Electrical resistivity of geopolymer concrete: Understanding microstructure and pore solution chemistry to establish transport-based specifications

Surface (AASHTO T 358) and bulk (AASHTO TP 119) electrical resistivity tests are widely recognised as rapid, user-friendly indicators of concrete durability. However, as both methods were developed and validated for Portland cement concrete (OPC), their applicability to geopolymer concrete (GPC) has not been verified. This paper examines how microstructure and pore-solution chemistry jointly influence the electrical resistivity of fly ash (FA)/ground granulated blast-furnace slag (GGBS)-based GPC systems. A total of 60 GPC mixtures with GGBS contents ranging from 15% to 85%, Na 2 O contents from 4.43% to 8.43%, and activator moduli from 0.91 to 1.80 were investigated under both one-part and two-part synthesis routes. All specimens were tested at 28 days under controlled vacuum saturation conditions (ASTM C1202). By identifying the key parameters (precursor chemistry, activator composition, and FA/GGBS ratio) that control resistivity and linking resistivity measurements to durability performance, this study proposes preliminary performance-based durability specifications for chloride resistance in the investigated GPC systems. To directly link resistivity to pore-solution chemistry, the elemental concentrations (Na, Si, K, Al, Ca, Fe, and Mg) of the extracted pore solution were measured via ICP-OES, along with porosity and pore size distribution via mercury intrusion porosimetry and apparent volume of permeable voids. The results demonstrate that lower porosity, reduced extracted pore-solution elemental concentrations, or higher pore-solution resistivity do not necessarily translate into higher overall GPC resistivity, underscoring differences from conventional OPC-based interpretations. For instance, increasing GGBS content reduced pore-solution resistivity while increasing GPC resistivity. However, thermal curing increased both pore-solution and GPC resistivity compared with ambient curing. To avoid misinterpretation, electrical resistivity in GPC must be evaluated by considering both pore-solution chemistry and porosity. The predictive capability of electrical resistivity was evaluated against chloride diffusion coefficients (ASTM C1556), using the adjusted formation factor ( FF a ) assessed as an indicator of transport behaviour in GPC. Both resistivity and FF a showed strong correlations with the apparent diffusion coefficient (R 2 ≈ 0.90), supporting the development of preliminary resistivity- and FF a -based criteria for different AS 3600 exposure classes within the investigated FA/GGBS-based GPC systems. By considering the combined influence of mix design, microstructure, and pore solution chemistry, this study presents a preliminary framework for interpreting resistivity-based durability performance of GPC and contributes toward bridging the gap between laboratory measurements and practical implementation.