Silicate production conclusion

No attempt is made here to explain the intricate temperature dependence of the hydrations of the three calcium silicates. Work on the kinetics of these hydration processes is not complete, so the authors could give at present only highly tentative and speculative explanations. The conclusion from Figures 2 to 7 for the purposes of this paper is that the surface area development in pastes of all three silicates is determined primarily by the degree of hydration of the silicate. For a simple reaction, in which the reactants have negligible surface areas compared with the reaction products, such a conclusion would be trivial. However, for as complex... 

Ft is doubtful whether XRD can distinguish a mixture of tobermorite and C-S-H from a uniform material of intermediate crystallinity the situation may lie between these extremes (A30). This question is discussed further in Section 11.7.4. Crystallization is probably favoured by low bulk density its extent is apparently minimal in calcium silicate bricks (P49), but considerable in aerated concretes (A30). In cement-silica materials, substantially all the AljOj appears to enter the C-S-H, which as its Ca/Si ratio decreases can accommodate increasing amounts of tetrahedrally coordinated aluminium (S70). NMR results (K34) support an early conclusion (K62) that 1.1-nm tobermorite, too, can accommodate aluminium in tetrahedral sites. Small amounts of hydrogarnet have sometimes been detected, especially in products made from raw materials high in AljOj, such as pfa or slag. Minor amounts of tricalcium silicate hydrate (jaffeite C, S2H,) have sometimes been detected (A29,K61). 

Diatoms have been used to look at the history of changing productivity, and hence nutrient inputs, in lake systems. The use of diatoms as palaeoenvironmental indicators in coastal environments is limited because of the preservation problem (Barker et al., 1994) and the paucity of areas which are continuously accreting sediment. Brown (1994) studied a number of sediment cores from the Wash for preservation of diatom material. The biogenic silica content of the sediments decreases with depth (Figure 5.6) which may indicate progressive corrosion with depth and/or increased input of silica to the sediments with time. Gross changes in the type of diatom present in the sediment may support the latter conclusion but the work is of a preliminary nature. Further examination of siliceous palaeoenvironmental indicators in intertidal sediments will be useful. 

To elucidate the Pt promoting effect in the Pt/Ga-si1icate we have comparethe reaction product distribution at similar conversions for Ga-sillcate and Pt/Ga-silicate. Comparable conversions have been obtained at the same temperatures by varying contact time (see the diagram on Fig.5). Again at lower temperature we can see prominent differences between the two catalysts. The following main conclusions have been drawn from the analysis of these features ... 

The results reported on the thermal and thermooxidative decomposition of layered silicate nanocomposites are rather contradictory and do not lead to unambiguous or consistent conclusions. The results vary from enhanced decomposition, to no significant influence, to a strong improvement depending on the source and system discussed. The influence on thermal decomposition differs strongly from nanocomposite to nanocomposite. What is more, often the product release is changed rather than the primary decomposition reactions. The diffusion of the products is hindered by the decreased permeability for nanocomposites 5 wt% layered silicate-polymer nanocomposites show a reduction in gas permeability of around 40 to 60%, even for small gas molecules such as nitrogen and... 

More systematic studies of silicon electrodeposition began in the 1930 s with Dodero s (17)(18) investigation of the electrolysis of molten silicates at temperatures of 800 to 1250 0. The very high potentials used in these studies would be expected to liberate not only silicon but also alkali and alkaline earth metals. There is no conclusive proof in Dodero s work that silicon was the primary cathode product or the result of a reduction of the silicon containing compounds by alkali or alkaline earth metals that had been produced by electrolysis. His best result was 72% silicon produced from a melt composition of 5 Si02 -1 Na20 - 0.2NaF electrolyzed at 1150 C. 

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