Viscosity and structure relationship with equimolar substitution of CaO with MgO in the CaO–MgO–Al2O3–SiO2 slag melts

Currently, the Al2O3 content in the high-alumina slag systems within blast furnaces is generally limited to 16wt%–18.5wt%, making it challenging to overcome this limitation. Unlike most studies that concentrated on managing the MgO/Al2O3 ratio or basicity, this paper explored the effect of equimolar substitution of MgO for CaO on the viscosity and structure of a high-alumina CaO–MgO–Al2O3–SiO2 slag system, providing theoretical guidance and data to facilitate the application of high-alumina ores. The results revealed that the viscosity first decreased and then increased with higher MgO substitution, reaching a minimum at 15mol% MgO concentration. Fourier transform infrared spectroscopy (FTIR) results found that the depths of the troughs representing [SiO4] tetrahedra, [AlO4] tetrahedra, and Si–O–Al bending became progressively deeper with increased MgO substitution. Deconvolution of the Raman spectra showed that the average number of bridging oxygens per Si atom and the $$X_{\rm{Q}^{3}}/X_{\rm{Q}^{2}}$$ ( $$X_{\rm{Q}^{i}}$$ is the molar fraction of Qi unit, and i is the number of bridging oxygens in a [SiO4] tetrahedral unit) ratio increased from 2.30 and 1.02 to 2.52 and 2.14, respectively, indicating a progressive polymerization of the silicate structure. X-ray photoelectron spectroscopy (XPS) results highlighted that non-bridging oxygen content decreased from 77.97mol% to 63.41mol% with increasing MgO concentration, whereas bridging oxygen and free oxygen contents increased. Structural analysis demonstrated a gradual increase in the polymerization degree of the tetrahedral structure with the increase in MgO substitution. However, bond strength is another important factor affecting the slag viscosity. The occurrence of a viscosity minimum can be attributed to the complex evolution of bond strengths of non-bridging oxygens generated during depolymerization of the [SiO4] and [AlO4] tetrahedral structures by CaO and MgO.