@article {534, title = {Atomistic and mesoscale simulation of sodium and potassium adsorption in cement paste}, journal = {The Journal of Chemical Physics}, volume = {14963}, year = {2018}, month = {Aug-21-2018}, pages = {074705}, abstract = {

An atomistic and mesoscopic assessment of the effect of alkali uptake in cement paste is performed. Semi-grand canonical Monte Carlo simulations indicate that Na and K not only adsorb at the pore surface of calcium silicate hydrates (C-S-H) but also adsorb in the C-S-H hydrated interlayer up to concentrations of the order of 0.05 and 0.1 mol/kg, respectively. Sorption of alkali is favored as the Ca/Si ratio of C-S-H is reduced. Long timescale simulations using the Activation Relaxation Technique indicate that characteristic diffusion times of Na and K in the C-S-H interlayer are of the order of a few hours. At the level of individual grains, Na and K adsorption leads to a reduction of roughly 5\% of the elastic moduli and to volume expansion of about 0.25\%. Simulations using the so-called primitive model indicate that adsorption of alkali ions at the pore surface can reduce the binding between C-S-H grains by up to 6\%. Using a mesoscopic model of cement paste, the combination of individual grain swelling and changes in inter-granular cohesion was estimated to lead to overall expansive pressures of up to 4 MPa\—and typically of less than 1 MPa\—for typical alkali concentrations observed at the proximity of gel veins caused by the alkali-silica reaction.

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}, issn = {0021-9606}, doi = {10.1063/1.5042755}, url = {https://aip.scitation.org/doi/10.1063/1.5042755}, author = {Dufresne, Alice and Arayro, Jack and Zhou, Tingtao and Katerina Ioannidou and Franz-Josef Ulm and Roland Jean-Marc Pellenq and B{\'e}land, Laurent Karim} } @article {632, title = {On the existence and origin of sluggish diffusion in chemically disordered concentrated alloys}, journal = {Current Opinion in Solid State and Materials Science}, volume = {22}, year = {2018}, month = {Jun-2018}, pages = {65-74}, abstract = {

Concentrated single phase solid solutions, including medium- and high-entropy alloys, represent a new class of materials that have recently attracted significant interest due to exceptional functional and structural properties. Their fascinating properties are mainly attributed to the sluggish atomic-level diffusion and transport, but its controlling mechanisms are largely unknown and there is certain skepticism about its very existence. By using microsecond-scale molecular dynamics, on-the-fly and conventional kinetic Monte Carlo, we reveal the governing role of percolation effects and composition dependence of the vacancy migration energy in diffusion. Surprisingly, an increase of concentration of faster species (Fe) in face-centered cubic Ni-Fe alloy may decrease the overall atomic diffusion. Consequently, the composition dependence of tracer diffusion coefficient has a minimum near the site percolation threshold, similar to 20 at.\%Fe. We argue that this coupled percolation and composition-dependent barriers for vacancy jumps within different subsystems in medium- and high-entropy alloys leads, indeed, to the sluggish diffusion. A fast method for preselecting materials with potentially desired properties is suggested.

}, issn = {13590286}, doi = {10.1016/j.cossms.2018.05.003}, url = {https://linkinghub.elsevier.com/retrieve/pii/S135902861830038X}, author = {Osetsky, N. and B{\'e}land, Laurent Karim and Barashev, Alexander V. and Zhang, Yanwen} } @article {573, title = {Thermodynamics, kinetics, and mechanics of cesium sorption in cement paste: A multiscale assessment}, journal = {Physical Review Materials}, volume = {2}, year = {2018}, month = {May-30-2018}, chapter = {Article Number: 053608 }, abstract = {

Cesium-137 is a common radioactive byproduct found in nuclear spent fuel. Given its 30 year half life, its interactions with potential storage materials-such as cement paste-is of crucial importance. In this paper, simulations are used to establish the interaction of calcium silicate hydrates (C-S-H)-the mam binding phase of cement paste-with Cs at the nano- and mesoscale. Different C-S-H compositions are explored, including a range of Ca/Si ratios from 1.0 to 2.0. These calculations are based on a set of 150 atomistic models, which qualitatively and quantitatively reproduce a number of experimentally measured features of C-S-H-within limits intrinsic to the approximations imposed by classical molecular dynamics and the steps followed when building the models. A procedure where hydrated Ca2+ ions are swapped for Cs1+ ions shows that Cs adsorption in the C-S-H interlayer is preferred to Cs adsorption at the nanopore surface when Cs concentrations are lower than 0.19 Mol/kg. Interlayer sorption decreases as the Ca/Si ratio increases. The activation relaxation technique nouveau is used to access timescales out of the reach of traditional molecular dynamics (MD). It indicates that characteristic diffusion time for Cs1+ in the C-S-H interlayer is on the order of a few hours. Cs uptake in the interlayer has little impact on the elastic response of C-S-H. It leads to swelling of the C-S-H grams, but mesoscale calculations that access length scales out of the range of MD indicate that this leads to practically negligible expansive pressures for Cs concentrations relevant to nuclear waste repositories.

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}, keywords = {CALCIUM-SILICATE-HYDRATE; C-S-H; MOLECULAR-DYNAMICS; CRYSTAL-STRUCTURE; MONTE-CARLO; IMMOBILIZATION; RADIONUCLIDES; SIMULATIONS; RELAXATION; COLLOIDS}, doi = {10.1103/PhysRevMaterials.2.053608}, url = {https://link.aps.org/doi/10.1103/PhysRevMaterials.2.053608}, author = {Arayro, Jack and Dufresne, Alice and Zhou, Tingtao and Katerina Ioannidou and Franz-Josef Ulm and Roland Jean-Marc Pellenq and B{\'e}land, Laurent Karim} } @article {624, title = {Accurate classical short-range forces for the study of collision cascades in Fe{\textendash}Ni{\textendash}Cr}, journal = {Computer Physics Communications}, volume = {219}, year = {2017}, month = {Oct-2017}, pages = {11 - 19}, abstract = {

The predictive power of a classical molecular dynamics simulation is largely determined by the physical validity of its underlying empirical potential. In the case of high-energy collision cascades, it was recently shown that correctly modeling interactions at short distances is necessary to accurately predict primary damage production. An ab initio based framework is introduced for modifying an existing embedded-atom method FeNiCr potential to handle these short-range interactions. Density functional theory is used to calculate the energetics of two atoms approaching each other, embedded in the alloy, and to calculate the equation of state of the alloy as it is compressed. The pairwise terms and the embedding terms of the potential are modified in accordance with the ab initio results. Using this reparametrized potential, collision cascades are performed in Ni50Fe50, Ni80Cr20 and Ni33Fe33Cr33. The simulations reveal that alloying Ni and NiCr to Fe reduces primary damage production, in agreement with some previous calculations. Alloying Ni and NiFe to Cr does not reduce primary damage production, in contradiction with previous calculations.

}, issn = {00104655}, doi = {10.1016/j.cpc.2017.05.001}, url = {https://linkinghub.elsevier.com/retrieve/pii/S0010465517301315}, author = {B{\'e}land, Laurent Karim and Tamm, Artur and Mu, Sai and Samolyuk, D. and Osetsky, N. and Aabloo, Alvo and Klintenberg, Mattias and Caro, Alfredo and Stoller, E.} }