Nanoscale mechanisms governing the mechanical degradation of C–S–H in seawater environments

Abstract Purpose: Calcium silicate hydrate (C–S–H) governs the mechanical behaviour of cement paste, yet the nanoscale mechanisms by which seawater ions modify its cohesion and strength remain insufficiently understood. This study investigates how interfacial distance and cation substitution (Na + , K + , Mg 2+ ) affect the shear cohesion and tensile properties of C–S–H. Methods: Non-equilibrium molecular dynamics simulations were performed to evaluate shear and tensile responses of C–S–H layers containing different seawater ions at cation/Si ratios of 0.1–0.3. Perfect-contact and expanded interfacial distances (up to 1 nm) were considered to quantify the roles of water confinement and mobility. Results: Pure C–S–H showed the highest shear and tensile strengths, with performance decreasing in the order: CSH ≈ CSH_Mg > CSH_Na > CSH_K. Shear strength dropped sharply with increasing interfacial distance and vanished beyond ~ 0.32 nm when confined water transitioned from bilayer to trilayer and became mobile. Stress relaxation occurred via stick–slip interlayer sliding while solid layers remained rigid. Tensile tests showed consistent trends, with pure C–S–H exhibiting the greatest strength, stiffness, and ductility. Conclusion: Cation chemistry and interfacial distance strongly control nanoscale cohesion in C–S–H. Increased water mobility at larger interfacial distances weakens shear and tensile responses, linking ion type, water structure, and confinement to mechanical degradation. These insights provide guidance for designing durable cementitious materials in marine environments.