The linear relationship between electrical resistivity of concrete and corrosion rate of steel in logarithmic coordinates (referred as R-C relationship) has been widely reported. However, R-C relationships are not universal but highly dependent on binder chemistry and corrosion environment, and there has been no consensus on the reasons behind the discrepancy of R-C relationships reported in the literature. In this work, the R-C relationships of alkali-activated slag (AAS) mortars are investigated and compared with that of reference ordinary Portland cement (OPC) mortars, aiming at uncovering the fundamental compositional parameters that cause differences in R-C relationships of various cementitious systems. The results show that the hydroxide ion concentration (pH value) of pore solution has a significant and decisive effect on the R-C relationship. With the decrease of pH, the material resistivity increases because of decreased pore solution conductivity while corrosion rate of steel increases due to the drop of [Cl−]/[OH−], leading to a shift of R-C relationship lines. A new mechanism is proposed to explain the differences in the R-C relationship caused by binder chemistry and corrosion type (chloride- versus carbonation-induced corrosion), which is further verified using the published experimental results in the literature.