Calcium silicate phase

As discussed previously in this chapter, there are spots where the lead content (and the elementary composition) is higher and so-called lead enrichments are formed. Table 2.15 shows that the composition of lead enrichments can be rather different in different enrichments. Besides the increase in lead concentration, other changes in elementary composition may also occur. There are examples of lead enrichment (Figure 2.27 Sample 3 in Table 4) where the average composition remains unchanged even though the lead concentration is elevated. However, there are lead enrichments where the increase in lead concentration is accompanied by a simultaneous increase in iron (Sample 4 in Table 2.15), or a simultaneous increase of calcium and lead (Sample 2). Lead enrichment in these cases is likely due to lead coprecipitation reactions with other minerals (iron oxide and other calcium silicate phases) that were present in the sample. 

In the attempt to synthesize molecular sieves with isomorphous substitutions of A1 and/or Si by the divalent calcium element in the tetrahedral positions, we obtained a new calcium silicate phase by inclusion of heteroatom calcium into silicate sols. The characterization results showed that as-synthesized calcium silicate, named CAS-1 (Calcium silicate No. 1), was a novel zeolite-like crystal material with the cation reversibly exchangeable and selectively adsorptive properties. In this paper, the effects of composition of raw materials, reaction temperature and the different alkali ion on the hydrothermal synthesis of calcosilicate crystal material CAS-1 were investigated and the uptake of different cation on the thermal stability of CAS-1 structure was also examined. The sample was characterized by XRD, TEM, SEM, DT-TGA, BET, AAS and chemical analysis. 

Portland cement and high-alumina cements contain, in addition to calcium silicate phases, calcium monoaluminate, CaAl204 (or CA in cement chemist s shorthand, where C = CaO and A = AI2O3). The Al NMR spectra of this compound, in which the Al is exclusively in tetrahedral coordination, and a number of other calcium alu-minates have been determined (Muller et al. 1986), and more recently, using satellite transition spectroscopy (SATRAS) which has allowed the multiple tetrahedral sites in the various calcium aluminates to be distinguished (Skibsted et al. 1993). The NMR parameters for the synthetic aluminates and a number of their hydration products are shown in Table 5.4. 

According to Taylor [160] the mechanism of hydrated calcium silicate phases formation is strongly related to the properties of silicate anion substmcture. The SiO tetrahedra condensate simultaneously with the calcium-oxygen polyhedral, influencing on one another. Alternatively, the calcium-hydroxide layers can be formed firstly, and the condensation of SiO tetrahedra occurs on this matrix. 

Portland cement reacts with jarosite in the presence of water to form various Ca-Al-Fe silicate-sulphate-hydrate phases (Figure 2) and Ca-Al-Fe oxide ( Ca4Al2Fe08.5 or Ca2(Al,Fe)205). Among the various cement reaction products, the Ca-Al-Fe silicate-sulphate-hydrate and Ca-Al-Fe silicate-hydrate phases appear to be the most common. Table II shows the average electron microprobe-determined compositions of the calcium silicate phase in the Portland cement and of the Ca-Al-Fe silicate-sulphate-hydrate cement reaction products. As the compositions of these reaction products vary widely, the values in Table II are only indicative. 

Table II - Average Electron Microprobe-Determined Compositions of the Calcium Silicate Phase in the Portland Cement and the Ca-Al-Fe Silicate-Sulphate-Hydrate Cement Reaction Products (wt%) ...

Belite (or belitic) cements are produced by grinding belitic clinkers with limited amounts of calcium sulfate (gypsum or anhydrite). Such clinkers contain belite (dicalcium silicate) as their sole or main calcium silicate phase. In addition, they contain tricalcium aluminate and the ferritic phase. Alite (tricalcium silicate) may also be present in some belitic clinkers, but only in very limited amounts. They differ both from ordinary Portland clinker and from Portland clinker with an elevated C2S content (see section 2.4) by having a lower CaO content, which results in a lime saturation factor of not more than LSF=80. 

It is immediately obvious that most of the iron-containing compounds melt in the area of 1200- 1350°C while most of the calcium silicate phases melt above 1485°C. It is also true that melting can typically begin below these indicated temperatures by as much as 50°C due to the effect of other impurities and due to the presence of multicomponent eutectics not seen on three-component diagrams. 

Figure 4.3 XRD scans showing the effect of selective dissolution treatments for a port-land cement. Top residues of SAM middle residues of KOSH treatment bottom original untreated portland cement. The KOSH treatment dissolves the aluminate, ferrite, sulfate and most minor phases, leaving only the calcium silicate phases. In contrast, the SAM treatment dissolves the calcium silicate phases and thus concentrates the aluminate, ferrite and minor phases in the residue. In this particular example, the SAM treatment led to the clear identification of goergeyite (K2Ca5(S04)6 H20) as a minor phase in the cement.

Poulsen, S. L., H. J. Jakobsen and J. Skibsted. 2010. Incorporation of phosphorus guest ions in the calcium silicate phases of Portland cement from P MAS NMR spectroscopy. Inorg. Chem. 49, 5522-5529. 

Skibsted, J., H. J. Jakobsen and C. Hall. 1995b. Quantification of calcium silicate phases in Portland cements by Si MAS NMR spectroscopy. /. Chem. Soc. Faraday Trans. 91, 4423-4430. 

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