A comprehensive investigation on the viscosity and structure of CaO-SiO2-Al2O3-MgO-BaO slag with different Al2O3/SiO2 ratios

•The viscosity of slag decreased with Al2O3/SiO2 ratio rising from 0.34 to 0.69.•Al2O3 substitution for SiO2 weakened the degree of polymerization of slag.•Comprehensive analysis of changes in slag structure through multiple detection methods. In this study, the effect of Al2O3/SiO2 mass ratios on the viscosity and structure of a CaO-SiO2-MgO-Al2O3-BaO slag system was comprehensively investigated using multiple characterization methods. The results indicated that viscosity gradually decreased as the Al2O3/SiO2 ratio increased from 0.34 to 0.69. FTIR analysis revealed that the Si-based and Al-based structures and Si-O-Al bonds were weakened. Raman spectroscopy further suggested that the complexity of the Si-based structure was reduced because of the transformation from polymerized Q3 and Q2 units to simpler Q0 and Q1 units. The changes in the numbers of bridged oxygen (O0), non-bridged oxygen (O-), and free oxygen (O2–) atoms with increasing Al2O3/SiO2 ratio were quantified through XPS; the results revealed that the content of O0 decreased, whereas the O- and O2– contents showed the opposite trend. 27Al MAS NMR spectroscopy indicated that the relative content of the [AlO5] and [AlO6] network modifiers increased and that of [AlO4] decreased with increasing Al2O3/SiO2 ratio. This confirmed that the substitution of SiO2 with Al2O3 depolymerized the complicated network structure to some extent, leading to a reduction in the slag viscosity.