Magnesium and calcium silicate hydrates, Part II Mg-exchange at the interface “low-pH” cement and magnesium environment studied in a C-S-H and M-S-H model system

International audience“Low-pH” cementitious materials have been developed in the context of nuclear wastes encapsulation to reduce the alkaline plume and the early hydration heat causing shrinkage of the Portland cement based-materials in contact with clayey rocks. This study follows the evolutions at an interface between calcium silicate hydrate (C-S-H) with a Ca/Si ratio of 0.8 and magnesium silicate hydrate (M-S-H) with a Mg/Si ratio of 0.8, while controlling the pore solution by using reservoirs. In a first step a simplified “low-pH” binder was mimicked by C-S-H with a low Ca/Si in the presence of magnesium. In a second step the impact of calcium on pure M-S-H was studied. Secondary electron microscopy observations show the fast deterioration of the C-S-H and the precipitation of M-S-H in the C-S-H disk and an uptake of calcium in the M-S-H disk together with a change of the reservoir compositions including pH values. The reactive transport modelling is in good agreement with the changes in both the solid phases and the composition of the solution reservoirs. © 2017 Elsevier Lt

Recoupling flow and chemistry in variably saturated reactive transport modelling - An algorithm to accurately couple the feedback of chemistry on water consumption, variable porosity and flow

International audienceMost reactive transport codes and algorithms decouple the flow from the reactive transport calculations. In some cases, geochemical reactions lead to significant modifications of porosity or water-content, which can have an impact on the flow. The flow problem is based on the continuity equation and is described in terms of pressure. However, most reactive transport codes do not model the pressure-evolution through mineral reactions. The aim of this study is to recouple the reactive transport and the flow, by providing an accurate description of the evolution of both the porosity and the water in the reactive system. We discuss a formulation of the geochemical solver, based on a mole-conservation, which allows an accurate computation of the volume and masses of all phases. This allows for a water and pore volume computation at the scale of the REV which can impact the fluid-pressure, hence the flow. Additionally, solving the geochemical equilibrium in terms of moles instead of concentrations is more accurate for problems involving important mineral reactions. Finally, this method is suited to saturated, unsaturated and two-phase flow. This method is easy to implement and can be used in different reactive transport simulators, regardless of their numerical approaches. We also test the numerical efficiency of this approach and apply it to fully-coupled problems involving variable porosity, variable saturation, water production/consumption

Ten years of Toarcian argilite -carbon steel in situ interaction

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Magnesium and calcium silicate hydrates

International audienceThe structure and chemistry of magnesium silicate hydrates (M-S-H) is significantly different from calcium silicate hydrates (C-S-H), although both phases are poorly crystalline and have a variable chemical composition. The molar Ca/Si ratio in synthetic C-S-H varies from approximately 0.7 to 1.5 and the Mg/Si ratio in M-S-H from 0.7 to 1.3. In M-S-H silica sheets are present, while the silica in C-S-H is organized in single chains. In addition, M-S-H contains more chemically bound water than C-S-H. Analyses of synthetic samples containing both magnesium and calcium with a total (Mg + Ca)/Si of 0.8 indicate the formation of separate M-S-H and C-S-H gels with no or very little uptake of magnesium in C-S-H or calcium in M-S-H. The clear difference in the silica structure and the difference in ionic radius of Ca2 + and Mg2 + make the formation of an extended solid solution between M-S-H and C-S-H gel improbable. © 2015 Elsevier Ltd. All rights reserved

Comparison of the chemo-mechanical behavior of low-pH cement exposed to calcareous water and to argillite pore water

This paper focuses on the impact of ground water on the chemo-mechanical behavior of low-pH cementitious materials. Two kinds of ground water are considered, calcareous water and argillite pore water. A sample of low-pH cement paste was immersed for two months in each solution. Chemo-mechanical characterizations (SEM, C-14 PMMA autoradiography, nano-indentation) were performed to estimate the degradation state of the samples. Then, geochemical modelling was performed to validate experimental observations. The decalcification of C–S–H, the dissolution of ettringite and a magnesium enrichment are the main phenomena in both cases. However, we observe a larger and deeper increase of porosity when the calcareous water is in contact with the cementitious material. The chemical modifications linked to the increase of porosity tend to locally decrease the mechanical strength of the cementitious matrix.Peer reviewe

Multi-scale modeling of the evolution of the transport properties of cement pastes subject to leaching and carbonation

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Physico-chemical investigation of clayey/cement-based materials interaction in the context of geological waste disposal: Experimental approach and results

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NUMERICAL REPRESENTATIVE ELEMENTARY VOLUME GENERATION OF A SIMPLIFIED CEMENT PASTE AND ESTIMATION OF ITS DIFFUSIVITY AND COMPARISON WITH DEDICATED EXPERIMENTS

International audienceCementitious materials are widely used in the concepts of radioactive waste disposal facilities. During the lifetime of these disposals, those materials will undergo physicochemical degradations. To assess their impacts, reactive transport modellingisused.Reactivetransportcodesmodifythetransportpropertiesbasedonthemodelledporosityevolutionby usingArchie’slawasafeedbackbetweenporosityanddiffusiveproperties.Theselawsarenotsuitedtocementitiousmaterials, whose pore structure is complex and expands over a wide range of pore sizes. The ultimate goal of this research is about developing a microstructure-based feedback relation for the diffusive properties of complex porous structures such as cementitious ones. Therefore, we developed an algorithm designed to generate numerical microstructures representative of simplified cement pastes and performed an experimental campaign consisting of dedicated experiments. Arandom-walkalgorithmisusedtocomputetheeffectivediffusioncoefficientsofournumericalmicrostructures.This paper investigates the description of the initial numerical microstructure and how transport properties are sensitive to different microstructural features that can be controlled from the designed algorithm. Simulations both on the experimental microtomograph and the generated microstructures allow to show that our models are complete to describe the microstructure and diffusion transport property of simplified cementitious materials. Sensitivity analysis is also provided, whose results show that a simple feedback relation cannot properly describe these transport properties. This gives confidence in our approach and its future extension toward the description of cementitious material degradations

Weathering of olivine under CO2 atmosphere A martian perspective

International audienceRecent analyses from the Curiosity rover at Yellowknife Bay (Gale crater, Mars) show sedimentary rocks deposited in a lacustrine environment and containing smectite clays thought to derive from the alteration of olivine. However, little is known about the weathering processes of olivine under early martian conditions, and about the stability of smectite clays in particular. Here, we present a 3-month experiment investigating the weathering of forsteritic olivine powders (Fo90) under a dense CO2 atmosphere, and under present-day terrestrial conditions for comparison. The experiment also evaluates the potential effects of hydrogen peroxide (H2O2), as a representation of the highly oxidizing compounds produced by photochemical reactions throughout martian history. The weathered samples were characterized by means of near-infrared spectroscopy (NIR), X-ray diffraction (XRD), transmission electron microscopy with energy dispersive X-ray spectrometry (TEM-EDX), Mössbauer spectroscopy and thermogravimetry. The results show that a Mg-rich smectite phase formed from the weathering of olivine under CO2 conditions, although in lower abundance than under terrestrial conditions. The main secondary phase formed under CO2 turns out to be a silica-rich phase (possibly acting as a "passivating" layer) with a non-diagnostic near-infrared spectral signature. The use of H2O2 highlights the critical importance of both the redox conditions and Fe content of the initial olivine on the nature of the secondary phases. © 2014 Elsevier Ltd