The viscosity of steel slag plays an important role in mass transfer, steel-slag reaction rate and slag discharge. The viscosity of CaO–SiO2–FeOt–(Al2O3)–BaO slag with different Al2O3 content was measured. The effect of Al2O3 on structure evolution was investigated. The results showed that the viscosity of steel slag increased by the addition of Al2O3. The FTIR and Raman analysis all showed that the structures of [SiO4] symmetric tetrahedron and [AlO4] asymmetric tetrahedron were more complex by the addition of Al2O3. The XPS analysis the showed that the overall relative fraction of O0 increased from 34.72% to 38.05%, O- decreased from 55.55% to 54.07%, and O2- decreased from 9.73% to 7.88% with the Al2O3 content from 0% to 6wt%. The above analysis meant the degree of polymerization (DOP) increased by the addition of Al2O3, which are agree well with the results of viscosity measurement.
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