Nonmonotonic Cation-Balance Regulation of Strength and Transport-Pathway Accessibility in Ternary Geopolymer Grouts

Geopolymer grouts designed for potential water-bearing fracture applications require coordinated fresh-state behaviour, structural stability, and resistance to connected liquid- and gas-transport pathways. This study investigates how precursor composition, alkali equivalent, and Na/K cation balance regulate reaction-product continuity and transport-pathway accessibility in a metakaolin–fly ash–GGBS ternary system. Ternary composition established a suitable baseline reaction framework, with Al/Si = 0.744 and Ca/Si = 0.520 giving the most favourable overall response. Alkali-equivalent regulation identified 7% as a kinetic–structural window balancing reaction-product development, structural continuity, and transport resistance. At constant alkali equivalent, varying the Na/K ratio produced a pronounced nonmonotonic response. The Na90K10 formulation (90% Na-based and 10% K-based alkali cations) exhibited the lowest final capillary water absorption, decreased the absolute gas-pressure-decay slope by approximately 38.9%, and produced the shallowest moisture-migration front among the tested formulations. Multiscale observations indicate that barrier performance is governed more strongly by reaction-product continuity and reduced accessibility of connected transport pathways than by maximising any individual structural indicator, identifying cation balance as an effective means of structural regulation.