Silicate inherent differences

The high Si Al ratio of the material, typically around 10, makes the material inherently hydrophobic. Changing the Si Al ratio of the material has, however, a marked influence on the hydrophobicity. The increased hydrophobicity of siliceous Beta has been demonstrated by competitive adsorption of toluene and water. The so-called hydrophobicity index32, the amount of toluene adsorbed divided by the amount of water adsorbed at 25 °C amounts to 1.4 to 2.2 for a Si Al = 10 sample and increases to 10.8 to 66 for the all-silica material (see Table 2). The large difference observed for the all-silica zeolites is most likely due to differences in the amount of defects in the material. These defects are essentially silanol pairs required for template charge compensation during synthesis as shown by van der Waal et al.12... 

Silicate anion structures in Portland cement pastes have been studied by the methods described in Section 5.3.2 for calcium silicate pastes. Trimethylsily- i lation (TMS) studies (L20,T12,S69,T36,L31,M43,M44) show that, as with C,S. the proportion of the silicon present as monomer decreases with age and that the hydration products contain dimer, which is later accompanied and eventually partly replaced by polymer (>5Si). Some results have i indicated that fully hydrated pastes of cement differ from those of CjS in that substantial proportions of the silicate occur as monomer (S69,L31), but the results of a study in which pastes of CjS, P-CjS and cement were compared (M44) suggest that the differences between the anion structures of cement and CjS pastes are probably within the considerable experimental errors inherent in the method. The recovery of monomer from unhydrated P-CjS was only 66% and results for cement pastes can only be considered semiquantitative. 

Some of the factors mentioned in the literature (see reviews by Bailey 1984 a,b and also Velde 1980) as potentially exerting controls on cation ordering in layer silicates includes (1) inherent structural differences of some sites, (2) composition via ion size and charge-balance considerations, and (3) P and T conditions of formation (including duration of time subjected to the given conditions). All of these, but especially (2) and... 

The application of pure siliceous materials is limited because of the inherent catalytically inactive nature of unmodified mesoporous materials. Mesoporous silica needs to be modified in order to expand its applications. For more information, readers are referred to a huge number of reviews dealing with the modification of mesoporous silicas and applications [88-91]. Nevertheless, in this section, we address a brief summary of the different strategies and some interesting applications in the development of cleaner technologies. 

Multimodal porous systems in the form of monolithic-shaped forms (bodies) were obtained from a process where a (true) liquid-crystal templating is combined with a sol-gel process [205]. One characteristic of such a process is the incompatibility of lyotropic surfactants with an alcohol, mostly an inherently produced alcohol, which is responsible for the phase separation during the sol-gel processing, stabilizing the resulting monolithic architecture of the monolith [206]. Different approaches use various components, namely, the alcohol source, the silicate sources (both are sometimes combined in one molecule), and molecular or supramolecular templating agents [207]. 

The difference in hydraulic activity of dicalcium silicates arises as a result of the difference in stability of inherent crystal structure. The usual form of dicalcium silicate in Ordinary Portland Cement (OPC) is p-CjS, which reacts more slowly with water and results in the lowest rate of heat... 

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