Nanoscale Hydration Mechanisms Governing Strength Evolution in Red Clay Aggregates: Insights from Molecular Dynamics

The distinctive engineering properties of red clay, arising from its unique colloidal interactions among oxides, clay minerals, and water, present both opportunities and challenges in geotechnical applications. This study employs molecular dynamics simulations to unravel the hydration-driven microstructural evolution and strength mechanisms of red clay aggregates. Results reveal that illite surfaces were coated with colloidal oxides [FeO(OH), Al 2 O 3 , Fe 2 O 3 ] and water molecules during aggregation, in which metal–oxygen coordination bonds impart mechanical stability. The porosity and strength exhibit a nonmonotonic dependence on water content: optimal hydration at 40% water content minimizes porosity (30.93%) and maximizes tensile (1,025 MPa) and shear (558 MPa) strengths through hydrogen-bonding networks; beyond this threshold, hydration repulsion dominates, causing particle separation and strength degradation. Chemical bond analysis identifies interfacial bridge failure as the primary mechanism of collapse, with hydrogen bonds being the most vulnerable. The competing roles of water were evaluated through statistical analysis of chemical bonds. These findings provide actionable strategies for strength optimization, including dehydration consolidation and the promotion of chemical coordination. This work bridges atomic-scale interactions with macroscopic behavior, offering critical insights for mitigating red clay-related geohazards.