This study investigates the composition–property relationships in ternary engineered geopolymer composites (EGCs) using a simplex centroid design method to optimize the synergy between ground granulated blast furnace slag (GGBS), fly ash (FA), and metakaolin (MK). Mechanical testing revealed that compressive strength (>85 MPa) peaked at 75% GGBS/25% MK, demonstrating MK’s dominant role in enhancing densification, while flexural strength showed a negative correlation with GGBS content but consistent improvement with MK addition. Strain-hardening behavior was most pronounced in 75% GGBS/25% MK and 83% GGBS/8% FA/8% MK mixtures, with the latter achieving optimal precursor synergy. Fiber dispersion uniformity showed a strong linear correlation (R² = 0.98) with tensile strain capacity, confirming its critical role in strain-hardening performance. Mercury intrusion porosimetry analysis demonstrated that the G83F8M8 mixture exhibited the lowest porosity (<10%) and finest pore size distribution (50–100 nm dominant), directly linking pore refinement to superior mechanical properties.

This study systematically investigates the effects of gold tailings (GT) sand substitution levels and particle fineness on the rheological behavior, mechanical properties, and microstructural characteristics of Engineered Cementitious Composites (ECC). The results demonstrate that incorporating moderate amounts of GT sand (≤75%) effectively enhances both the compressive strength and fracture toughness of the composite. However, complete substitution (100% GT) significantly increases matrix porosity, leading to a consequent reduction in comprehensive mechanical strength. A critical size-dependent decoupling effect was observed: while the full replacement of fine aggregates with coarse GT severely degrades compressive strength due to excessive void formation, it unexpectedly increases the matrix fracture toughness by inducing significant crack deflection and tortuosity. Conversely, the incorporation of fine GT optimizes the distribution of matrix flaws and fosters a stable fiber pull-out mechanism. Consequently, despite a reduced pseudo-strain hardening (PSH) index, the fine GT-blended ECC achieves a remarkable tensile ductility of up to 3.0% by satisfying the fundamental multiple-cracking energy criteria while maximizing frictional energy dissipation. Furthermore, substituting natural silica sand with GT yields a highly sustainable composite, reducing carbon dioxide emissions by 41% and material costs by 20% without compromising core mechanical performance.