Molecular-Scale Regulation of Mg(OH)2 Morphology and Hydration Kinetics via Steric Engineering of Carboxylate Anions

Abstract The hydration of magnesium oxide (MgO) to brucite-type Mg(OH)2 is the fundamental reaction governing strength development, microstructure evolution, and reactivity in MgO-based low-carbon cement systems. However, this process is strongly influenced by the poorly understood interfacial regulation of anisotropic crystal growth and defect evolution, which complicates the rational control of hydration kinetics and product morphology. Here, we establish a molecular-scale regulatory strategy using simple carboxylate anions with systematically varied steric size to modulate MgO hydration pathways. Electrochemical kinetics, multiscale structural characterization, and density functional theory calculations reveal a clear size-dependent bifurcation in hydration behavior. Hydration initiates through preferential dissolution of highly reactive MgO (101) facets, followed by outward reprecipitation into lamellar Mg(OH)2 architectures, while simultaneous layer-by-layer dissolution at (001) facet edges produces stacked platelet structures. Small formate anions can be incorporated during early brucite-layer stacking, inducing interlayer expansion, stacking faults, and fragmentation along the [001] direction. This intercalation accelerates hydration and promotes partial amorphization but destabilizes defect retention, limiting sustained lattice reactivity. In contrast, bulkier acetate and propionate anions are excluded from the interlayers and preferentially adsorb on lateral crystal facets, enforcing anisotropic platelet growth, enhancing crystallinity, and kinetically stabilizing oxygen vacancies that act as intrinsic reactive sites during hydration. Importantly, a delayed anion addition strategy mitigates early-stage surface passivation of MgO particles, resulting in a maximum hydration degree of 37.6% when SF was added after 0.5 h and a maximum BET surface area of 18.0 m2/g when SF was added after 4 h. These two optima occur at different addition times, indicating that MgO conversion and surface-area development are governed by related but nonidentical temporal windows. These results elucidate the coupled interfacial, structural, and defect-mediated mechanisms underlying the controllable hydration of MgO in cementitious systems.