Impact of Na+ and Ca2+ on Strength Development and Hydration Mechanisms in Alkali-Activated Slag Cementitious Materials

Abstract

This study reveals the distinct activation mechanisms of sodium and calcium chloride additives in alkali-activated slag cementitious systems. Through pH monitoring, X-ray diffraction (XRD), thermogravimetric analysis (TG), scanning electron microscopy (SEM), and Pearson correlation analysis, the hydration processes are systematically investigated. Results demonstrate that calcium ions exhibit superior activation effects, achieving 187% and 117% higher 3-day and 28-day compressive strengths at 4.5% Cl − content, with strength progressing from 23.65 MPa to 32.37 MPa (37% growth). In contrast, sodium ions show limited enhancement (103% and 20% improvements at 1.5%  Cl − ), stagnating at 17.9 MPa after 28 days (7% gain). Both ions accelerate early hydration, but calcium sustains activation through later stages by promoting aluminum phase dissolution and Friedel’s salt (F’s) formation. This process consumes chloride ions while stimulating C-S-H gel production, revealing dual benefits of strength development and chloride immobilization. The findings highlight calcium-based additives’ advantages in structural durability and corrosion resistance for alkali-activated cement systems.