The effect of basicity (CaO/SiO2 mass ratio) and FetO on the viscous behavior of CaO–SiO2–FeO–Al2O3–MgO–MnO slag was investigated via stepped-temperature viscometry (1873–1473 K) to predict the thermophysical properties of high-FetO-bearing molten slag. The basicity and FetO contents were varied to determine the constitutional effect of the slags on the physical properties and ionic structure. At low FetO contents (below 15 wt%), the viscosity of high-FetO-bearing slags exhibited a strong dependence on basicity. However, at high FetO contents (above 15 wt%), the viscosity decreased as the FetO content increased, indicating a strong FetO dependency. This behavior was attributed to the change in activation energy as the dominant polymeric structural unit (T–O–T; T = Si, Al, and Fe) varied depending on the equilibrated primary phase. In addition, to understand the change in the dominant polymeric structural unit depending on the FetO content, the relationship between the polymeric structural units and thermophysical properties of high-FetO-bearing slags was studied by analyzing the Fe3+/Fe2+ and nonbridging oxygen fractions of the slag melts.
