A realistic molecular model of cement hydrates (C-S-H) has been developed to understand its structural and mechanical properties. The model, based on atomistic simulations, considers the chemical composition of C-S-H as the key factor. The model's chemical composition is (CaO)₁.₆₅(SiO₂)(H₂O)₁.₇₅, which matches experimental data on the calcium-to-silicon ratio (C/S = 1.7) and density (2.6 g/cm³). The model incorporates short silica chains distributed as monomers, dimers, and pentamers, and predicts structural features and physical properties that can be validated experimentally. The model also includes both glass-like short-range order and crystalline features of the mineral tobermorite. The model was validated against experimental data, including extended X-ray absorption fine structure (EXAFS) spectroscopy, X-ray diffraction, infrared spectroscopy, and nanoindentation measurements. These results confirm the presence of short-range structural disorder, characteristic of a glassy phase. The model also shows that the first peak in the experimental EXAFS signal is broader than in simulations, suggesting a higher volume fraction of short-range structural disorder in real C-S-H. The model was further tested for mechanical properties, including elastic constants and rupture strength. The results showed excellent agreement with experimental values, indicating that the model accurately predicts the mechanical behavior of C-S-H. The model also demonstrates that the presence of water lowers the strength of C-S-H, consistent with experimental observations. The model was used to study the shear localization behavior of C-S-H under affine shear deformation. The results show that the shear response of the C-S-H model is strain localization in the interlayer region, facilitated by the lubricating action of water molecules. The presence of water reduces the shear strength of C-S-H, consistent with the concept of hydrolytic weakening in other crystalline and glassy silicates. The study provides an atomistic-level structural model for C-S-H, developed from a bottom-up perspective and validated against experimental data. This model could enable future developments focused on understanding fundamental deformation mechanisms, diffusive properties, electrical properties, and other characteristic material parameters. The insights gained from this model could help in the development of mechanism-based models and multiscale simulation methods to study inelastic deformation, flow, and fracture in cement-based materials. The existence of an atomistic-level model of the C-S-H nanostructure is crucial to enable advances in our understanding of how specific structural arrangements at the nanoscale relate to resulting material properties.A realistic molecular model of cement hydrates (C-S-H) has been developed to understand its structural and mechanical properties. The model, based on atomistic simulations, considers the chemical composition of C-S-H as the key factor. The model's chemical composition is (CaO)₁.₆₅(SiO₂)(H₂O)₁.₇₅, which matches experimental data on the calcium-to-silicon ratio (C/S = 1.7) and density (2.6 g/cm³). The model incorporates short silica chains distributed as monomers, dimers, and pentamers, and predicts structural features and physical properties that can be validated experimentally. The model also includes both glass-like short-range order and crystalline features of the mineral tobermorite. The model was validated against experimental data, including extended X-ray absorption fine structure (EXAFS) spectroscopy, X-ray diffraction, infrared spectroscopy, and nanoindentation measurements. These results confirm the presence of short-range structural disorder, characteristic of a glassy phase. The model also shows that the first peak in the experimental EXAFS signal is broader than in simulations, suggesting a higher volume fraction of short-range structural disorder in real C-S-H. The model was further tested for mechanical properties, including elastic constants and rupture strength. The results showed excellent agreement with experimental values, indicating that the model accurately predicts the mechanical behavior of C-S-H. The model also demonstrates that the presence of water lowers the strength of C-S-H, consistent with experimental observations. The model was used to study the shear localization behavior of C-S-H under affine shear deformation. The results show that the shear response of the C-S-H model is strain localization in the interlayer region, facilitated by the lubricating action of water molecules. The presence of water reduces the shear strength of C-S-H, consistent with the concept of hydrolytic weakening in other crystalline and glassy silicates. The study provides an atomistic-level structural model for C-S-H, developed from a bottom-up perspective and validated against experimental data. This model could enable future developments focused on understanding fundamental deformation mechanisms, diffusive properties, electrical properties, and other characteristic material parameters. The insights gained from this model could help in the development of mechanism-based models and multiscale simulation methods to study inelastic deformation, flow, and fracture in cement-based materials. The existence of an atomistic-level model of the C-S-H nanostructure is crucial to enable advances in our understanding of how specific structural arrangements at the nanoscale relate to resulting material properties.