Alite polymorphism

On present evidence, it is thus probable that the only alite polymorphs found in normal production clinkers are Mj, M, and, occasionally, T, and that those that have been described as R are really M,. However, both XRD and thermal evidence show that some alites formed in the presence of fluoride are rhombohedral (M7). XRD and optical evidence indicates that the commonest polymorph in production clinkers is almost certainly M the presence of which in such a clinker has also been demonstrated using single crystal electron diffraction (HI). 

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... 

Some of the optical characteristics of alite polymorphs and their correlative changes with differential thermal analysis (DTA) and X-ray diffraction (XRD) analysis have been the subjects of reports by Maki and Kato (1982) and Chromy and Carin (1980/81). 

Alite polymorphs R and Mji were said to represent endmembers of a continuous solid solution series, rather than discontinuous thermal polymorphs Mj and M3 of Maki and Chromy (1978). 

Figure 4.5 XRD scans of alite (C3S) polymorphs showing clear differences in characteristic peak profiles with changing crystal structure. The comparison illustrates a clear increase in the number of diffraction peaks with decreasing crystal symmetry from monoclinic to triclinic (M3 > T3 > T). In industrial portland cements, solid solution of magnesium and sulfate into the C3S structure normally leads to the stabilisation of monoclinic (MI and M3) alite polymorphs.

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. 

Fig. 1.2 Dependence of the polymorphic modification of alite in production clinkers on the MgO and SO, contents of the clinker. Maki and Goto (M6).

It is doubtful whether the polymorphs can be reliably distinguished by DTA, since both the M, and the M3 forms are metastable at low temperatures and transform to Tj at about 700°C (M4,M5). This effect is normally swamped by the endothermic p to aY transition of the belite. Due to the effect of substituents, neither T3 nor Mj appears to form on heating, and endothermic transitions occur at 790-850 C, giving M, and at 950 C, giving M3. These could occur irrespective of the form initially present however, clinkers high in MgO, in which the alite is present entirely as M3, normally show no thermal effects attributable to the alite until more than lOOO C, when transition to R occurs. 

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. 

Reactions during cooling. The liquid crystallizes, giving mainly aluminate and ferrite. Polymorphic transitions of the alite and belite occur. 

Alite may be expected to form in clinkers as the R polymorph, and belite as u l, ot n or a, according to the temperature. Chromy (C9) and Hofmanner (H21) discussed DTA curves of raw meals (Fig. 3.4). The main exotherm is due to belite formation, and the main endotherm to melting. A small... 

Fig. 2.28 Simultaneous influence of MgO and SO on stabilization of alite different polymorphic modification in industrial clinkers. (After [42])...

Different 38 phases, in the case of low content of belite in clinker, is most convenient to identify using light microscopy. Among others, Metzger [161] applying this method found the presence of phase a. The striations of belite crystals always observed in industrial ehnkers due to polysynthetic twinning, are distinctly differentiate this phase from alite crystals and are formed in the polymorphic transformation process. 

The medium-temperature polymorph is monoclinic and biaxial negative X is parallel to c (trigonal), y is parallel to a (trigonal), and Z is perpendicular to a (trigonal), with the latter two relationships less firmly established. The 2 Vis 20° to 60°. In horizontal sections, alite may show irregular patches of threefold cyclic twins formed in the trigonal to monoclinic inversion uniform extinction occurs in vertical sections. 

Triclinic alite, the low-temperature polymorph, is biaxial negative. The 2 Vranges from 20° to 60°, with X... 

Hirano and others (1991) combined microscopy and x ray diffraction to evaluate alpha and beta belite polymorphs. After treatment with a KOH sugar solution to remove the matrix, a heavy liquid (d=3.22) was used to separate alite from belite. Alpha C S was possibly detected microscopically and with x ray diffraction. 

Also in 1965, Ono, Uno, and Kanai reported data on five laboratory-made polymorphs of tricalcium silicate that persisted at room temperature. These authors described monoclinic, triclinic, and rhombohedral forms of alite. 

Segregation of belite, digitation, gamma polymorph, exsolution in alite, reduced areas with metallic inclusions and oldhamite (Ca, MnS) variation in phase percentages Assimilation of high silica and alumina coal ash (Kihara, 1988)... 

The use of X-ray diffraction (XRD) data can clearly be supportive to microscopy in almost all materials investigations. One obvious advantage of XRD analysis is that it is a reliable mode of identification and quantification for almost all of the major phases commonly seen in raw feed, clinker, and cement. However, XRD analysis suffers one major disadvantage it cannot tell you where, in the examined mass of clinker, the belite is located or how the belite is related to alite. XRD analysis can indicate the presence of belite and, with some calibrated standards, the amoimts of belite polymorphs, but only the microscope can tell you that the belite occurs mostly as decomposition of alite crystals or that the belite occurs only as inclusions in alite. XRD analysis may indicate the presence and quantity of more than... 

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