Alite solid solution

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%. 

Portland cement clinker is composed of fora principal phases alite, which is close to tricalcium orthosilicate Ca3[Si04]0, belite close to dicalcium orthosilicate Ca2[SiOJ, tricalcium aluminate Ca3[Al20g] and ferrite C2(A, F). Chemical composition of these phases is complicated by solid solutions, which have the decisive influence on their reactivity with water. For the elements from which chrrker phases are composed isomorphism is typical and very developed phenomenon. The highest concentration of isomorphic elements is encountered in tricalcium aluminate (about 12-13%), and then in alumino-ferrites (about 10-11%), belite (about 6%), and the lowest in alite (about 4 %). 

Except earlier mentioned elements others minor components do not form own phases, but solid solutions in principal clinker phases. The use of industrial by-products and untypical raw materials are introducing to clinker other foreign elements, which distribution in clinker phases was studied. In Table 2.14 the distribution of some foreign elements is presented [190]. As it results from these data vanadium is concentrated in alite, however, chromium and titanium principally in interstitial matter. 

According to Taylor [218] Bogue s formitlae give to low values in the case of alite and tricalcium aluminate, too high for belite and relatively correct for ferrite phases. More close to reahty results can be obtained with modified by Taylor [218] Bogue s formulae, which take into account the presence of minor components solid solutions in clinker phases. Taylor simplifying assumed that in clinker phases the quantity of minor components are on the level given in Table 2.17. 

It is known, that the monoclinic tricalcium aluminate with alkalis in solid solution reacts with water slowly (see Chap. 2). Therefore this phase can be used as a source of aluminate ions in reaction of ettringite formation. The fast-setting cements are known which contain 35 % NCj A, apart from the high belite content of 57% or calcium orthosilicate phases belite and alite (C2S—38%, 038—21%) [1]. 

Alite is a solid-solution series of trigonal, monoclinic, and triclinic modifications of impure tricalcium silicate, which is generally termed C S in the cement industry. Substitution of magnesium and aluminum for silicon causes triclinic pseudotrigonal forms to change to monoclinic pseudotrigonal forms other substitutions may involve iron and sodium. Alite may include up to approximately 4 percent impurity (Ghosh, 1983). It comprises 40 to 70 percent of normal portland cement clinker. Its density is 3.13 to 3.22 Mg/m. ... 

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). 

Alite crystal chemistry was discussed in a paper by Ono (1974) in which he described changes in the atomic structure of alite in response to such variables as solid solution, exsolution, thermal vibration, states of disorder, inversion, and partial decomposition. 

In response to Fundal (1982), Lee (1983) pointed out the bimodality of alite birefringence determined by Maki and Kato (1982), who demonstrated relationships with magnesium oxide content in the crystal. Lee restated the conclusion by Maki and Kato that the M, (monoclinic) phase appears pseudotrigonal in an X-ray powder-diffraction pattern. Lee believed that Fundal s comparison of alite compositional and optical data with a tectosilicate solid-solution series, such as plagio... 

Alite with ultra-fine particles under 0.5 pm (inclusions) Excess solid solution (Ono, 1995)... 

Asakura, E. Uda, S. and Kawabata, H., "Difference of Characteristics Between Alite in Portland Cement Clinker and CjS Solid Solution," Reviews, 38th General Meeting, Cement Association of Japan, 1984, pp. 24-27. 

Ikabata, T. Honda, M. and Yoshida, H., "The Effect of Solid Solution of Minor Components in Alite and Belite to the Hydration Properties of Cement," Reviews, 42nd General Meeting of the Cement Association of Japan, 1988, pp. 26-29. 

Nagashima,M. and others, "Characteristics of Alite in Portland Cement Clinker and CjS Solid Solution," 8th International Congress on the Chemistry of Cement, Rio de Janeiro, Brazil, 1988, pp. 199-204. 

Alite is defined as C3S with about 4% C3A in solid solution. 

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.

XRE gives the total amount without differentiating between the amounts in exsolved regions and in solid solution in alite. The microscopy techniques permit the selection of regions without clusters. With SEM techniques, because of the large interaction volume, subsurface clusters not visible from the surface may be included, but the relatively large distance between exsolved regions makes this unlikely. The STEM technique allows a localised analysis of the composition, since the interaction volume is small because of the small thickness of the sample. Table 8.1 compares the results obtained with the different techniques to the calculated amount of each element from the synthesis recipe. 

---