An integrate study of the effects of CaF2 on the viscous behavior and structure of CaO-SiO2-MgO-Al2O3-CaF2 blast-furnace slag

Understanding the physical and chemical characteristics of fluorine-containing blast furnace slag is crucial for using fluorine-bearing ores in blast furnace melting. This paper comprehensively analyzes the effects of CaF2 on the viscous flow characteristics and structural changes of CaO-SiO2-MgO-Al2O3-CaF2 blast furnace slag at 1773K. Viscosity measurement results suggest that a higher concentration of CaF2 contributes to a reduction in slag viscosity. Increasing the CaF2 content consistently decreases the viscosity of the slag, though the magnitude of this effect diminishes with higher CaF2 levels. FT-IR shows changes in structure: the diminishing intensity of the [SiO4]-tetrahedral vibration band and the T-O-T bond strength, shallowing of the groove depth. Besides, a slight narrowing of the shoulder width of the [AlO4]-tetrahedral structure, but the change is insignificant. Raman results indicate a depolymerization of Si-based structural units. As the CaF2 content increases from 0 % to 4 %, there is a rise in simpler Q0 and Q1 structural units and a reduction in more complex Q2 units. The Q3 structural units show a decreasing trend, without significant change upon further increase. The ratio of non-bridging oxygen to silicon (NBO/Si) increases from 1.95 to 2.28 with the addition of CaF2, suggesting a lower level of slag polymerization. Fluorine ion serves as a network modifier within the slag structure. XPS results show a decline in the relative concentration of bridging oxygen, whereas the relative concentrations of NBO and free oxygen increase. Finally, 27 Al MAS NMR reveals that adding CaF2 reduces the proportion of [AlO4] and raises the proportion of [AlO5] and [AlO3F] units.