The reason for this is the above mentioned hydrophobic nature of PVAc, which requires alkaline pH values for saponification before the polymer can be dissolved. Thus, the XRD measurements are consistent with the NMR measurements. Additionally, approx. 10% more amorphous material is formed in the case of c/PVAc. This is again a result of the hydrophobic nature of PVAc. As discussed in the NMR section, c/PVAc swells at a lower rate than c/PVA. This limits the access of water to the clinker phases, which hampers the formation of larger crystallite sizes. 

The influence of reducing atmosphere on phase composition and clinker properties was studies by several authors [76-81]. They state unanimously the gradual iron reduction, which leads to ferrite phase decomposition with C3A and free CaO formation. Fe can substitute Ca in all clinker phases, obviously in accordance with its solubility in individual phases. Frequently the tendency of clinker to seU-powdering, linked with Y-C2S formation is listed. 

Fig. 2.43 Enthalpy of elinkering process 1, 3, 5, 7, P heating of raw mix, 2 dehydroxylation kaolinite, 4 decarbonisation of CaCOj, 6 mullite crystallization, 8 melt formation, 10 crystallization of clinker phases, 11 clinker cooling, 12 cooling of gaseous CO 13 eooling of water vapour...

The reaction of clinker phases with water, together with the crystalUzation of hydrates, brings about the substantial changes of rheological properties of the paste. The formation of hydrated phases and, according to Locher [28], the morphology of hydrates crystallized in the liquid phase or on the surface of cement grains, leads to the reduction of distance between them and the viscosity of paste is enhanced. At... 

Dreizler, I. Strunge, J. and Knbfel, D., "Effect of Alkalies and Sulfur on the Formation of Clinker Phases and on the Cement Properties," Proceedings ofthe Seventh International Conference on Cement Microscopy, International Cement Microscopy Association, Ft. Worth, Texas, 1985, pp. 100-126. 

Blanco-Varela, M.T, Palomo, P., and Vazquer, T. (1984) Effect of fluorspar in the formation of clinker phases. Cement and Concrete Research 14,397 06. 

Some indication of the respectively contributions of the clinker phases to the strength development of cement is given in Fig. 9. However, these results obtained for individual phases cannot be directly applied to the conditions actually occurring in cement paste, as is apparent also from the heat of hydration values given in Table 10. Fig. 17 schematically shows the sequence of formation of the hydrate phases and the structure development in the setting and hardening of Portland cement. 

Mineralizers are added to the raw feed to aeeelerate the kinetics of reactions by modifying the solid and liquid state sintering. The temperature of decomposition of calcium carbonate is lowered in the presence of mineralizers. In the synthesis of C3S and C2S, DTA has shown that some phosphates, earbonates, sulfates, and ehlorides deerease the deearbonation temperature and that of the formation of the silieates.[ °l Caleium fluoride acts both as a flux and a mineralizer in promoting the formation the tricalcium silicate phase. DTA thermograms have been applied to study the effect of mineralizers on the melting behaviors and crystallization temperatures of clinker. Early formation of liquid can be established by thermal techniques. 

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. 

U.S. imports for consumption, 4 528t Calcium(II), concentration formation constant of chelates, 5 717t 12-Calcium-7-aluminate, phase in Portland cement clinker, 5 472t Calcium absorption, 26 292 Calcium addition, in silicon production, 22 505-506... 

Portland cement, which was discovered in 1824, sets from within. It is made by firing limestone ( 75%) with clays at about 1450 °C, causing partial fusion and the formation of a clinker, which is then powdered. This material typically contains the equivalents of about 67% CaO, 22% SiC>2, 5% AI2O3, 3% Fe2C>3, and the balance other materials, and the four chief components are the following anhydrous phases ... 

Supercritical C02 treatment affects the microstructure of the cement paste. In the first stage of the sc C02 treatment, free water in the cement pores is extracted. As a consequence of this dehydration process, channels of about 50-pm diameter develop. Dissolved calcium in the free water reacts with the C02 and crystallizes with the C02 as calcite along the channel walls. In the second stage, the structural water of the hydrated cement phases is extracted. The carbonation of the portlandite to form more calcite takes place. Water, bound to the CSH surrounding the partially hydrated cement clinker particles, is partially replaced by a carbonate formation. The short fibers of the CSH-cement framework, which are responsible for the physical properties of the cement, are not affected (Hartmann et al., 1999). 

The relative reactivity of the different mineral phases of cement with water is usually given as C A>C S>C S>C AF. Aluminate phases and their hydration products therefore play an important role in the early hydration process. Because of the high reactivity of calcium aluminate, the aluminate hydration reaction is carried out in the presence of sulfate ions. The latter provide control of the reaction rate through the formation of mixed aluminum sulfate products (ettringite and monosulfoaluminate) Calcium sulfate which is added to the cement clinker hence controls the properties of the aluminate hydration products. Sulfates thus play a crucial role in cement hydration and the influence of chemical admixtures on any process where sulfates are involved may be expected to be significant [127],... 

Pollitt and Brown (P2) were unable to prepare an analogue of the orthorhombic phase with as the sole substituent, but obtained evidence that could stabilize it in clinker, probably because other substituents were also present. Maki (M12) also failed to prepare K forms of the orthorhombic or monoclinic phases under equilibrium conditions, but by moderately rapid cooling of melts he obtained orthorhombic crystals having cell parameters close to those of the corresponding sodium-containing phase. He considered that the presence of silicon in the clinker liquid would favour supercooling and thereby also non-equilibrium formation of the orthorhombic or monoclinic phase. 

Production clinkers have been found to contain cubic or orthorhombic forms of the aluminate phase, alone or in combination. The monoclinic modification has not been observed. The orthorhombic modification is also known as the prismatic, dark interstitial material, and is sometimes pseudo-tetragonal. It can arise only if sufficient alkali is available, but its formation appears to be favoured also by rapid cooling and by bulk compositions potentially able to yield a relatively high proportion of aluminate phase (M12). 

Fig. 3.1 Schematic diagram showing the variations in typical contents of phases during the formation of Portland cement clinker. Loosely based on a figure by Wolter (W9).

Nodulization, chemical reactions and evaporation of volatiles are interdependent. Alite formation and loss of volatiles affect nodulization, and the compaction that the latter entails must affect the kinetics of the former processes. Alite formation thus occurs in a changing environment. Initially, the clinker liquid is not a continuous and uniform medium enveloping the grains oflime, belite and other solid phases rather, it fills separate pores and capillaries, forming thin films on the particles, and there are large local variations in composition (Tl). As compaction proceeds, it becomes more continuous and more uniform. 

Some limestones contain substantial proportions of SrO. Small contents favour alite formation, but phase equilibrium studies show that it increases the range of compositions at which free lime, with SrO in solid solution, is present at the clinkering temperature (B34,G27,G28). This limits the permissible content of SrO in mixes of normal LSFs to about 3%. 

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