Coupling effect of TiO₂ and Al₂O₃ on the structure of CaO–SiO₂–MgO–xwt%Al₂O₃–ywt%TiO₂ slag systems

This study analyzes the influence of TiO₂ and Al₂O₃ contents on the microstructure of CaO–SiO₂–MgO–xwt%Al₂O₃–ywt%TiO₂ (14 ≤ x ≤ 22, 0 ≤ y ≤ 10) blast furnace slag systems based on the change of slag viscosity, Raman spectroscopy, and molecular dynamics. The Raman spectroscopy results indicate that an increase in TiO₂ content leads to the gradual depolymerization of complex silicate structures ( \mathrmQ_\mathrmS\mathrmi^3 and \mathrmQ_\mathrmS\mathrmi^2 ) into simpler structures ( \mathrmQ_\mathrmS\mathrmi^0 and \mathrmQ_\mathrmS\mathrmi^1 ) in the slag. At the same time, the Al–O–Al bonds in the aluminate structures of the slag also depolymerize into simpler Al–O⁻ forms, resulting in a decrease in the degree of polymerization of both silicates and aluminates. In contrast, an increase in Al₂O₃ content generally results in an increased degree of polymerization for the silicates and aluminates. Molecular dynamics simulations of the polymerization and depolymerization processes in the microstructure of the blast furnace slag reveal that Si and Al mainly exist in tetrahedral SiO₄⁴⁻ and AlO₄⁴⁻, while Ti mainly exists in the form of simple pentacoordinate TiO₅⁶⁻ and hexacoordinate TiO₆⁸⁻. TiO₂ exhibits basic properties in this system, whereas Al₂O₃ demonstrates acidic behavior. The addition of TiO₂ introduces free oxide ions into the system, causing the bridging oxygens to break into non-bridging oxygens, leading to the depolymerization of complex structures \mathrmQ_\mathrmS\mathrmi^4 and \mathrmQ_\mathrmS\mathrmi^3 , which simplifies the slag structure. On the other hand, an increase in Al₂O₃ content tends to capture or share the oxide ions within the system to form AlO₄⁴⁻, resulting in the polymerization of free oxygens into non-bridging oxygens, which further polymerize into bridging oxygens and lead to the consolidation of simple structures \mathrmQ_\mathrmS\mathrmi^0 and \mathrmQ_\mathrmS\mathrmi^1 , resulting in a more complex slag structure. Both Raman spectroscopy analysis and molecular dynamics simulation results indicate that the degree of polymerization of SiO₄⁴⁻ and AlO₄⁴⁻ in the slag network structure is a crucial factor determining the fluidity of the slag.