Common opal was dissolved in NaOH lyes in rotating autoclaves. The starting material\nwas characterized by X-ray diffraction and adsorption spectroscopy, thermal\nand chemical analysis, electron and atomic force microscopy. The opal proved to be\nan Opal-CT with a microstructure consisting of microcrystalline tridymite, traces of\nlow-quartz, and amorphous parts built up by random packings of size distributed\namorphous silica colloids. The dissolution conditions have been inspired by the\ntechnological process of hydrothermal water glass synthesis by dissolving silica.\nTemperature and time of the dissolution process as well as initial molar SiO2:Na2O\n(Rm) ratios of the starting materials were varied systematically. The particle size of\nthe samples was varied, too, but due to the nanoscopic microstructure without greater\nimpact on dissolution kinetics. The process products were analyzed chemically.\nAdditionally, some of them were characterized by X-ray diffraction, viscosimetry and\ndynamic light scattering. Already after short dissolution times, water glasses with\nquite high silica concentrations of up to 27 wt.% and SiO2:Na2O ratios of up to 3.7\nwere obtained. At longer dissolution times low-quartz and analcime precipitated and\nthe SiO2 contents were reduced to about 22 wt.% and Rm to about 2.7. The silica contents\nin equilibrium with low-quartz were almost independent on temperature.
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