Multi-criteria optimisation of fly ash-kaolin geopolymer coatings: correlating chloride transport, interface stability, and corrosion kinetics

This study investigates the optimisation of fly ash–kaolin geopolymer coatings for corrosion protection of structural steel under chloride exposure. A Taguchi L9 design was used to evaluate the effects of fly ash–to–kaolin ratio, Portland Pozzolana Cement (PPC), and silica-based modifiers on rapid chloride penetration (RCPT), charge-transfer resistance (Rct), and corrosion current density. Electrochemical results indicate that increasing kaolin content, combined with micro-silica modification, may reduce RCPT from ∼920 C to ∼460 C, increase Rct from ∼22 to ∼105 kΩ cm², and decrease corrosion current density from ∼5.0 × 10−⁷ to ∼2.8 × 10−⁸ A/cm². Regression models show high predictive capability (R² > 97%), with the fly ash–kaolin ratio emerging as the dominant factor. Multi-criteria evaluation using MEREC, MARCOS, and DNMA methods consistently identifies the optimal formulation with improved transport resistance and electrochemical stability. The observed improvements may be attributed to enhanced gel cross-linking and pore refinement, which might limit ionic transport and interfacial reactions. The results provide a statistically supported basis for designing geopolymer coatings for chloride-exposed steel systems.