Alkali-activated slag (AAS) is a cementitious material prepared by using blast furnace slag as the precursor, triggered by alkaline activators to undergo dissolution, polycondensation, and solidification reactions. Compared to traditional cement-based materials, inherent defects of AAS, such as rapid setting and hardening, significant volumetric shrinkage, and susceptibility to cracking, severely limit its engineering applications. The hydration of magnesium oxide (MgO) generates magnesium hydroxide, with a solid-phase volume expansion of approximately 118%, which effectively compensates for the volumetric shrinkage caused by AAS hydration. However, existing studies are predominantly confined to single-dimensional analyses, such as the impact of MgO reactivity and content, fixed activator parameters, or specific curing ages on isolated performance, lacking systematic investigation into the coupling effects of multiple factors, microstructural evolution, and macro-performance development. This paper systematically investigates the regulatory mechanisms of MgO as a modifier on the hydration kinetics, hardening behavior, and microstructure evolution of AAS.

The precursor used in this experiment was slag with a specific surface area of 426 m²/kg. The alkaline activator was liquid sodium silicate which adjusted to a modulus of 1.5 using sodium hydroxide and deionized water. The sand employed was ISO standard sand. The additives included high-reactivity HR-MgO (reactivity time: ~20 s) and medium-reactivity MR-MgO (reactivity time: 70~90 s). The experimental parameters were set as follows: alkali content (Na₂O as a mass percentage of slag) = 6%, modulus (Ms = SiO₂/Na₂O molar ratio) = 1.5, water-to-solid ratio (including water in the activator and mixing water) = 0.4, and MgO dosage (as a mass percentage of slag) = 0%, 4%, 8%, and 12%. The specimens were molded at room temperature, demolded after resting for 6 h, and then cured under sealed conditions at a temperature of (20 ± 2) ℃ and relative humidity ≥ 95% until the specified curing age. The flowability, setting time, hydration heat, compressive strength, TG, XRD, FTIR, and SEM tests were conducted.

During hydration, MgO consumes partial free water, significantly reducing the fluidity of AAS mortar, with the fluidity loss intensifying as MgO reactivity and dosage increase. Concurrently, MgO incorporation prolongs the setting time of AAS paste, particularly with HR-MgO exhibiting stronger retardation effects. This phenomenon primarily arises from the formation of a passivation layer—composed of Mg(OH)₂ and M-S-H gel generated via MgO hydration—on slag particle surfaces, which inhibits further slag dissolution and thus delays setting. Moderate MgO content enhances the early hydration exothermic rate of AAS, shortening the duration of the induction period, with MR-MgO exhibiting superior enhancement effects compared to HR-MgO. While MgO minimally impacts the peak exothermic rate during the acceleration stage, it significantly increases cumulative heat release over 72 h. For instance, the HR-12 group achieved 178.84 kJ/kg, representing a 13.8% improvement over the control group. This indicates that MgO synergistically promotes hydration by accelerating slag dissolution and ion exchange processes, though excessive dosage diminishes early activation efficiency. MgO incorporation improves the early compressive strength of AAS. The HR-8 group attained a maximum 8 h compressive strength of 51.76 MPa, representing a 38.82% increase compared to the control group. However, strength growth in MgO-doped samples lags behind the control group at later ages, with 28 d compressive strength decreasing as MgO reactivity and dosage increase. MgO incorporation reduces the geometric crystal growth index (n) during the nucleation and growth (NG) stage, shifting hydration from homogeneous to heterogeneous nucleation. This accelerates crystal nucleation/growth rates, enabling earlier transition to the phase boundary reaction (I) stage. During Stage I, the synergistic interaction between MgO's secondary reactions and slag particle phase boundary reaction dominates hydration kinetics, causing actual exothermic curves to deviate upward from fitted curves and resulting in reduced curve fitting accuracy. As hydration progresses, the hydration product layer gradually covers slag particles, inhibiting ion diffusion and thereby reducing reaction rates during the diffusion-controlled (D) stage. Additionally, MgO alters AAS hydration product composition: M-S-H gel (with lower hydration heat than C-A-S-H gel) further decreases the D-stage reaction rate constant, impacting long-term hydration.

MgO markedly reduces the fluidity of fresh paste through free water absorption. During the initial hydration phase, the low solubility of Mg (OH)₂ under high alkalinity enables the formation of a passivation layer on slag particles, effectively delaying slag dissolution and prolonging setting time. As hydration progresses, pH reduction enhances Mg(OH)₂ solubility, releasing Mg²⁺ and OH^- ions that exhibit synergistic effects: 1) Elevated alkalinity accelerates slag dissolution; 2) Mg²⁺ reacts with dissolved [SiO₄]^4- and [Al(OH)₄]^- to generate secondary hydration products (M-S-H gel and hydrotalcite, Ht), substantially improving early compressive strength (8 h strength increased by up to 38.82% with MgO content of 8%). However, excessive Mg²⁺ competitively consumes [SiO₄]^4- and [Al(OH)₄]^- with Ca²⁺, suppressing the crystallinity of C-(A)-S-H gels and reducing their aluminum substitution, ultimately leading to structural loosening and compromised long-term strength development (28 d strength reduction up to 15.99%). Krstulovic-Dabic hydration kinetic modeling demonstrates that MgO incorporation enhances the nucleation and crystal growth (NG) rate but hinders the diffusion-controlled (D) stage by forming a shielding product layer. Notably, the total hydration heat exhibits positive correlation with MgO reactivity and dosage, attributed to continuous formation of magnesium-bearing phases (Mg (OH)₂, Ht, M-S-H). This work provides theoretical insights for optimizing AAS formulations to balance setting regulation, early strength enhancement, and long-term strength sustainability.