A Kinetic Study on Crystallization in TiO2-SiO2-CaO-Al2O3 Glass under Nucleation Saturation Conditions for the High Value-Added Utilization of CaO-SiO2-Based Solid Wastes

A kinetic study of the non-isothermal crystallization of CaO-SiO2-Al2O3-TiO2 glass was carried out using the Matusita-Sakka equation and differential thermal analysis. As starting materials, fine-particle glass samples (<58 mu m), case defined as "nucleation saturation" (i.e., containing such a large number of nuclei that the nucleus number is invariable during the DTA process), became dense bulk glass-ceramics through heat treatment, demonstrating the strong heterogeneous nucleation phenomenon at the juncture of particle boundaries under "nucleation saturation" conditions. Three types of crystal phase are formed during the heat treatment process: CaSiO3, Ca3TiSi2(AlSiTi)(3)O-14, and CaTiO3. As the TiO2 content increases, the main crystal shifts from CaSiO3 to Ca3TiSi2(AlSiTi)(3)O-14. The EG values (activation energy of crystal growth) are in the 286-789 kJ/mol range. With increasing TiO2, EG initially decreases (the minimum appears at 14% TiO2), and then, increases. When added within 14%, TiO2 is shown to be an efficient nucleating agent that promotes the growth of wollastonite in a two-dimensional mechanism. As TiO2 further increases to exceed 18%, it is no longer just a nucleating agent but becomes one of the major components in the studied glass, so, in turn, it undermines the crystallization of wollastonite by forming Ti-bearing compounds, resulting in a tendency toward surface crystallization and higher activation energy of crystal growth. For glass samples with fine particles, it is important to note the "nucleation saturation" case for a better understanding of the crystallization process.