Inconsistencies in rapid chloride permeability testing of geopolymer concrete: Insights into binder chemistry, pore structure, and test cell configuration

The Rapid Chloride Permeability Test (RCPT) is commonly used to evaluate chloride resistance in concrete; however, its applicability to geopolymer concrete (GPC) remains highly disputed due to inconsistent and often contradictory results under standard testing conditions (60 V, 6 h). This study addresses this critical gap by conducting a comprehensive experimental investigation, analysing the RCPT performance of 34 GPC mixes while systematically varying nine key mix design parameters (including precursor composition, activator modulus, FA/GGBS ratio, synthesis method, and others). In addition to RCPT results, the porosity and pore solution chemistry of selected mixes were characterised to uncover the mechanistic origins of charge transport behaviour in GPC. The findings reveal that CaO/SiO₂ ratio, modulated by precursor and activator chemistry, plays a dominant role in controlling capillary porosity and ultimately the charge passed. GPC mixes in this study showed premature test termination when the CaO/SiO₂ ratio was ≤0.28, except in one-part or thermally cured systems. Test termination also occurred at 85 % FA content, but this threshold dropped to 65 % with low-CaO fly ash and varied further with the synthesis method. Notably, the study identifies an unexpected negative correlation between pore solution conductivity and charge passed, challenging conventional assumptions based on OPC systems. Furthermore, the results demonstrate that standard RCPT protocols are often unreliable for GPC, especially in mixes with high porosity and low pore solution conductivity, where premature test termination is triggered by excessive current flow. This work provides new mechanistic insights into the electrically driven chloride transport in GPC and establishes a foundational framework for developing more reliable durability testing methods for alkali-activated materials.