Title: Simulation of the cement strength retrogration
organised by Dr. Moon Ju Hyuk
Cementitious materials are widely used by the mankind. Research is made to reduce its production costs, its environmental impact and to increase its durability. For this, several approaches are used including experiments and theoretical works. At the DIPC/CFM we are simulating the different phases of the cement paste at the atomic scale using advanced methods. In a collaborative project with Tecnalia Research and Innovation and INTEVEP‐PDVSA, we study the C‐S‐H gel which is the most common phase of cements. This phase is important because it gives the strength to the cement and glue its structure. At high temperature, the C‐S‐H gel goes through an irreversible phase transition that reduces the lifetime of the cement. The product of the phase transition is a crystal (see figure) that is more compact than the C‐S‐H gel; cracks appears in the cement due to the contraction of this phase. We investigate if this transition can be avoided looking at the mechanism of transition and at the energetical properties of the system. In this presentation, we will briefly present methods that we use to study the C‐S‐H gel. Using those, we understood the mechanism of transition which consist in multiple dissociations of Si‐O‐Si bonds. Moreover, we can suggest a way to push back the dissociation and consequently the phase transition.
19/01/2016 – Ce séminaire à été donné à l’INSP (Institut des NanoSciences de Paris) afin de présenter l’importance de prendre en compte les effets nucléaires quantiques lorsque l’on s’intéresse à des systèmes composés d’atomes légers (H, Li) ou sous contraintes. La méthode des intégrales des chemins a été présentée puis ses points forts ont été mis en évidence au travers de deux exemples : le fractionnement isotopique du Li entre un minéral et une solution ; le déplacement des atomes d’hydrogène dans la Portlandite (hydroxyde) sous très haute pression.
1/ Title: Quantum Nuclear Effects in Portlandite under Pressure
2/ Title: Strength retrogression in cements: stability of alpha-C2SH
Titled: Hydrogen tunnelling in Portlandite [Ca(OH)2] under pressure
Fractionation of silicon isotopes in liquids: the importance of configurational disorder
Dupuis, Romain ; Benoit, M. ; Nardin, E. ; Méheut, M.
Abstract Silicon isotopes are a promising tool to assess low-temperature geochemical processes such as weathering or chert precipitation. However, their use is hampered by an insufficient understanding of the fractionation associated with elementary processes such as precipitation or dissolution. In particular, the respective contributions of kinetic and equilibrium processes remain to be determined. In this work, equilibrium fractionation factors for silicon isotopes have been calculated using first-principles methods for quartz, kaolinite, and dissolved silicic acid (H4SiO4 and H3SiO4−) at 300 K. The two liquid systems are treated both as realistically as possible, and as consistently with the solids as possible. They are first simulated by ab initio molecular dynamics, then individual snapshots are extracted from the trajectories and relaxed, giving inherent structures (IS) and their fractionation properties are calculated. The fractionation properties of these IS are then calculated. A significant variability of the fractionation properties (σ= 0.4‰) is observed between the independent snapshots, emphasizing the importance of configurational disorder on the fractionation properties of solutions. Furthermore, a correlation is observed between the fractionation properties of these snapshots and the mean Si-O distances, consistent with calculations on minerals. This correlation is used to identify other parameters influencing the fractionation, such as the solvation layer. It is also used to reduce the number of configurations to be computed, and therefore the computational cost. At 300 K, we find a fractionation factor of + 2.1±0.2‰ between quartz and H4SiO4, + 0.4±0.2‰ between kaolinite and H4SiO4, and -1.6±0.3‰ between H3SiO4− and H4SiO4. These calculated solid-solution fractionations show important disagreement with natural observations in low-temperature systems, arguing against isotopic equilibration during silicon precipitation in these environments. On the other hand, the large fractionation associated with the de-protonation of silicic acid suggests the importance of speciation, and in particular pH, for the fractionation of silicon isotopes at low temperature.
