Molecular Dynamics Study on the Effect of Calcite Deposition on the Interfacial Bonding Performance Between Shotcrete and Surrounding Rock

During the service life of karst tunnels, groundwater containing Ca2+ and CO32− can migrate along the shotcrete–surrounding rock interface and induce calcite deposition, thereby changing the interfacial material composition and load transfer path. To clarify the effect of this process on interfacial bonding performance, this study used molecular dynamics simulations to construct CSH–SiO2, SiO2–calcite, CSH–calcite, and CSH–calcite–SiO2 interface models. The interfacial density distribution, radial distribution function, number of hydrogen bonds, interaction energy, and normal tensile failure behavior were analyzed. The results show that all four models reached stable energy plateaus after relaxation, and clear atomic density overlap and short-range RDF peaks appeared in the interfacial regions. These descriptors indicate short-range contact and possible Ca–O electrostatic attraction, hydroxyl-related hydrogen bonding, and carbonate-associated interactions between calcite and both SiO2 and CSH surfaces. Approximately 80 hydrogen bonds were formed at the SiO2–calcite interface, approximately 32 at the CSH–calcite interface, and approximately 59 in total for the two hydrogen bond subtypes at the SiO2–CSH interface, indicating that the hydroxyl state of different substrate surfaces controls the interfacial hydrogen bond network. Interaction energy analysis shows that the single CSH–calcite interface has the strongest interaction (−51,753.6 kcal/mol), approximately 1.90 times that of the SiO2–CSH interface and 16.43 times that of the SiO2–calcite interface. However, in the three-layer composite model, the interaction energy on the CSH–calcite side is only approximately 28.0% of that on the SiO2–calcite side, suggesting that a continuous calcite interlayer introduces asymmetric interfacial constraints. Tensile simulations further show that the SiO2–calcite model has the highest peak stress (approximately 3.23 GPa) and exhibits brittle failure, whereas failure in CSH-containing systems is more likely to transfer into the CSH layer or weakly connected regions. These results indicate that calcite deposition does not simply strengthen or weaken the interface. Instead, within the two idealized endpoint configurations tested here, its effect depends on deposition continuity, the surface chemistry of the two substrates, and the weak links within the serial interface. This study provides a nanoscale theoretical basis for evaluating relative trends in the long-term service performance of shotcrete–surrounding rock interfaces, and for guiding future multiscale validations of drainage and waterproofing measures in karst tunnels.