Synergistic Effects of Mixed Na ⁺ /K ⁺ Alkalis on the Dissolution of Metakaolin Basal Surfaces: A ReaxFF Molecular Dynamics Study

The initial dissolution of metakaolin (MK) governs the supply and stoichiometry of dissolved aluminosilicate species for subsequent geopolymerization, yet the ion-specific roles of mixed alkalis at chemically heterogeneous MK basal surfaces remain poorly understood. In this work, ReaxFF reactive molecular dynamics simulations were performed at 350 K for 2.0 ns to investigate the dissolution of Al-rich and Si-rich MK (001) basal surfaces in 5 M NaOH, 5 M KOH, and a 5 M 1:1 NaOH/KOH mixed solution, using the ReaxFF parameter sets of Aryanpour et al. and Fedkin et al. The results reveal distinct cation-specific pathways: K + more effectively facilitates Al–O bond cleavage and promotes the formation of Al-bearing oligomers, whereas Na + more strongly perturbs the silicate network and sustains Si release. Most importantly, the 5 M 1:1 NaOH/KOH mixed solution exhibits a synergistic effect arising from the spatiotemporal complementarity of Na + and K + adsorption, which suppresses premature local passivation and yields a balanced dissolved Si/Al ratio of approximately 0.9. This mixed solution also gives the lowest residual fraction of CN = 4 Al species (14.6%), indicating enhanced framework destabilization and sustained early-stage bond-breaking reactivity; the associated framework response should be understood as structural relaxation rather than true diffusion of the solid phase. These findings provide atomistic insight into cation-regulated MK dissolution and offer a mechanistic basis for designing mixed-alkali activators for alkali-activated aluminosilicate materials.