Reactive magnesia (MgO)-activated ground granulated blastfurnace slag (GGBS), with fixed GGBS dosages but varying MgO/GGBS ratios, was used for stabilization of two soils and compared with brucite (Mg(OH)2)-activated GGBS and hydrated lime (Ca(OH)2)-activated GGBS. A range of tests, including unconfined compressive strength testing, X-ray diffraction, and scanning electron microscopy, was conducted to study the mechanical, chemical, and microstructural properties of the stabilized soils, and then to investigate the mechanism of MgO–GGBS soil stabilization. Results indicate that the Mg(OH)2 had a minimal activating efficacy for GGBS-stabilized soil, while the reactive MgO yielded a higher activating efficacy than the Ca(OH)2. The activator–soil reactions in the stabilized soil slowed down the activating reaction rate for GGBS; this effect was less significant in MgO–GGBS-stabilized soil than in Ca(OH)2–GGBS-stabilized soil, and hence the GGBS hydration rate in the former was less reduced by the soil than the latter. The Mg2+ and OH− ions produced from MgO dissolution participated in the GGBS hydration reactions without precipitating Mg(OH)2. The common hydration products in all GGBS-stabilized soils were calcium silicate hydrate–like compounds. Additionally, hydrotalcite and calcite could be produced in MgO–GGBS- and Ca(OH)2–GGBS-stabilized soils, respectively, especially with a high activator/GGBS ratio.
