Alkali-Equivalent-Dependent Dual Role of Sodium Chloride in Alkali-Activated Slag Cement: From Synergistic Activator to Competitive Inhibitor

The cement industry is a major contributor to global CO2 emissions, necessitating the development of low-carbon alternatives, such as alkali-activated slag cement (AAS). This study investigates the feasibility of using NaCl and NaOH as co-activators for granulated blast furnace slag (GBFS), focusing on the alkali-equivalent-dependent role of NaCl. At a low-alkali equivalent (2% Na2O), incorporation of ≤4 wt% NaCl enhanced ionic strength, promoted slag dissolution, and accelerated C-(A)-S-H gel formation, increasing 28-day compressive strength by up to 21%. In contrast, at a high-alkali equivalent (4% Na2O), NaCl addition induced competitive binding of Cl− with aluminate species, inhibiting C-(A)-S-H formation and reducing strength by up to 18% at 10 wt% NaCl. The optimal NaCl dosage for strength improvement was 1–4 wt% under low alkalinity and 1–2 wt% under high alkalinity. Microstructural analyses (XRD, FTIR, TG-DTG, SEM-EDS) confirmed that NaCl promotes Friedel’s salt formation under both conditions, but its effect on the primary gel phase is alkalinity dependent. This work provides a theoretical basis for utilizing industrial NaCl by-products in low-carbon cement design and highlights the importance of alkalinity control in achieving synergistic activation.