Effect of the Al2O3 Content and MgO/Al2O3 Ratio on the Viscosity and Structure of CaO–SiO2–MgO–Al2O3-Based Furnace Slag with CaO/SiO2 = 1.2

The effects of the Al₂O₃ content and MgO/Al₂O₃ ratios on the structure and viscosity of CaO–SiO₂–MgO–Al₂O₃ silicate-based slag were studied by the rotating cylinder method. The evolution of the silicate structure was analyzed by Fourier transform infrared spectroscopy. The results show that when the CaO/SiO₂ ratio is 1.20 and the MgO content is 10 mass pct, the Si–O–Al structure and [AlO₄]⁵⁻ tetrahedron structure of the slag increase with increasing Al₂O₃ content (17 to 22 pct), which increases the viscosity, break-point temperature, and activation energy of the slag. In addition, when the CaO/SiO₂ ratio is 1.20 and the Al₂O₃ content is 18, 20, and 22 mass pct, respectively, with the increase of MgO/Al₂O₃ ratio in the range of 0.4 to 0.7, the viscosity, break-point temperature, and activation energy of slag decrease. The analysis of the Raman spectra in the 800 to 1200 cm⁻¹ range indicates that complex silicate structures ([Si₂O₅]²⁻, [Si₂O₆]⁴⁻) disaggregate into simpler structures ([Si₂O₇]⁶⁻ and [SiO₄]⁴⁻) with the MgO/Al₂O₃ ratio increase. The proportion of structural units Q³ and Q² decreases, the proportion of Q¹ and Q⁰ increases, and the proportion of silicate structural units (Q⁰ + Q¹)/(Q² + Q³) increases, which indicates that the non-bridging oxygen content in the slag increases, resulting in a decrease in slag polymerization.