Invited PhD at the DIPC (San Sebastian, Spain) from september 2014 to december 2014.
Postdoctoral contract at DIPC starting from 10 Dec. 2014 on the study of cementitious materials with ab initio calculations.
Seminar titled : Realistic calculations of the isotopic fractionation factors of Si and Li for equilibriums involving liquid phases on the 17th of July at the DIPC of Donostia.
Homepage of the DIPC : http://dipc.ehu.es/
Efficient calculation of free energy differences associated with isotopic substitution using Path Integral Molecular Dynamics
Marsalek, Ondrej ; Chen, Pei-Yang ; Dupuis, Romain ; Benoit, Magali ; Méheut, Merlin ; Bačić, Zlatko ; Tuckerman, Mark E.
The problem of computing free energy differences due to isotopic substitution in chemical systems is discussed. The shift in the equilibrium properties of a system upon isotopic substitution is a purely quantum mechanical effect that can be quantified using the Feynman path integral approach. In this paper, we explore two developments that lead to a highly efficient path integral scheme. First, we employ a mass switching function inspired by the work of Ceriotti and Markland [ J. Chem. Phys. 2013, 138, 014112] that is based on the inverse square root of the mass and which leads to a perfectly constant free energy derivative with respect to the switching parameter in the harmonic limit. We show that even for anharmonic systems, this scheme allows a single-point thermodynamic integration approach to be used in the construction of free energy differences. In order to improve the efficiency of the calculations even further, however, we derive a set of free energy derivative estimators based on the fourth-order scheme of Takahashi and Imada [ J. Phys. Soc. Jpn. 1984, 53, 3765]. The Takahashi–Imada procedure generates a primitive fourth-order estimator that allows the number of imaginary time slices in the path-integral approach to be reduced substantially. However, as with all primitive estimators, its convergence is plagued by numerical noise. In order to alleviate this problem, we derive a fourth-order virial estimator based on a transferring of the difference between second- and fourth-order primitive estimators, which remains relatively constant as a function of the number of configuration samples, to the second-order virial estimator. We show that this new estimator converges as smoothly as the second-order virial estimator but requires significantly fewer imaginary time points.
Development of Monte-Carlo simulations for nano-patterning surfaces associated with MM-EPES analysis: Application to different Si(111) nanoporous surfaces
EPES (elastic peak electron spectroscopy) allows measuring the percentage of elastic backscattered electrons ηe from a surface excited by primary electrons. However, this method must be combined with Monte Carlo simulations to get quantitative information. After a brief description of the algorithm used in this work (named MC2), we focused on the adaptation of this simulation for nanoporous surfaces (named MC2-NP). The theoretical results obtained put in evidence the dependence of ηe versus pore diameter (d), depth of the pores (h) and covering rate (CR) of the pores on the surface. Results obtained on surfaces having cylinder-shaped and cone-shaped holes with nanometer dimensions are presented too. To validate theoretical results obtained with MC2-NP, silicon(111) nanoporous surfaces have been prepared with an anodized aluminum oxide (AAO) template and by argon ion bombardment in an UHV chamber. Uniform nanohole arrays were formed as a replica of ordered lattice pattern of the template. Then EPES experimental measurements have been performed on planar and nanoporous Si(111) surfaces using a retarding field analyzer (RFA). The experimental results put in evidence that the percentage of the elastically backscattered electrons is influenced by the patterning of the surface. Then comparing values of ηe obtained experimentally with those obtained with MC2-NP simulations, we show the sensitivity of the EPES method for studying nanoporous surfaces. In this way, we expect fast estimation of nanohole’s dimensions by in-situ MM-EPES (Multi-Mode EPES) without other techniques such as for example scanning electron microscopy.
- Nanoporous alumina mask;
- Surface plasmon;
- Monte Carlo simulation
Silicon isotope fractionation implying liquid phases at 300K: importance of configurational disorder