ENHANCED DEHYDRATION AND AMORPHIZATION OF MG(OH)2 IN THE PRESENCE OF ULTRAFINE SIO2 UNDER MECHANOCHEMICAL CONDITIONS

Mechanochemical amorphization and dehydration of fine particulate crystalline Mg(OH)2 was substantially accelerated by the presence of amorphous SiO2. The TG-DTA profile of the mixture consisting of fine powders of Mg(OH)2 and SiO2 changed significantly with grinding time whereas only a slight change was observed on grinding pure Mg(OH)2. Enhanced mechanochemical dehydration of Mg(OH)2 in the mixture was explained by assuming the following complex processes: intimate mixing, agglutination at the contact points of unlike particles promoted by the higher affinity of silica over magnesia towards hydroxyl groups and initiation of simultaneous solid-state reactions, i.e. addition-reaction between two ingredients and dehydration to form a precursor of magnesium silicate in an amorphous state. The proposed mechanisms were compared with those for the mechanochemical change of talc.