Alkali silicate phases

There should be little free lime. What there is should occur as rounded grains, typically 10-20 pm in size, and associated with alite and interstitial material. Lime appears cream in sections etched with HF vapour. Its presence may be confirmed by a microchemical test using White s reagent (5 g of phenol in 5 ml of nitrobenzene + 2 drops of water) long, birefringent needles of calcium phenate are formed. The test also responds to CH. Alkali sulphates occur in the clinker pore structure they are etched black with HF vapour, and inhibit the etching of silicate phases with which they are in contact. 

Several studies on the quaternary systems of CaO-SiOj-HiO with NajO or K,0 have been reported (K19,S52,M52). Alkali greatly lowers the concentrations of CaO in the solution and raises those of Si02. The solid phase compositions are difficult to study. Determinations based on changes in concentration on adding CH to alkali silicate solutions are subject to considerable experimental errors, while direct analyses of the solid are difficult to interpret because the alkali cations are easily removed by washing. Suzuki ei al. (S52) considered that they were adsorbed. Macpheeef /. (M52) reported TEM analyses of the C-S-H in washed preparations obtained by reaction ofCjS (lOg) in water or NaOH solutions (250 ml). The C-S-H obtained with water had a mean Ca/Si ratio of 1.77 that obtained with 0.8 M NaOH had a mean Ca/Si ratio of 1.5 and a mean NujO/SiOj ratio of 0.5. These results do not appear to be directly relevant to cement pastes. The pore solutions of the latter may be 0.8 M or even higher in alkali... 

The reaction of the aluminate phase in a cement paste is influenced by the presence of the silicate phases and of alkali (Section 7.6). Ghorab and El Fetouh (G62) studied the effect of the latter on reactions in the pure system. 

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. 

The simplest models for the composition of the planets presume that the differences between them can be explained in terms of an equilibrium condensation. At the highest temperatures a sequence of mixed oxides of calcium, titanium, and aluminum would be found (>1,400 K). This would be followed, at lower temperatures, by metal and silicate fractions. At temperatures somewhat greater than 600 K alkali metals enter the silicate phase along with sulfur, which combines with iron at 650 K to form triolite... 

XRD (Cu Ka) data for (A) raw Portland cement clinker, (B) Residue 1 from which most of the C3S has been removed and (C) Residue 2 from which C3S and C2S have been removed. Silicate phase extraction was conducted using a salicylic acid methanol (SAM) mixture using the method described by Taylor." In Residue 2 the presence of the alkali sulfate phases [denoted Arc for arcanite K2SO4 and Ap for aphthitalite K3Na(S04)2] is clearly evident. Notably, the material used in these plots was derived from a different cement plant to the one illustrated in Figure 10. 

Alkali Sulfates. Since the silicate phases (C3S and C2S), present in total at up to 85 wt.%, normally dominate clinker samples, all minor phases are significantly concentrated in Residue 2. This includes the important alkali sulfate phases which can (i) affect setting times and final strength, and (ii) be used to assess kiln operating conditions. Since these are normally present at a total of about 0.5 wt.% in clinker, and are often distributed across several Na and K sulfate phases, they are not easily identified in raw clinker XRD patterns. However, their presence may be more easily detected in the XRD pattern of Residue 2. By optimizing the parameters of the alkali sulfates from Residue 2 data, and then constraining them in the on-line analysis system, these phases can be measured at the <0.5wt.% level (Madsen, Scarlett and Storer 2001, unpublished results) even when rapidly collected on-line data is used. 

Phase studies in the siliceous portion of the lithia system and the lithia-soda system resulted in the synthesis of mordenites with the same coexisting phases—analcimes, phillipsites, quartz, opaline silica, and crystalline alkali silicates—as had been found in the soda system. Whereas the starting materials used as reactants are not critical parameters in the synthesis of these zeolites in the soda system, the choice of reactants is a predominant factor in lithia-containing systems to produce these phases. The mechanism is not understood yet but the sensitivity of these... 

Critical temperatures often follow a simple trend for systems of related compositions. The critical temperature of alkali silicate melts, for example, decreases in the order of increasing radius of the alkali ion present, such that lithium and sodium silicate melts clearly exhibit metastable immiscibility, while the existence of immiscibility in potassium silicate glasses has not been conclusively established. There is no evidence for the existence of phase separation in the rubidium or cesium silicate systems. 

Addition of modifier ions to silica fills the interstices, preventing bond bending, and hence increases the thermal expansion coefficient. The thermal expansion coefficients of binary alkali silicate glasses increase in the order Li < Na < K. The thermal expansion coefficient is virtually independent of the existence of phase separation, increasing linearly with increasing alkali oxide content for all three oxides over the... 

Huang, C. and Cormack A. N. (1991) Structural Difference and Phase Separation on Alkali Silicate Glasses, J. Chem. Phys., 95, 3634-3642. 

Because of their sizes, neither K+ nor Si + can enter into solid solution with the magnetite and so if some silica is present in the iron oxide used, small occlusions of alkali silicates are present as separate phases in the fused catalyst. Microscopic investigations of the milled catalyst showed that whereas the larger particles still contain alkali silicate occlusions, the finer particles consist of a mixture of separate alkali silicate and magnetite particles (20). Hence, the distribution of alkali in the milled, fused catalyst is heterogeneous. During reduction and FT synthesis, however, the alkali does to some extent spread over the catalyst surface ((7), chapter 3). 

Gittleman et al. studied in more detail the synthesis of all-silica SSZ-24 [108]. In agreement with earlier studies they found that the use of the trimethyl-1-aminoadamantyl cation and fiuned silica as Si02 source is essential for the formation of pure SSZ-24. Moreover, they also confirmed that the molar ratios template/Si02 and OH-/Si02 should exceed values of 0.15 and 0.25, respectively [108]. Especially for lower pH or alkali content, the formation of non-zeolitic silicate phases such as quartz, cristobalite and layered silicates are reported to be favored. 

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