Molecular dynamics analysis of the microstructural behavior of fluoride ions in CaO–SiO2-based and CaO–Al2O3-based slag systems

To further clarify the structural behaviour of F− in CaO–SiO2–CaF2–MgO and CaO–Al2O3–CaF2–MgO slags, the microstructure of F ions in slags with different CaF2 contents was resolved by molecular dynamics simulation. The results showed that no stable coordination structure was formed between Si and F, and no chemical bond could be formed; while Al and F and Ca and F could form relatively stable structures, both of which could be bonded. With the increase of CaF2 content, the average distance between Si–Si and Al–Al atom pairs increases, and CaF2 plays the role of network diluent. F− ions are mainly coordinated with Ca2+, and there is a dynamic equilibrium between Ca2+ and coordination anions (O2− and F−) in both systems, and the total coordination number is maintained between six and seven, which is distributed in the network structure in the form of Ca–F clusters. As F− replaces the position of O2− in the Ca–O structure, Ca2+ ions in the silicate slag system are not sufficient to balance the negative charge, and the charge balance can only be maintained by sharing the O2− at the top angle, leading to more interconnected silicate network structures, resulting in a shift in the oxygen type in the melt and the appearance of more highly polymerised Qn structural units. In aluminates, the electrically neutral [AlO3F]4− tetrahedral structure is formed as F replaces O atoms in [AlO4]5− tetrahedra, where the released O atoms are absorbed by other [AlO4]5− tetrahedra to form unstable highly coordinated Al. However, since the formed [AlO3F]4− tetrahedral structure is neutral, it can, together with Ca2+, balance the Al–O network structure with negative charges, making the system contain a large amount of Al–O–Al bridging oxygen structure and the effect of CaF2 in reducing viscosity is diminished.