Magnesium oxide is gaining an interest as an agent for stabilizing fluoride and some heavy metals in contaminated soils. When it is mixed with contaminated soils, it hydrates to form brucite into which fluoride and heavy metals are incorporated substituting hydroxyl and octahedral magnesium ions, respectively. There is a report that magnesium oxide reacts also with SiO_2 to form sepiolite, which may induce when inhaled by human. We aged mixtures of magnesium oxide-silica-water for up to 50 days and characterized the reaction product by X-ray diffraction, Fourier transform infrared spectroscopy and electron microscopy. The magnesium oxide and silica in the starting mixture disappeared rapidly in 5 days but a significant amount of brucite did not form. Instead, poorly crystalline layer silicate formed. The crystallinity did not improve to a noticeable extent in 50 days. The infrared spectra and morphology under electron microscope suggested that the formed silicate is poorly ordered smectite but not sepiolite.
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