2018 Volume 67 Issue 6 Pages 603-608
The crystallization and transformation behaviors from amorphous and crystalline silica to tridymite were investigated. Silica glass and precipitated silica as an amorphous silica and quartz as a crystalline silica were used for silica source. These silica were well mixed with tridymite as a seed crystal and fired at a temperature from 1100°C to 1400°C for 4, 6 and 12 h to form tridymite phase. The effect of CaO as a dopant on the formation of tridymite was also examined. Regardless to the presence or absence of CaO, the seed crystal tridymite was needed for phase transformation from amorphous silica or quartz to tridymite. In addition, it was found that silica glass was easier to transform to tridymite phase than precipitated silica and quartz. For samples without CaO, tridymite could be directly achieved from amorphous silica without the assistance of liquid phase. For samples doped with CaO, on the other hands, the dominant reaction for transformation to tridymite was the precipitation from liquid phase that was caused by the reaction between doped CaO and cristobalite crystallized from amorphous silica. The significant behaviors that occurred in the samples with CaO were (1) the formation of cristobalite and CaSiO3 which reduced tridymite amount and (2) the liquid phase formation which led the precipitation of tridymite.
