JOURNAL OF SOLID STATE CHEMISTRY 138, 169 — 177 (1998) ARTICLE NO. SC987768 Role of Water in the Mechanochemical Reactions of MgO–SiO 2 Systems J. Temuujin,* K. Okada,- and K. J. D. MacKenzie‡ *Institute of Chemistry of the Mongolian Academy of Sciences, Ulaanbaatar 51, Mongolia; -Department of Inorganic Materials, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152, Japan; and ‡New Zealand Institute for Industrial Research and Development, Lower Hutt, New Zealand Received September 15, 1997; in revised form January 23, 1998; accepted January 29, 1998 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 29 Si MAS NMR, but their thermal reaction to form forsterite at 890°C was slightly en- hanced by grinding. In contrast, mixtures containing more highly hydrated silicic acid produce a layer-lattice magnesium silicate with an 8.4-A s basal spacing and a 29 Si MAS NMR spectrum similar to that of the product obtained when the same mixture is subjected to hydrothermal reaction conditions. A possible ex- planation in terms of a mechanochemically induced hydrother- mal reaction mechanism is discussed.  1998 Academic Press 1. INTRODUCTION Grinding of solid mixtures of inorganic compounds is a basic technique in solid-state mechanochemical synthesis (1). The grinding can be carried out under either wet or dry conditions; dry grinding is expected to be influenced by the presence of water or hydroxyl groups in the starting mater- ials (2— 4). The influence of water or hydroxyl groups on mechanochemical reactions (soft mechanochemical reac- tions) (4) has been studied extensively and systematically by Senna and co-workers (4 —11). Mechanochemical reactions of hydroxides or hydrous compounds have also been re- ported (12—14). Avvakumov et al. reported (15, 16) that mechanochemical reactions in mixtures of hydrated oxides occur faster than those in anhydrous oxide mixtures. The acceleration of soft mechanochemical reactions by water or hydroxyl groups has been interpreted in terms of acid—base reactions between oxides (4 — 6, 8, 9, 11, 16). Liao and Senna (5, 6) reported the formation of an amorphous phase during reaction of amorphous SiO  with magnesium hydroxide. They suggested that the solid-state reaction between Mg(OH)  and SiO  begins at the contact points between these dissimilar particles through solid state charge transfer, i.e., between basic hydroxide and acidic oxide. However, previous studies of the mechanochemical reaction of hydrated mixtures of Mg(OH)  and SiO  have not been reported. The aim of the present investigation was to examine the mechanochemical reactions of three mixtures (magnesium hydroxide/silica gel, magnesium hydroxide/hydrous silicic acid, and magnesium oxide/silicic acid) to elucidate the effect of hydration water on the mechanochemical interac- tion in these systems. The mechanochemical product ob- tained in this study was shown by XRD and solid-state MAS NMR to have the characteristics of a layer-lattice magnesium hydroxysilicate, and its similarity to the prod- ucts of hydrothermal reaction of magnesium hydroxide with silicic acid was noted. 2. EXPERIMENTAL PROCEDURE The starting materials were silica gel, silicic acid, and magnesium oxide (Wako Pure Chemical Industries, Ltd) and magnesium hydroxide (Kanto Chemical Co., Inc). The average particle sizes (d  ) of the magnesium hydroxide, silicic acid, and silica gel measured with a laser analyzer (Nikkiso, Microtrac) were 3, 12.4, and 20 m, respectively, and the specific surfaces of these reagents, determined by the BET method in nitrogen at 77 K, were 23.5, 142, and 580 m g, respectively. Thermogravimetry of the starting materials up to 1200°C showed that Mg(OH)  loses its structural water at 400°C, the associated mass change of 32.3% being satisfactorily close to the theoretical loss (31%). The mass loss of the silica gel (17.4%) occurred at 88—140°C, representing the removal of surface adsorbed water. The silicic acid was in a hydrated form, SiO  ) nH  O, and had an ignition loss of 46%, most of which (the hy- dration water) was lost below 150°C with a further small continuous loss up to 1200°C due to the dehydroxylation of silanol groups. Before use, the magnesium oxide was 169 0022-4596/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.