Optimization of metakaolin-wood fly ash-mine tailings alkali-activated binders by Taguchi, GRA, and PCA

The valorization of locally available mine tailings and biomass combustion residues in alkali-activated binders offers a credible pathway to reduce dependence on conventional clinker-based binders and to manage high-volume industrial waste streams with useful mechanical performance. The combination of these two highly variable, low-reactivity residue streams with metakaolin presents a compositional complexity that requires systematic multi-response optimization to identify formulations capable of sustaining gel formation and matrix densification. This study evaluates alkali-activated metakaolin-wood fly ash-Cu-Mo mine tailings (MK-WFA-MT) binders as a low-clinker construction-material pathway for valorizing locally available residue streams. A Taguchi L9 design combined with grey relational analysis (GRA) and principal component analysis (PCA) was used to examine the effects of MK content, WFA content, NaOH molarity, and curing regime on flowability and 7- and 28-day compressive strength. XRD, TGA/DTG, and SEM-EDS were used to relate mechanical performance to reaction-product development and matrix morphology. MK content was the dominant factor controlling strength. Increasing MK from 20% to 30–40% substantially improved matrix continuity, increasing the 28-day strength from 3.36 to 15.40 MPa for the 20% MK mixtures to 25.77–30.04 MPa and 32.88–39.44 MPa for the 30% and 40% MK mixtures, respectively. Among the original L9 mixtures, the highest 28-day strength was obtained for M9 (40% MK, 10% WFA, 50% MT, 10 M NaOH, ambient curing), whereas the GRA-PCA procedure identified 40% MK, 5% WFA, 55% MT, 10 M NaOH, and steam curing as the optimized multi-response combination. Verification experiments confirmed the benefit of this high-MK steam-cured formulation, achieving 36.56 MPa at 7 days and 49.26 MPa at 28 days. Microstructural evidence was consistent with an N-A-S-H-dominated matrix with localized Ca- and/or K-modified aluminosilicate products, as interpreted from the convergence of XRD, TGA/DTG, and SEM-EDS observations.