The paper presents a molecular model of calcium-silicate-hydrate (C-S-H), the primary binder phase in concrete, developed through a bottom-up atomistic simulation approach. The model aims to explain the interplay between chemical composition and density, which have been challenging to understand due to the complex nature of C-S-H. The authors use neutron scattering measurements to determine the calcium-to-silicon (C/S) ratio and density of C-S-H particles, which are 1.7 and 2.6 g/cm³, respectively. By considering only the chemical specificity of the system, the model predicts realistic values for the C/S ratio and density, with a chemical composition of (CaO)₀.₆₅(SiO₂)(H₂O)₁.₇₅. The model also predicts other structural features and physical properties, such as short-range order and crystalline features, that are consistent with experimental validation. The mechanical properties of the model, including stiffness, strength, and hydrolytic shear response, are compared with experimental data, showing good agreement. The study highlights the potential of treating cement on par with metals and ceramics in mechanism-based models and multiscale simulations, which can help understand inelastic deformation and cracking in cementitious materials.The paper presents a molecular model of calcium-silicate-hydrate (C-S-H), the primary binder phase in concrete, developed through a bottom-up atomistic simulation approach. The model aims to explain the interplay between chemical composition and density, which have been challenging to understand due to the complex nature of C-S-H. The authors use neutron scattering measurements to determine the calcium-to-silicon (C/S) ratio and density of C-S-H particles, which are 1.7 and 2.6 g/cm³, respectively. By considering only the chemical specificity of the system, the model predicts realistic values for the C/S ratio and density, with a chemical composition of (CaO)₀.₆₅(SiO₂)(H₂O)₁.₇₅. The model also predicts other structural features and physical properties, such as short-range order and crystalline features, that are consistent with experimental validation. The mechanical properties of the model, including stiffness, strength, and hydrolytic shear response, are compared with experimental data, showing good agreement. The study highlights the potential of treating cement on par with metals and ceramics in mechanism-based models and multiscale simulations, which can help understand inelastic deformation and cracking in cementitious materials.