Hydration, carbonation, and microstructural evolution of MgO- and Mg(OH)2-activated blast furnace slag under respective moist and CO2 curing regimes

This study investigates the hydration, carbonation, and microstructural evolution of ground granulated blast furnace slag (GGBFS) activated by magnesium oxide and magnesium hydroxide under both moist and CO 2 curing conditions. A two-stage curing regime, initial moist curing followed by CO 2 exposure-was adopted to examine the synergistic effects on reaction kinetics and phase assemblage. The MgO-activated binder exhibited significantly higher compressive strength than the Mg(OH)₂-activated counterpart, with the 28-day strength being approximately 30–60% higher. For MgO-based binders, short-term CO 2 exposure resulted in an approximately 57% increase in the degree of carbonation; however, despite similar early-age strength development, this enhanced carbonation was accompanied by a strength reduction of about 23% at 28 days. CO 2 curing promoted the formation of aragonite and calcite, but partially suppressing continued hydration. XRD and TGA analyses confirmed the presence of C-(A)-S-H, M-S-H, ettringite, brucite, and hydrotalcite, while NMR spectra revealed higher silicate polymerization and Al incorporation in the MgO system. In contrast, Mg(OH) 2 -activated system showed limited reactivity, mainly functioning as a filler. The results demonstrate that MgO is a more effective alkali-free activator for slag, offering improved hydration, carbonation reactivity, and strength development, thus supporting its potential in low-carbon binder applications with enhanced CO 2 sequestration capacity.