208 research outputs found
Atomistic Simulation and Theory of Nanoporous Carbons and Nanostructures
International audienc
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 (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 (~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
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
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
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
Molecular Simulations of Supercritical Fluid Permeation through Disordered Microporous Carbons
International audienceFluid transport through microporous carbon-based materials is inherent in numerous applications, ranging from gas separation by carbon molecular sieves to natural gas production from coal seams and gas shales. The present study investigates the steady-state permeation of supercritical methane in response to a constant cross-membrane pressure drop. We performed dual control volume grand canonical molecular dynamics (DCV-GCMD) simulations to mimic the conditions of actual permeation experiments. To overcome arbitrary assumptions regarding the investigated porous structures, the membranes were modeled after the CS1000a and CS1000 molecular models, which are representative of real microporous carbon materials. When adsorption-induced molecular trapping (AIMT) mechanisms are negligible, we show that the permeability of the microporous material, although not significantly sensitive to the pressure gradient, monotonically decreases with temperature and reservoir pressures, consistent with diffusion theory. However, when AIMT occurs, the permeability increases with temperature in agreement with experimental data found in the literature
Freezing point depression and freeze-thaw damage by nanofluidic salt trapping
A remarkable variety of organisms and wet materials are able to endure temperatures far below the freezing point of bulk water. Cryotolerance in biology is usually attributed to “antifreeze” proteins, and yet massive supercooling (10 MPa) required to damage concrete, the observed correlation between pavement damage and deicing salts, or the FT damage of cement paste loaded with benzene (which contracts upon freezing). In this work, we propose a different mechanism—nanofluidic salt trapping—which can explain the observations, using simple mathematical models of dissolved ions confined between growing ice and charged pore surfaces. When the transport time scale for ions through charged pore space is prolonged, ice formation in confined pores causes enormous disjoining pressures via the ions rejected from the ice core, until their removal by precipitation or surface adsorption at lower temperatures releases the pressure and allows complete freezing. The theory is able to predict the nonmonotonic salt-concentration dependence of FT damage in concrete and provides some hint to better understand the origins of cryotolerance from a physical chemistry perspective
- …