Alite phase

Aliphatic solvents, alkyllithium compounds and, 14 250-251 Aliphatic sulfonates, 26 145 Aliquot samples, 13 413-415 analysis of, 13 416 Aliskren, 5 158 Alitame, 12 42 24 232 Alite, phase in Portland cement clinker, 5 471, 472t, 473t Alitretinoin, 25 790 Alizarin, color of, 7 331 Alizarin derivatives, 9 337 Alizarin pure Blue B, 4 361t Alkadienes, metathesis of, 26 923 Alkali/alkaline-earth cation recognition,... 

Maki, I., and Chromy, S., "Characterization of the Alite Phase in Portland Cement Clinker by Microscopy," il cemento. No. 3, Luglio-Settembre, 1978a, pp. 247-254. 

Nagashima,M. Asakura,E. and Uda,S., "Influenceof Burning Conditions on the Alite Phase in Portland Cement Qinker," Reviews, 37th General Meeting, Cement Association of Japan, 1983, pp. 31-33. 

Generally, divalent and tiivalent cations enhance the effectiveness of these anions more than monovalent ions do. This category of additives affects mainly the hydration of the alite phase, thus accelerating the development of early strength. The setting time is also shortened, but less effectively than with admixtures that increase the pH of the hquid phase. 

The primary phases all contain impurities. In fact these impurities stabilize the stmctures formed at high temperatures so that decomposition or transformations do not occur during cooling, as occurs with the pure compounds. For example, pure C S exists in at least six polymorphic forms each having a sharply defined temperature range of stability, whereas alite exists in three stabilized forms at room temperature depending on the impurities. Some properties of the more common phases in Portland clinkers are given in Table 2. 

Calculate the overall Ca and Si contents (expressed as CaO and Si02) of a Portland cement clinker that has 55% alite, 30% belite, 5% alumi-nate phase, and 10% ferrite (assume ideal compositions for the latter two). Would you expect this to behave as a fast or a slow setting cement ... 

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. 

The aqueous phase in contact with Ca3Si05 and alite becomes saturated with... 

Alite is the most important constituent of all normal Portland cement clinkers, of which it constitutes 50-70%. It is tricalcium silicate (CajSiOj) modified in composition and crystal structure by incorporation of foreign ions, especially Mg ", AP and Fe. 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. 

The ferrite phase makes up 5 15% of normal Portland cement clinkers. It is tetracalcium aluminoferrile (CajAIFeOj) substantially modified in composition by variation in Al/Fe ratio and incorporation of foreign ions. The rale 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 ages. 

A number of experimentally determined XRD powder patterns have been reported for CjS and alite polymorphs (PI), but because of uncertainties in the indexing and sometimes in identification of the polymorph, these are usefully supplemented by patterns calculated from the crystal structures (Appendix). The patterns of the polymorphs are closely alike, and in clinkers, interpretation is complicated by the fact that many peaks overlap ones of other phases. The pattern depends not only on the polymorph or... 

XRD powder evidence shows that in the majority of clinkers the belite is predominantly or entirely of P-CjS structure (GI,YI), though some peaks are broadened (Gl) and the presence also of both a and a (presumably u l) forms has been reported (GI,Yl,RI,OI). Characterization of the polymorphic form is rendered difficult by the similarities between their powder patterns (Fig. 1.5) and by overlaps between the peaks and ones of other phases, especially alite, but has been aided by examination of fraetions in which the belite has been concentrated by chemical (Rl) or heavy liquid (Yl) separation. 

Table 2.3 lists some phases containing MgO that are in varying degrees relevant to cement chemistry. It is not a complete list of phases with essential MgO in the CaO-MgO-AljOj-SiOj system. As seen in Chapter 1, some MgO is also taken up by all four of the major clinker phases, typical contents being 0.5-2.0% for alite, 0.5% for belite, 1.4% for the aluminate phase, and 3.0% for the ferrite phase. Magnesium oxide (periclase), like calcium oxide, has the sodium chloride structure it is cubic, with a = 0.4213 nm, space group Fm3m, Z = 4, = 3581 kgm (S5) and refrac-... 

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. 

In principle, the clinker liquid can behave in any of three ways during cooling, with intermediate possibilities. In the first, equilibrium between the liquid and the pre-existing solid phases (alite and belite) is continuously maintained. This implies the possibility of material transfer between these solids and the liquid in either direction for as long as any liquid remains. We shall call this situation equilibrium crystallization . In the second, the liquid does not crystallize but forms a glass. In the third, the liquid crystallizes independently, i.e. without interacting with the solid phases already present. Intermediate modes include, for example, equilibrium crystallization down to a certain temperature and independent crystallization below it. 

All the effects described above indicate that rapid cooling is desirable the aluminate phase reacts more slowly with water when finely grained and intimately mixed with ferrite, making it easier to control the setting rate (S24), decrease in alite content either from reactions involving the interstitial material or from decomposition is avoided, a higher MgO content can be tolerated, and the clinker is easier to grind. 

Much of the SO3 is present at the clinkering temperature in a separate liquid phase, immiscible with the main clinker liquid. The alkali cations are distributed between the two liquids and the alite and belite. During cooling, some redistribution of alkali cations and sulphate ions between the liquids may be expected to occur, the sulphate liquid finally solidifying below 900 C to yield alkali or potassium calcium sulphates. 

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