<p>This study investigated the effects of K₂O content and Na₂O substitution for K₂O on the viscosity, free running temperature, and structure of high-alumina CaO–SiO₂–MgO–Al₂O₃–FeO–TiO₂ slag system generated from red mud smelting in the HIsmelt process. Spectroscopic analysis revealed that as the K₂O content increased from 0 to 4 wt pct, the preferential charge compensation of K⁺ for Al³⁺ led an increase in bridging oxygen content. Concurrently, the relative proportions of highly polymerized Q⁴(Al), Q²(Si), and Q³(Si) units in the tetrahedral structure increased, thereby enhancing the slag’s degree of polymerization and resulting in higher viscosity. However, the weak charge compensation capability of Na₂O facilitated the formation of simpler structural units such as Q²(Al), Q⁰(Si), and Q¹(Si). As a result, the slag viscosity progressively decreased with the substitution of Na₂O for K₂O. However, it was noteworthy that the slag containing 4 wt pct Na₂O still exhibited higher viscosity than the reference slag due to its relatively higher degree of polymerization. Additionally, the low melting point characteristics of alkali metal oxides contributed to a continuous decrease in the free running temperature of the slag.</p>