Structure and Crystallization Kinetics of Glassy CaO-Al₂O₃-SiO₂-CaF₂-Na₂O Mold Fluxes with Varying Basicity

The structure and the crystallization kinetics of CaO-Al₂O₃-SiO₂-CaF₂-Na₂O mold fluxes with varying basicities were investigated by solid-state ²⁹Si nuclear magnetic resonance with magic angular spinning (MAS-NMR) and differential thermal analysis (DTA) technique, respectively. ²⁹Si MAS-NMR study indicated that the increase of basicity decreased the degree of polymerization of mold fluxes. With the increasing basicity, Q ⁰, Q ², and Q ³ gradually decreased, while Q ¹ gradually increased, and the overall degree of polymerization was reduced. Crystallization analysis showed the cuspidine first crystallized from glass, and wollastonite crystal crystallized at elevated temperature for the samples with basicity (defined as CaO/SiO₂ mass ratio) values of 0.9 and 1.0, respectively. Only cuspidine was found to crystallize from glass for the samples with basicity values of 1.1 and 1.2, indicating that the crystallization of wollastonite was suppressed with the increase of basicity. Crystallization kinetics analysis by DTA and field emission scanning electron microscopy equipped with energy dispersive spectroscopy investigation showed that growth mechanism of cuspidine is mainly of the diffusion-controlled three-dimensional growth with the increasing number of nuclei during heating. Activation energies for growth of cuspidine decreased with the increasing basicity of mold flux, which indicated that the crystallization ability was enhanced with the increase of basicity. The relationship between structure and crystallization of mold fluxes was established.