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 16-18.5wt%, 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. 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 Q3/Q2 ratio increased from 2.30 and 1.02 to 2.52 and 2.14, respectively, indicating a progressive polymerization of the silicate structure. 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 degree of polymerization 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.