Romain Dupuis
77 Massachusetts Ave, E19-722, Cambridge, MA 02139

Molecular simulation of silica gels: Formation, dilution, and drying

Romain Dupuis, Laurent Karim Béland, and Roland J.-M. Pellenq

The formation and ageing of gels is a complex issue that has to be resolved to investigate manifold synthetic materials, among them: porous materials such as cement, high-quality glass fiber, and geomaterials for radioactive waste sealing. Herein, a coupling between a grand canonical Monte Carlo and the parallel tempering methods is developed. The gain in simulation time is of, at least, two orders of magnitude; therefore, we are able to move at will on the water to silicon ratio axis and to observe the restructuring of gels during dilution and drying. At high water to silicon ratio, a colloidal-like structure is obtained, mostly constituted of silicate chains. As humidity is an essential aspect of gels, affecting their physical and mechanical properties, the effect of drying is herein investigated. In agreement with experiments, the structure becomes denser, crosslinks between silicate chains increase and glasslike structures are observed locally.


Doping as a Way To Protect Silicate Chains in Calcium Silicate Hydrates

Dupuis, Romain ; Jorge Dolado ; Jose Surga ; Andres Ayuela


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A critical challenge in reducing anthropogenic impacts on the environment is to decrease the carbon footprint of the cement industry. A key concern in the search for more sustainable cement designs is the understanding and control of the depolymerization process that eventually determines the integrity of their silicate chains under mechanical, chemical, or thermal stresses. Herein, we use metadynamics to show that the depolymerization of cement silicate skeletons consists of hydroxylation followed by bond-breaking. We then clarify the local effects of doping the silicate chains: a stable pentacoordinate state following hydroxylation is promoted by aluminum atoms but restrained by phosphorus additions, the presence of two dopants being related to energy landscapes less favorable to bond-breaking. The role of these dopants is explained in cement-based materials and is key to the quest for low-cost opportunities to preserve the strength of cement for high temperatures or even over time.


Tracing Polymerization in Calcium Silicate Hydrates Using Si Isotopic Fractionation

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.

Oral contribution at Ipolymorphs

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:

List of the speakers:

Concours meilleur article 2015 de la SFIS

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.