Silica formation, composition, physical

Silica encapsulation has been widely studied both in the open literature and in the patent literature. The first studies were reported by Hergeth et ah, who described the elaboration of composite particles made from quartz powders and polyvinyl acetate through seeded emulsion polymerization [85]. These authors showed that the number of seed particles must exceed a minimal value to prevent formation of new particles and, thus promote seed particles growth. The polymerization was proved to take place in the vicinity of the surface according to the admiceUar polymerization mechanism described previously, and the so-produced interfacial polymer was shown to display physical properties different than those of the bulk polymer. 

These two distinct processes lead to the formation of secondary minerals mainly phyl-losilicates such as clays, of soluble products (e.g., carbonates or silica) lixiviated by percolating waters and of colloids usually iron and aluminum sesquioxides complexed by humic acids. While physical degradation involves mechanical (e.g., abrasion, impact) or thermal (e.g., thermal shock) processes, alteration involves only chemical reactions such as hydrolysis influenced by pH conditions and/or the oxidation of primary materials depending on the Eh (redox potential) conditions. Whatever the type of underlying rock, the end product is always a clay except when silica is totally absent from the bedrock, the composition of the clay depending on the type of climate and the time over which the evolution process takes place. These conditions are summarized in Table 14.1. 

The role of silica-only systems on adhesion has been studied using model compounds with squalene [59]. It was shown that the mechanism for increased adhesion to brass-coated wire-to-rubber was not just a simple improvement of the physical properties of the rubber, but that silica moderated the thickness and composition of the interfacial layer by a chemical interaction. SEM-EDX (scanning electron microscopy with energy dispersive analysis of X-rays), XPS, AES and PIXE (proton induced X-ray emission spectroscopy) revealed that silica affected the relative concentrations of compounds present in the interfacial layer, promoting zinc oxide formation in particular. 

However, the physical properties, chemical composition and mineralogy of the fly ash largely influence its conversion to zeolites, and their resulting grade and overall characteristics. Further, the formation of one or more combination of zeolites mainly depends on the available contents of silica and alumina in amorphous glass form. 

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