Molecular-scale origins of shear-thinning in SiO₂-Al₂O₃-CaONa₂O slag: High-temperature rheology and non-equilibrium molecular dynamics

Abstract

SiO₂-Al₂O₃-CaO Na₂O slag represent the basic constituents of high-alkali coal ash, and its non‑Newtonian rheology critically affects ash deposition and transport. In this study, their shear‑thinning behavior was investigated by combining high‑temperature rheometry (0–500 s⁻¹) with non‑equilibrium molecular dynamics (NEMD) simulations (1–1000 ns⁻¹). The effects of shear rate and Na₂O/CaO mass ratio (0.08–0.56) on flow behavior and melt structure were examined at 1400 °C under comparable degrees of polymerization. It was found that the Na₂O/CaO ratio had only a minor influence on the shear‑thinning index, whereas the imposed shear field played a dominant role in governing the non‑Newtonian rheology. The NEMD results indicate a preferential loss and rearrangement of Si-O-Al and Al-O-Al bridging-oxygen connectivity, accompanied by increases in less-polymerized tetrahedral units and non-bridging and free oxygen species. Consistently, 27 Al MAS NMR analysis of the post-shear quenched samples revealed a decrease in Al(IV) and increases in Al(V) and Al(VI), providing experimental support for shear-induced rearrangement of the Al-containing network. An empirical exponential correlation between the Q 4 (Al) disruption ratio and normalized viscosity was further obtained for the S1 and S5 systems, providing a structure-sensitive description of shear thinning within the investigated NEMD regime.