The thermal and hydrothermal behavior of silica-adsorbed magnesium hydroxide.

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Silica-adsorbed Mg(OH)2 was treated thermally and hydrothermally. The resulting change in the adsorption layer was followed, and the effects of the adsorbed silica on the change in the bulk (Mg(OH)2, MgO) were examined. (1) Thermal treatment: The dehydration temperature of Mg(OH)2 increased with an increase in the amount of silica adsorbed, and it was higher by 25 °C for Mg(OH)2 adsorbing silica above 0.4 mmol/g than for pure Mg(OH)2. The amorphous silicate of the adsorption layer on the Mg(OH)2 surface was transformed into forsterite (Mg2SiO4) and chain or sheet-like silicate over the temperature range of 870–1100 °C, and the presence of the silica inhibited the sintering and crystallite growth of the MgO grains. At 1400 °C, these products were all transformed into forsterite, and the presence of the silica had little effect on the crystallite growth. (2) Hydrothermal treatment: Below 360 °C, the amorphous silicate on the surface crystallized as serpentine (Mg3Si2O5(OH)4), and the crystallite growth of Mg(OH)2 was inhibited. Above 420 °C, on the other hand, the silicate was transformed into forsterite through serpentine-like silicate, and the crystallite growth of Mg(OH)2 was promoted. The change in the crystallite size of the bulk by the treatment was interpreted mainly in terms of the behavior of the silicate on the surface.
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