The alkaline activators play a pivotal role in geopolymer concrete (GPC) by controlling the dissolution of aluminosilicate precursors and the subsequent geopolymerisation process. Existing studies have reported significant inconsistencies in the performance of the Rapid Chloride Permeability Test (RCPT, ASTM C1202) when applied to GPC, with variations in activator chemistry hypothesised as a key contributing factor. This study investigates the influence of activator chemistry on the performance of RCPT, surface resistivity (AASHTO TP 95), and bulk resistivity (AASHTO TP 119), tests that are inherently sensitive to concrete chemistry. To this end, the alkali content (expressed as Na₂O) of the activator was varied from 4.43% to 8.43% by mass of precursor. Additionally, the potential contribution of alkalis from aggregates was examined by partially replacing well-graded coarse sand with fine sand at two substitution levels, hypothesising that fine sand may influence test outcomes through alkali leaching. The findings show that GPC with higher alkali content (8.43% Na₂O) underwent premature termination during RCPT, indicating the test’s unreliability for high-alkali systems. Conversely, the incorporation of fine sand had a minimal effect on the measured charge passed. Resistivity-based methods showed that high-alkali GPC (8.43% Na₂O) exhibited lower chloride resistance than low-alkali GPC (4.43%). However, discrepancies were observed between surface and bulk resistivity classifications for a same GPC mix, highlighting the need to critically evaluate and potentially recalibrate existing Portland cement-based frameworks before their application to GPC.