This study examines the influence of fine aggregates on early-age mortar pore solution chemistry and electrical resistivity. Specifically, this paper considers the potential adsorption of Na+ and K+ on the aggregate. Ten aggregates from various locations in the United States were used in this study. It was shown that fine aggregates can adsorb Na+ and K+ ions, which can change the ionic concentration of the pore solution and increase the pore solution resistivity. The resistivity is also influenced by the absorbed water in the aggregate, which dilutes the pore solution. A test methodology was proposed to characterize the aggregate adsorption of Na+ and K+ from the pore solution. The alkali adsorption was quantified by developing adsorption isotherms for each aggregate. Using the adsorption isotherms, the concentrations of Na+ and K+ adsorbed on aggregates in mortars were estimated.