Dupuis, Romain ; Jorge Dolado ; Jose Surga ; Andres Ayuela
Silicate-chains polymerization is a crucial process in calcium silicate hydrate minerals, with large relevance for improving the durability and reducing the environmental impact of cement-based materials. To better understand the evolutionary mechanisms underlying the polymerization of silicate-chains in layered calcium silicate hydrates, we herein propose to trace the evolution of the polymerization degree by using silicon isotopes. The method requires tabulating the isotopic fractionation of several basic chemico–physical mechanisms that we obtained by performing atomistic simulations. The calculations reveal that the highly polymerized structures have longer Si–O bonds and that the Ca2+ cations play a dual role in the stretching and bending mode properties of silicates, such as isotopic fractionation is able to discern not only between the polymerization order of calcium silicate hydrate minerals, but even between cement gels suffering calcium leaching. Silicon isotopic fractionation can, therefore, be used to quantify the different evolutions of calcium silicon hydrate phases in a sample of man-made gel cement in order to improve its sustainability along lifetime stages in the quest for green cement.
Dupuis, Romain ; Benoit, M. ; Tuckerman, M. E. ; Merlin, M.
Equilibrium fractionation of stable isotopes is critically important in fields ranging from chemistry, including medicinal chemistry, electrochemistry, geochemistry, and nuclear chemistry, to environmental science. The dearth of reliable estimates of equilibrium fractionation factors, from experiment or from natural observations, has created a need for accurate computational approaches. Because isotope fractionation is a purely quantum mechanical phenomenon, exact calculation of fractionation factors is nontrivial. Consequently, a severe approximation is often made, in which it is assumed that the system can be decomposed into a set of independent harmonic oscillators. Reliance on this often crude approximation is one of the primary reasons that theoretical prediction of isotope fractionation has lagged behind experiment. A class of problems for which one might expect the harmonic approximation to perform most poorly is the isotopic fractionation between solid and solution phases.
In order to illustrate the errors associated with the harmonic approximation, we have considered the fractionation of Li isotopes between aqueous solution and phyllosilicate minerals, where we find that the harmonic approximation overestimates isotope fractionation factors by as much as 30% at 25 °C. Lithium is a particularly interesting species to examine, as natural lithium isotope signatures provide information about hydrothermal processes, carbon cycle, and regulation of the Earth’s climate by continental alteration. Further, separation of lithium isotopes is of growing interest in the nuclear industry due to a need for pure 6Li and 7Li isotopes. Moving beyond the harmonic approximation entails performing exact quantum calculations, which can be achieved using the Feynman path integral formulation of quantum statistical mechanics. In the path integral approach, a system of quantum particles is represented as a set of classical-like ring-polymer chains, whose interparticle interactions are determined by the rules of quantum mechanics. Because a classical isomorphism exists between the true quantum system and the system of ring-polymers, classical-like methods can be applied. Recent developments of efficient path integral approaches for the exact calculation of isotope fractionation now allow the case of the aforementioned dissolved Li fractionation properties to be studied in detail. Applying this technique, we find that the calculations yield results that are in good agreement with both experimental data and natural observations. Importantly, path integral methods, being fully atomistic, allow us to identify the origins of anharmonic effects and to make reliable predictions at temperatures that are experimentally inaccessible yet are, nevertheless, relevant for natural phenomena.
Dupuis, Romain; Dolaso, J. S. ; Benoit, M. ; Surga J.; Ayuela A.
Studies of the structure of hydroxides under pressure using neutron diffraction reveal that the high concentration of hydrogen is distributed in a disordered network. The disorder in the hydrogen-bond network and possible phase transitions are reported to occur at pressures within the range accessible to experiments for layered calcium hydroxides, which are considered to be exemplary prototype materials. In this study, the static and dynamical properties of these layered hydroxides are investigated using a quantum approach describing nuclear motion, shown herein to be required particularly when studying diffusion processes involving light hydrogen atoms. The effect of high-pressure on the disordered hydrogen-bond network shows that the protons tunnel back and forth across the barriers between three potential minima around the oxygen atoms. At higher pressures the structure has quasi two-dimensional layers of hydrogen atoms, such that at low temperatures this causes the barrier crossing of the hydrogen to be significantly rarefied. Furthermore, for moderate values of both temperature and pressure this process occurs less often than the usual mechanism of proton transport via vacancies, limiting global proton diffusion within layers at high pressure.
Title: Formation of a Quasi 2D-layer of Protons in Hydroxides at High Pressure
Co-authors: Romain DUPUIS, Jorge Dolado, Jose Surga, Magali Benoit, Andres Ayuela
Title of the talk: “Ca(OH)2 under pressure” (R. Dupuis, DIPC)
Chairman of thursday afternoon and friday sessions
Title: Etude du fractionnement isotopique du lithium et de la diffusion des protons dans la portlandite
Co-authors: Romain Dupuis , Andrés Ayuela , Jorge Dolado , Surga Jose , Merlin Méheut, Mark Tuckerman, Magali Benoit
Title: Aluminium content in polymorphs of calcium-silicate-hydrate
Co-authors: Romain Dupuis, J. Moon, J. Dolado, H. Manzano, P. Monteiro, A. Ayuela
I Polymorphs is a conference organized by DIPC members: http://ipolymorphs.dipc.org/
List of the speakers: http://ipolymorphs.dipc.org/lecturers
Title: Path Integral Methods for Isotopic Fractionation of Li and Proton Diffusion in Hydroxides
Co-authors: Romain Dupuis, J. Dolado, J. Surga, M. Méheut, M. Tuckerman, M. Benoit and Andrés Ayuela
Notre article sera présenté pour le prix du meilleur article 2015 de la Société Française des IsotopeS (SFIS).
Dupuis R, Benoit M, Nardin E, Meheut M (2015) Fractionation of silicon isotopes in liquids: The importance of configurational disorder. Chem Geol 396:239-254, doi:10.1016/j.chemgeo.2014.12.027
Les géochimistes se demandaient depuis un certain temps pourquoi les eaux des rivières et des océans avaient un silicium isotopiquement plus lourd que les roches de la croûte continentale? Cette observation avait été prédite depuis longtemps, puis confirmée lorsque les développements de la spectrométrie de masse avaient permis d’analyser la composition isotopique du silicium dissous dans les eaux des océans et des rivières. Les mécanismes responsables de ces observations restaient cependant mal compris.
En trouvant le moyen de combiner leurs premiers calculs de dynamique moléculaire d’espèces aqueuses de silicium à des calculs ab initio de silicates, Dupuis et al. (2015) ont pu montrer que les facteurs de fractionnement à l’équilibre entre quartz ou kaolinite et acide silicique étaient contraires aux valeurs mesurées sur des cas naturels. Ils démontrent ainsi que ces fractionnements isotopiques reflètent des phénomènes d’altération cinétiques des roches magmatiques qui produisent les argiles. Cet article me semble donc bien répondre à l’objectif du prix du meilleur article de l’année de la SFIS, c’est à dire qu’il ne concerne pas simplement la présentation d’une nouvelle idée, mais il présente plutôt le résultat d’un travail de fond, par ailleurs confirmé par des travaux expérimentaux récents, qui restera pour longtemps une référence dans le domaine.
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.