Order and disorder in calcium–silicate–hydrate

Despite advances in the characterization and modeling of cement hydrates, the atomic order in Calcium–Silicate–Hydrate (C–S–H), the binding phase of cement, remains an open question. Indeed, in contrast to the former crystalline model, recent molecular models suggest that the nanoscale structure of C–S–H is amorphous. To elucidate this issue, we analyzed the structure of a realistic simulated model of C–S–H, and compared the latter to crystalline tobermorite, a natural analogue of C–S–H, and to an artificial ideal glass. The results clearly indicate that C–S–H appears as amorphous, when averaged on all atoms. However, an analysis of the order around each atomic species reveals that its structure shows an intermediate degree of order, retaining some characteristics of the crystal while acquiring an overall glass-like disorder. Thanks to a detailed quantification of order and disorder, we show that, while C–S–H retains some signatures of a tobermorite-like layered structure, hydrated species are completely amorphous.ICoME2 Labex (ANR-11-LABX-0053)A*MIDEX projects (ANR-11-IDEX-0001-02)Program “Investissements d’Avenir

Unravelling CSH atomic structure via computational and experimental physical chemistry

Calcium Silicate Hydrate (CSH) is the main binding phase for the cement paste, which is responsible for its strength and creep behavior. This is a nonstoichiometric hydration phase with calcium to silicon ratio (C/S) ranging from 1 to 2.2. At low C/S ratios, the molecular structure of CSH resembles to that of Tobermorite minerals, whereas in high C/S ratios it mostly looks like disordered glasses. By taking advantage of tools of statistical physics, it is shown that CSH at a given C/S can be associated with degenerate molecular structures called CSH polymorphs. Polymorphs are energetically competitive, i.e., they have the same free energy content, which means they can coexist under equilibrium conditions. To start, SiO2 groups are randomly removed from the layered atomic structure Tobermorite 11A. One hundred and fifty structures are created. Grand Canonical Monte Carlo simulation of water adsorption is performed to adsorb water in the interlayer spacing and nanoscale porosities in defected CSH structures. The amount of adsorbed water scales linearly with the number of defects in the calcium–silicate layer. Samples are relaxed using a reactive potential in canonical and isothermal–isobaric ensembles. We observe that the confined water reacts with the free interlayer calcium atoms and nonbridging oxygen to form hydroxyl groups. The number of hydroxyl groups scales linearly with the amount of defects. The amount of water in CSH and Ca‑OH content match well with drying and Neutron Scattering experiment. Although the reactive modeling of CSH impacts the water molecules in CSH’s nanoconfinement environment, it does not significantly affect the silica chain length. This means that the reactive atomistic modeling does not affect the calico-silicate backbone of CSH structures. The silica mean chain length from atomistic simulation aligns perfectly with experimental NMR data. The elastic properties and hardness of all CSH polymorphs are measured at a given C/S and are directly compared with nano-chemo-mechanical testing via coupled nanoindentation and X-ray WDS. Atomistic simulation matches with the experimental data in both elastic and plastic regimes. The correlation of mechanical properties to structural observables of the molecular structures such as dimer content, mean silicate chain length, density, basal distance, water content, number of hydroxyl groups, and topological constraints parameter are calculated. No direct correlations were found at short ranges. The search was extended to the medium range order analysis and it is found that the polymorphism is closely related to the medium range order of Si‑O bonds

Physical Origins of Thermal Properties of Cement Paste

Despite the ever-increasing interest in multiscale porous materials, the chemophysical origin of their thermal properties at the nanoscale and its connection to the macroscale properties still remain rather obscure. In this paper, we link the atomic- and macroscopic-level thermal properties by combining tools of statistical physics and mean-field homogenization theory. We begin with analyzing the vibrational density of states of several calcium-silicate materials in the cement paste. Unlike crystalline phases, we indicate that calcium silicate hydrates (CSH) exhibit extra vibrational states at low frequencies (<2  THz) compared to the vibrational states predicted by the Debye model. This anomaly is commonly referred to as the boson peak in glass physics. In addition, the specific-heat capacity of CSH in both dry and saturated states scales linearly with the calcium-to-silicon ratio. We show that the nanoscale-confining environment of CSH decreases the apparent heat capacity of water by a factor of 4. Furthermore, full thermal conductivity tensors for all phases are calculated via the Green-Kubo formalism. We estimate the mean free path of phonons in calcium silicates to be on the order of interatomic bonds. This satisfies the scale separability condition and justifies the use of mean-field homogenization theories for upscaling purposes. Upscaling schemes yield a good estimate of the macroscopic specific-heat capacity and thermal conductivity of cement paste during the hydration process, independent of fitting parameters.Portland Cement AssociationNational Ready Mixed Concrete Association (Research and Education Foundation

Rigidity Transition in Materials: Hardness is Driven by Weak Atomic Constraints

Understanding the composition dependence of the hardness in materials is of primary importance for infrastructures and handled devices. Stimulated by the need for stronger protective screens, topological constraint theory has recently been used to predict the hardness in glasses. Herein, we report that the concept of rigidity transition can be extended to a broader range of materials than just glass. We show that hardness depends linearly on the number of angular constraints, which, compared to radial interactions, constitute the weaker ones acting between the atoms. This leads to a predictive model for hardness, generally applicable to any crystalline or glassy material

Topological Origin of Fracture Toughening in Complex Solids: the Viewpoint of Rigidity Theory

In order to design tougher materials, it is crucial to understand the relationship between their composition and their resistance to fracture. To this end, we investigate the fracture toughness of usual sodium silicate glasses (NS) and complex calcium--silicate--hydrates (CSH), the binding phase of cement. Their atomistic structure is described in the framework of the topological constraints theory, or rigidity theory. We report an analogous rigidity transition, driven by pressure in NS and by composition in CSH. Relying both on simulated and available experimental results, we show that optimally constrained isostatic systems show improved fracture toughness. The flexible to stressed--rigid transition is shown to be correlated to a ductile-to-brittle transition, with a local minimum of the brittleness for isostatic system. This fracture toughening arises from a reversible molecular network, allowing optimal stress relaxation and crack blunting behaviors. This opens the way to the discovery of high-performance materials, designed at the molecular scale

Applying Tools from Glass Science to Study Calcium-Silicate- Hydrates

To explain the similarities between a glass and amorphous C-S-H, a C-S-H molecular structure with stoichiometry of (CaO)1.7(SIO 2)1(H2O)1.9 is produced using a mixed reactive-nonreactive force field modeling. As the consequence of reactive modeling using REAXFF potential, part of water molecules in the interlayer spacing dissociate into hydroxyl groups and proton, which produces Ca-OH bonds. In addition, it is shown that monomers condensate to produce dimmers. This reduces the monomer content and increases the mean silicate chain length. Comprehensive topological analysis is performed to identify the local environment of each atom, which is indicative of short range order in C-S-H. Specially, the topological analysis is shown to be essential to distinguish between oxygen atoms in water, hydroxyl groups, silica chain and calcium oxide sheets. The medium range order in C-S-H is shown to exist using first sharp diffraction pattern derived from structure factor calculations. © 2013 American Society of Civil Engineers