Effect of MgO and BaO on viscosity and structure of blast furnace slag

•The increment of MgO content reduces the viscosity and complex structures of slag.•The addition of BaO increases the degree of polymerization of blast furnace slag.•Infrared showed the trough depth of [SiO4]-tetrahedron reduces with MgO adding.•Raman showed a decrease in bridge oxygen at the BaO content from 0% to 3.5%. The viscosity was measured by the cylinder method to investigate the effect of MgO and BaO on the viscous behavior of CaO-SiO2-Al2O3-MgO-BaO slags (CaO/SiO2 = 1.05, 7–10 wt%MgO, 0–3.5 wt%BaO). Based on the experimental data, the free running temperature and the activation energy were calculated to analyze the changes of characteristic of viscosity flow. The Fourier transformation infrared (FTIR) and Raman spectra were used to provide the information about the variation of slag structure. The results indicate that the viscosity, free running temperature, as well as activation energy decrease with the increase of MgO content in range of 7 wt%–10 wt%, and increase at BaO content from 0 wt% to 3.5 wt%. MgO provides free oxygen (O2−) to reduce the polymerization degree of slag and decreases the viscosity. However, the addition of BaO tends to form complicated structure and increase the viscosity. When BaO is added into slag, Ba2+ and O2− simultaneously influence the slag structure. O2− can break the Si-O bonds, and thus decrease the polymerization degree of slag. On the other hand, the charge compensation of Ba2+ can form complex aluminate-silicate structure and increase the viscosity of the slag. And the strong charge compensation effect that BaO acted is more dominated in the changes of slag structure.