Molecular Dynamics Study of the Interfacial Mechanical Behavior and CO2 Adsorption Characteristics of MgO-Reinforced Expansive Soil

Abstract Reactive magnesium oxide (MgO) is an increasingly attractive low-carbon binder for expansive soil stabilization, yet the molecular mechanisms coupling MgO hydration products with clay minerals and CO2 retention remain unclear. Using molecular dynamics simulations, this study examines the interfacial mechanical behavior and CO2 adsorption characteristics of brucite-montmorillonite systems representing MgO-reinforced expansive soil, across five montmorillonite models with cation exchange capacity (CEC) values from 18.43 to 178.20 cmol·kg–1. Increasing CEC markedly strengthens the brucite-montmorillonite interface, with a more than threefold increase in interfacial interaction energy and a more than 50% increase in interfacial hydrogen bonds, raising the peak tensile stress from approximately 0.7 to more than 2.0 GPa and the energy absorption capacity from approximately 30 to 400 MJ·m–3. CO2 preferentially accumulates near both mineral surfaces, but the montmorillonite interface becomes increasingly dominant at higher CEC values. The montmorillonite-CO2 interaction energy decreases from approximately −45 to −65 kcal·mol–1, and the CO2 diffusion coefficient decreases from approximately 17.5 × 10–9 to 12.0 × 10–9 m2·s–1, confirming stronger interfacial confinement. An overall response desirability (Ddes) increases monotonically with CEC and enters the good response regime at approximately 99.38 cmol·kg–1, establishing CEC as a governing molecular-scale descriptor for low-carbon, MgO-based stabilization of expansive soil.