Role of Sodium- and Potassium-Based Activators on the Engineering Properties of Low-Alkali-Activated Binders

Despite extensive research on alkali-activated materials, the comparative performance of sodium- and potassium-based hydroxide–silicate activator systems under low-alkali conditions remains insufficiently understood, particularly across different precursor compositions and engineering properties. This study comparatively evaluated practically workable sodium- and potassium-based activator systems in alkali-activated binders incorporating ground granulated blast-furnace slag, Class F fly ash, and silica fume. Eleven binary and ternary paste formulations were investigated at hydroxide molarities of 0.5–3 M and silicate-to-hydroxide ratios of 0.25–0.67 using NaOH–Na2SiO3 and KOH–K2SiO3 activator systems. Flowability, setting time, temperature evolution, 28- and 90-day compressive strength, 32-week drying shrinkage, water absorption, permeable void content, rapid chloride permeability test (RCPT), and Fourier transform infrared (FTIR) spectroscopy were evaluated. Increasing hydroxide molarity and silicate dosage generally accelerated reaction development and enhanced strength while reducing flowability. Potassium-activated binders exhibited 3–23% higher measured flow, shorter setting times, earlier temperature evolution, and 11–46% lower drying shrinkage. Conversely, sodium-activated binders generally achieved higher 28- and 90-day compressive strengths, together with 4–16% lower water absorption and permeable void contents and lower electrical charge passed during RCPT. FTIR spectroscopy revealed differences in the Si–O–T band positions between the activator systems, indicating differences in aluminosilicate network development. Overall, the findings establish important performance trade-offs between low-alkali sodium- and potassium-based formulations and provide a comparative basis for activator selection in the development of high-performance alkali-activated cementitious materials.