New-to-old concrete interfaces, which are prone to shear failure, represent a weak point in structures. Current research concerning the shear performance of these interfaces primarily relies on experimental and microscopic methods, such as X-ray diffraction or Scanning Electron Microscopy, and hence provides little data at the nanoscale level. To reduce the knowledge gap, the presented research utilized molecular dynamics simulations to study the shear bonding performance of two models: an interface comprising calcium silicate hydrate CSH-a (H2O/Si=1.68) and CSH-b (H2O/Si=1.0) and a CSH-a-to-SiO2 interface. Analyses of the mechanical response, ionic interactions, and chemical bond breakage behaviors of the interfaces provided nanoscale-level insight concerning the shear characteristics of new-to-old concrete interfaces. Furthermore, this paper explores how shear rate influences the shear resistance of the interface. The research reveals that the CSH-a-to-CSH-b interface mainly involves Ca-O bonds, while hydrogen bonds are more prevalent at the CSH-a-to-SiO2 interface. Both of the models share consistent shear failure modes, more specifically, the shear failure surface occurs within the weaker CSH-a substrate rather than at the interface between substrates, which aligns with experimental observations, as shear failure at new-to-old concrete interfaces is often accompanied by the spalling of low-strength concrete in close proximity to the interface. Additionally, when the shear rate decreases from 0.01 Å/fs to 0.008 Å/fs, and then to 0.005 Å/fs, shear strength declines by 30.2 % and 40.5 %, respectively. The findings of this study clarify the molecular-level mechanisms which govern the shear performance of new-to-old concrete interfaces as well as offers theoretical support for the shear-resistant design and optimization of these interfaces.