Effect of Relative Humidity on the Crack-Bridging Mechanism of CNTs at the CNT/C–S–H Interface

High Resolution Image Download MS PowerPoint Slide Carbon nanotubes(CNTs) play a crucial role in enhancing the crack resistance of cementitious material, especially for mass concrete structures. However, the crack-bridging effect of CNTs remains unclear under different internal relative humidity (RH) conditions. In the present work, three-point bending experiments, molecular dynamics (MD) simulations, and theoretical analysis were systematically applied to investigate the crack-bridging effect of CNTs on cement composites. The results indicated that the crack-bridging effect of CNTs was highly dependent on RH and was primarily reflected by their strengthening and toughening contributions. With increasing RH, CNTs reinforced the Calcium-Silicate-Hydrate (C–S–H) system primarily by inducing structural compaction of the interlayer regions through water migration and silicate chain rearrangement. In contrast, the toughening effect was progressively suppressed as a continuous water film formed at the CNT/C–S–H interface, lubricating the interfacial interaction and weakening crack-bridging capability during crack propagation. Moreover, the fracture toughness of the CNT/C–S–H system was theoretically evaluated using Irwin formula, which bridged MD simulation results with macroscopic mechanical properties. Overall, this study identified an optimal humidity condition (RH ≈70%) where the strengthening and toughening effects of CNTs operate synergistically. These insights provide a theoretical foundation for the rational design of CNT-reinforced cementitious composites from a nanoscale perspective.