Creep of Bulk C--S--H: Insights from Molecular Dynamics Simulations

Understanding the physical origin of creep in calcium--silicate--hydrate (C--S--H) is of primary importance, both for fundamental and practical interest. Here, we present a new method, based on molecular dynamics simulation, allowing us to simulate the long-term visco-elastic deformations of C--S--H. Under a given shear stress, C--S--H features a gradually increasing shear strain, which follows a logarithmic law. The computed creep modulus is found to be independent of the shear stress applied and is in excellent agreement with nanoindentation measurements, as extrapolated to zero porosity

Freezing point depression and freeze-thaw damage by nano-fuidic salt trapping

A remarkable variety of organisms and wet materials are able to endure temperatures far below the freezing point of bulk water. Cryo-tolerance in biology is usually attributed to "anti-freeze" proteins, and yet massive supercooling ($< -40^\circ$C) is also possible in porous media containing only simple aqueous electrolytes. For concrete pavements, the common wisdom is that freeze-thaw damage results from the expansion of water upon freezing, but this cannot explain the large pressures ($> 10$~MPa) required to damage concrete, the observed correlation between pavement damage and de-icing salts, or the damage of cement paste loaded with benzene (which contracts upon freezing). In this Letter, we propose a different mechanism -- nanofluidic salt trapping -- which can explain the observations, using simple mathematical models of dissolved ions confined to thin liquid films between growing ice and charged surfaces. Although trapped salt lowers the freezing point, ice nucleation in charged pores causes enormous disjoining pressures via the rejected ions, until their removal by precipitation or surface adsorption at a lower temperatures releases the pressure and allows complete freezing. The theory is able to predict the non-monotonic salt-concentration dependence of freeze-thaw damage in concreter and provides a general framework to understand the origins of cryo-tolerance.Comment: 5 figure

Atomistic Simulation and Theory of Nanoporous Carbons and Nanostructures

International audienc

Multiscale Poromechanics of Wet Cement Paste

Capillary effects such as imbibition-drying cycles impact the mechanics of granular systems over time. A multiscale poromechanics framework was applied to cement paste, that is the most common building material, experiencing broad humidity variations over the lifetime of infrastructure. First, the liquid density distribution at intermediate to high relative humidities is obtained using a lattice gas density functional method together with a realistic nano-granular model of cement hydrates. The calculated adsorption/desorption isotherms and pore size distributions are discussed and compare well to nitrogen and water experiments. The standard method for pore size distribution determination from desorption data is evaluated. Then, the integration of the Korteweg liquid stress field around each cement hydrate particle provided the capillary forces at the nanoscale. The cement mesoscale structure was relaxed under the action of the capillary forces. Local irreversible deformations of the cement nano-grains assembly were identified due to liquid-solid interactions. The spatial correlations of the nonaffine displacements extend to a few tens of nm. Finally, the Love-Weber method provided the homogenized liquid stress at the micronscale. The homogenization length coincided with the spatial correlation length nonaffine displacements. Our results on the solid response to capillary stress field suggest that the micronscale texture is not affected by mild drying, while local irreversible deformations still occur. These results pave the way towards understanding capillary phenomena induced stresses in heterogeneous porous media ranging from construction materials, hydrogels to living systems.Comment: 6 figures in main text, 4 figures in the SI appendi

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

Poromechanics of Microporous Carbons: Application to Coal Swelling during Carbon Storage

International audienceCoal seams are naturally filled with natural gas. Enhanced Coal Bed Methane recovery (ECBM) is a technique which consists in injecting carbon dioxide (CO2) in coal seams in order to enhance the recovery of the methane (CH4) present in the coal seams. A major issue for the industrial development of this technique is the loss of permeability of the reservoirs during injection. In a coal bed, most of the transport of fluids occurs in a network of natural frac- tures. The loss of permeability is attributed to the closure of the fractures induced by the swelling of the coal ma- trix during the progressive replacement of CH4 by CO2. Since both fluids are mostly adsorbed in the microporous matrix of coal, this particular problem raises the funda- mental question of how adsorption impacts the mechanics of a microporous solid. In this work, we present a porome- chanical modeling valid for microporous solids under ad- sorption and we apply this modeling to the specific case of ECBM. The first section presents the theoretical derivation of general constitutive equations of poromechanics which are valid for generic pore sizes and morphologies. In the second section, we apply this general poromechanics to the specific case of CH4 adsorption in coal. We use molecu- lar simulations to calibrate the derived constitutive laws. In the third section we validate this calibration by analyz- ing results of adsorption experiments in unjacketed condi- tions. The fourth section is dedicated to the case of CO2 adsorption in coal. Finally in the last section, we use this modeling to predict the swelling of coal in the context of ECBM

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

Fracture Properties of Kerogen and Importance for Organic-Rich Shales

International audienceOil and gas produced from organic-rich shales have become in the last ten years one of the most promising sources of unconventional fossil fuels. The oil and gas are trapped in rocks of very small permeability, but hydraulic fracturing enables to operate those reservoirs with competitive costs. The global reserves of shale oil and gas that are potentially recoverable are equivalent to tens of years of world con- sumption. However, hydraulic fracturing is facing many challenges regarding the productivity but also the security and the environment. One of those challenges is to un- derstand how the fractures propagate underground. The propagation depends on the mechanical stress prevailing in the reservoir and on the fracture properties of the rocks. Regarding the fracture properties, the oil and gas indus- try developed brittleness indicators to distinguish between brittle rocks (containing mostly calcite and silica) and duc- tile rocks (containing a significant proportion of clay and kerogen). During fracturing, a brittle rock shatters easily leading to a well-distributed network of fractures, whereas a ductile rock deforms instead of shattering leading to few fractures and in some situations acting as a barrier to the fracture propagation. In this work, we study the role of kerogen in the ductility of shale. The ultimate objective is to develop a fine understanding of the fracture properties of shales

Bottom-up model of adsorption and transport in multiscale porous media

We develop a model of transport in multiscale porous media which accounts for adsorption in the different porosity scales. This model employs statistical mechanics to upscale molecular simulation and describe adsorption and transport at larger time and length scales. Using atom-scale simulations, which capture the changes in adsorption and transport with temperature, pressure, pore size, etc., this approach does not assume any adsorption or flow type. Moreover, by relating the local chemical potential μ(r) and density ρ(r), the present model accounts for adsorption effects and possible changes in the confined fluid state upon transport. This model constitutes a bottom-up framework of adsorption and transport in multiscale materials as it (1) describes the adsorption-transport interplay, (2) accounts for the hydrodynamics breakdown at the nm scale, and (3) is multiscale.France. Investissements d'avenir (ICoME2/ANR-11-LABX-0053)France. Investissements d'avenir (A*NUDEX/ANR-11-IDEX-0001-02)Schlumberger FoundationShell Oil Compan