Early-age in-situ chemical composition evolution of alkali-activated slag under different activator systems

Understanding the early-age evolution of surface chemical composition in alkali-activated slag is critical for controlling interfacial reactions, phase development, and durability-related processes. This study investigates the early-age surface chemistry of alkali-activated slag under six representative activator systems: sodium silicate–sodium hydroxide (SS–SH), sodium hydroxide (SH), sodium silicate (SS), hydrated lime (CH), hydrated lime–sodium silicate (CH–SS), and sodium carbonate–sodium hydroxide (SC–SH), over the first 24 h of reaction. Surface-sensitive characterization using scanning electron microscopy coupled with energy-dispersive spectroscopy and Raman spectroscopy was employed to track time-dependent changes in elemental composition and bonding environments. The findings from this study showed that activator chemistry strongly governs early-stage reaction mechanisms, leading to distinct chemical regimes. Sodium hydroxide-based systems exhibit the highest alkali dominance, with Na/Si ratios exceeding 2.4, indicating highly alkaline conditions and accelerated slag dissolution. In contrast, sodium silicate systems display significantly lower Na/Si ratios (0.46–0.49) and higher silicate availability, promoting more controlled network formation. Calcium-rich systems show elevated Ca/(Si+Al) ratios (greater than 3), reflecting strong calcium participation in early reaction products, while blended systems exhibit intermediate behaviour depending on the relative contributions of hydroxide, silicate, and calcium sources. Raman analysis reveals the development of Si–O–T (T = Si or Al) bands in the 1070–1090 cm⁻¹ region, confirming early gel formation, alongside carbonate-related features between 1340 and 1560 cm⁻¹ , whose intensity varies significantly across activator systems. These observations indicate that early-stage carbonation is influenced by both alkali availability and calcium content, with alkali-rich systems showing greater susceptibility.