This chapter explores how the chemical composition of C-S-H affects its cryogenic stability, specifically examining its nanostructure and nanomechanical properties. It provides a comprehensive analysis of C-S-H’s structural and physical stability under cryogenic conditions, focusing on the following aspects: the microscopic morphological variations; the transformation process and mechanisms affecting its nanostructure; the influence of Ca/Si ratios on the stability of nanomechanical properties; the evolution of C-S-H pore structures across scales during cryogenic exposure. Through multiscale characterization techniques, we analyzed the multiscale structural features of C-S-H under cryogenic attack, revealing a bottom-up degradation pathway under this harsh condition.