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The surface properties of three SiO2 samples, one crystalline (quartz) and two amorphous, with a large difference in particle size, have been investigated by thermal analysis, adsorption calorimetry and infrared spectroscopy. The variation in the silanol group population upon thermal treatment has been followed via the evolution of the i.r. bands at 3745–3750 cm−1 (free hydroxyl groups) and 3650-3550 cm−1 (adjacent pairs of SiOH) and the evolution of the heat of adsorption of water with coverage on the outgassed samples.

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To determine the role of the forms of fixed water in the relaxation of the silica structure, silica as treated with hot water vapour in an autoclave, and the kinetics of its dehydration were studied by thermogravimetry. Five successive stages of relaxation were established:1.Flocculation of globules, accompanied by evaporation of the polymolecular water layer situated in the interglobular space.2.Formation of a glassy structure, accompanied by further water evaporation.3.Transition of the glassy structure to cristobalite with the formation of large particles.4.Formation of quartz seeds within each particle.5.Aggregation of oriented seeds into monocrystals of quartz.

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Several commercial and laboratory-synthesized specimens of silica gel have been studied, using thermogravimetry. Two forms of adsorbed water have been found: the first form, which evaporates in the temperature range 100–200°, is polymolecular water layer in open pores and loosely-bonded molecules on the surface and in the bulk of the globules; the second form, which evaporates in the temperature range 200–700°, comprises water molecules hydrogen-bonded to silanol groups, and evidently coordinated to silicon atoms. The dehydration heat of the first form, determined from the DTA curves, depends on the ratio of the two forms, and varies from 3.7±0.6 kcal/mole H2O for coarse-grain silica gel produced at pH=7 to 12±1 kcal/mole H2O for fine-grain silica gel of commercial “chromatography” grade.

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]. Thus, Co catalysts supported on mesoporous silica (SBA-15) and amorphous silica with similar average pore sizes were compared to investigate the influence of pore regularity in FT synthesis in the present work. Experimental

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Abstract  

Naturally occurring opals from three different regions in Australia were studied for their thermal characteristics. All the opals showed initial expansion followed by contraction in thermomechanical analysis (TMA) although the temperature at which the change from expansion to contraction occurred depended on their provenance. Thermogravimetric analysis (TG) revealed different rates and temperatures of dehydration for these opals. A general correlation between the temperature at which there was a zero thermal expansion and that of the maximum rate of dehydration was observed. A dehydration–sintering mechanism is proposed with the effect of sintering being more pronounced following total dehydration.

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Abstract  

The dehydration of samples of a Coober Pedy, South Australian sedimentary white opal, displaying play of colour, was investigated using TMA by heating the samples of the specimen to a range of temperatures between room temperature and 1000 at 200C intervals followed by cooling to room temperature. Etched fracture surfaces of the samples were then examined using SEM. The samples showed the typical expansion at low temperature up to 210C before contraction was observed. The contraction of the opals was ascribed to both sintering, supported by morphological change observed in the SEM micrographs, and dehydroxylation of the silanol groups producing silicon-oxygen-silicon bridges resulting in a more dense silica network.

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Abstract  

The dehydration of an opal specimen was investigated by thermogravimetric analysis (TG) in powder and bulk forms. The change in geometry resulted in a significant difference in the temperature range in which dehydration occurred with peak temperatures in the differential TG (DTG) curve for the hand ground opal at 203°C and for the bulk opal at 340°C. This difference was attributed to time taken for diffusion of free water in the bulk opal to the specimen surface prior to evolution as a registered mass loss. A model was proposed to account for the diffusion of water and was used to estimate the diffusion coefficient.

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Pozzolanic reaction simply consists of an acid–base reaction between calcium hydroxide (Ca(OH) 2 ), and silicic acid (H 4 SiO 4 , or Si(OH) 4 ). The reactivity of pozzolans, in the case of lime-based mortars, is attributed to their content in amorphous silica

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Abstract  

The products of dickite heated in air at 1000 to 1300°C were studied using curve-fitting of transmission and photoacoustic infrared and micro-Raman spectra. The spectra were compared with those of mullite, Al-spinel, corundum, cristobalite, amorphous silica and meta-dickite. Bands that characterize crystalline phases appeared at 1100°C and became stronger with increasing temperature. Mullite, Al-spinel, corundum and amorphous silica were identified by their characteristic bands. The characteristic IR bands of cristobalite overlap those of mullite and amorphous silica, and its presence was therefore established from intensity ratios of the appropriate bands. The research clearly demonstrated the advantage of using curve-fitting for the identification of high temperature phases in the study of the thermal treatment of kaolin-like minerals by infrared and Raman spectroscopy. This technique seems to be a useful method for materials analysis in the ceramic industry.

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