Role of water in the mechanochemical reactions of MgO-SiO2 systems

The products of mechanochemical activation of mixtures in the magnesia-silica system ground in a planetary pot mill for up to 20 h. were found to depend on the water content of the siliceous starting materials. Mixtures containing silica gel produced no new products detectable by XRD and Si-29 MAS NMR, but their thermal reaction to form forsterite at 890 degrees C was slightly enhanced by grinding. In contrast, mixtures containing more highly hydrated silicic acid produce a layer-lattice magnesium silicate with an 8.4-Angstrom basal spacing and a Si-29 MAS NMR spectrum similar to that of the product obtained when the same mixture is subjected to hydrothermal reaction conditions. A possible explanation in terms of a mechanochemically induced hydrothermal reaction mechanism is discussed. (C) 1998 Academic Press.