Cation induced microstructure and viscosity variation of molten synthetic slag analyzed by solid-state NMR

The viscosity and reaction behavior of ternary RO (R[dbnd]Na₂, Mg, Ca)-Al₂O₃-SiO₂ slag were investigated using high temperature viscometer and “FactSage”. Particularly, more extensive experiments regarding the micro-structural characteristics of slags were clearly evaluated by combining several solid-state NMR methods. The interactional districts among the major cations, especially the variation of microstructure, were clarified. Results show that the viscosity of slag is dictated by the microstructural change. The function of cations in the microstructure of slag is related to the cation field strength (CFS). The Na cation with relatively smaller size could enter and loosen the network structure to a smaller one, and then the charge of [AlO₄]⁻ and [SiO₄]⁻ groups could be balanced by the Na cations to form more stable T (tetrahedron)–O–R structures. The Ca²⁺ could present both charge compensator and network modifier; likely, the Mg cation with a greater CFS in the slag preferred to act as network modifier. Additionally, Mg cation could penetrate into the Al-bearing framework deeper than Ca cation, inducing more disordered distribution of A–O structure. Consequently, the melting structure of slag with Na cation tended to form short segments. Nevertheless, a large amount of T–O–T structure could still exist in the Ca-bearing slag, and their polymeric branch structures were easily broken down and modified by Ca²⁺ forming chain-like structural units. Differently, Mg cation in slag could generate T–O–T clusters with more branches, eventually resulting in the interaction of such species and leading to a rapid increase of viscosity during cooling.