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Viscosities of CaO-Al2O3 based non-reactive mold fluxes and their relationship to the structure of these slags
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DOI：10.1016/j.jnoncrysol.2026.124152.png)
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Abstract
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This study examines the viscosity and melt structure of low-silica calcium aluminate mold fluxes with compositions characterized by varying wt.(CaO)/wt.(Al2O3) ratios in the range of 1.00 to 1.50. The viscosity of the fluxes was measured using a rotating viscometer, while the melt structure was characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and 27Al and 11B MAS NMR. The viscosity at 1400 °C decreased significantly from 0.36 Pa·s to 0.143 Pa·s with increasing wt.(CaO)/wt.(Al2O3) ratio. The transformation of Newtonian to non-Newtonian fluid occurs between 1340 °C and 1384 °C for varying compositions. This trend corresponds to a reduction in the apparent activation energy for viscous flow from 176.09 kJ·mol-1 to 105.68 kJ·mol-1. Raman spectral deconvolution revealed the presence of Qn(Al) (n = 3, 4) and Qn(Si) (n = 0–4) structural units, with the non-bridging oxygen-to-tetrahedron ratio (NBO/T) increasing as the wt.(CaO)/wt.(Al2O3) ratio increased, indicating progressive depolymerization of the aluminosilicate network. XPS analysis further confirmed the increase in non-bridging oxygens with higher CaO content. This structural depolymerization leads to reduced viscosity at higher basicity. The 27Al and 11B MAS-NMR results further support this interpretation, demonstrating that Al3+ and B3+ predominantly exist as [AlO4] tetrahedral and [BO3] trigonal units, respectively. Comparisons of experimental data with predictions from several viscosity models showed that the NPL model provided the best agreement, demonstrating its reliability for calcium aluminate-based mold flux systems.
Keywords:
viscosity
melt structure
CaO-Al2O3 flux
non-bridging oxygen
slag structure
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