This research addresses a critical gap in sustainable binder development by integrating experimental optimization and multi-response analysis to assess the performance of ternary Red Mud–Ground Granulated Blast Furnace Slag–Metakaolin (RM–GGBS–MK) geopolymer mortars. Although geopolymers are recognized for their potential to reduce cement consumption and CO₂ emissions, limited studies have systematically optimized ternary waste-based binders considering both fresh and hardened properties. In this study, a hybrid Taguchi–Grey Relational Analysis (GRA) framework was employed to simultaneously optimize the compressive strength, flowability, and setting times of geopolymer mortar mixes under three criteria: balanced performance, strength, and workability. The experimental design followed an L16 orthogonal array comprising five parameters—ternary composition, NaOH molarity, activator-to-binder (AL/B) ratio, Na₂SiO₃/NaOH (SS/SH) ratio, and binder-to-sand (B/S) ratio—each at four levels. The GGBS-rich mixes attained the maximum compressive strength of 49.09 MPa at 28 days, whereas the RM and MK dominant mixes demonstrated diminished strengths comparatively. ANOVA results indicated the most significant factors affecting the strength and flowability as SS/SH and Al/B ratios, whereas the binder compositions regulated initial and final setting times. Across all three criteria, GRA identified 14 M NaOH and a 1:1.5 B/S ratio as common significant parameters.
