Impact of a 70°C temperature on an ordinary Portland cement paste/claystone interface: An in situ experiment

International audienceRadioactive wastes in future underground disposal sites will induce a temperature increase at the interface between the cementitious materials and the host rock. To understand the evolution of Portland cement in this environment, an in situ specific device was developed in the Underground Research Laboratory in Tournemire (France). OPC cement paste was put into contact with clayey rock under water-saturated conditions at 70°C. The initial temperature increase led to ettringite dissolution and siliceous katoite precipitation, without monosulfoaluminate formation. After one year of interaction, partial decalcification and diffuse carbonation (calcite precipitation) was observed over 800 μm in the cement paste. At the interface, a layer constituted of phillipsite (zeolite), tobermorite (well-crystallised C-S-H), and C-(A)-S-H had formed. Globally, porosity decreased at both sides of the interface. Geochemical modelling supports the experimental results, especially the coexistence of tobermorite and phillipsite at 70°C, minerals never observed before in concrete/clay interface experiments

Periprosthetic shoulder infection: an overview

Periprosthetic shoulder infection (PSI) is rare but potentially devastating. The rate of PSI is increased in cases of revision procedures, reverse shoulder implants and co-morbidities. One specific type of PSI is the occurrence of low-grade infections caused by non-suppurative bacteria such as Propionibacterium acnes or Staphylococcus epidemermidis.Success of treatment depends on micro-organism identification, appropriate surgical procedures and antibiotic administration efficiency. Post-operative early PSI can be treated with simple debridement, while chronic PSI requires a one- or two-stage revision procedure. Indication for one-time exchange is based on pre-operative identification of a causative agent. Resection arthroplasty remains an option for low-demand patients or recalcitrant infection

Origin of Cl− and High δ37Cl Values in Radiocarbon Dated-Fracture Groundwaters at the Tournemire URL (France)

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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

On coupling variable porosity and water activity with two-phase flow in reactive transport modelling

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Understanding oxidizing transient conditions in clayey rocks

International audienceLarge amounts of carbon steels are foreseen in most deep geological disposals of high-level radioactive waste. The disposal cells will be initially aerated and the ventilation will renew oxygen at their front edge for years to decades. Excavation damaged zones (Edz) along underground works (tunnels, drifts) may promote oxygen migration. The diffusion and reactivity of oxygen within such zones is still poorly known despite that steel corrosion is significantly enhanced in the presence of oxygen. With this respect, four tests were implemented in the Tournemire Underground Research Laboratory, France. In each test, the oxygen partial pressure and the total pressure within a chamber isolated from the atmosphere by a packer were recorded over 90 to 150 days. Oxygen was never completely depleted in the chambers, likely due to oxygen diffusion through the Edz, whereas pyrite oxidation within the Edz was themain oxygen sink. Four in situ tests have been carried out, with differences in terms of geometries (volume and surface of chambers) and drilling modes (compressed air flushing vs. pressurized argon flushing). A sequential global optimization of the relevant parameters (diffusion and reaction) has been developed, aiming at reproducing as closely as possible the measured four in situ tests data by modelling. The key controlling parameters are shown to be the Edz thickness and the ratio De/kre (effective diffusion coefficient over oxygen consumption first-order effective rate), whose optimum was found between 23 m 2 and 45 m 2 . The Edz extent active for oxygen diffusion ranges between 0.6 and 1.0 borehole radius, with mean water saturation of 0.7, a mean D e of 6×10 −7 m 2 .s −1 and a mean k re of 2×10 −8 s −1 (or k r = 6×10 −7 s −1 per equivalent pore volume). With respect to the safety assessment of radioactive waste disposals, any plug system that would not intercept the Edz could not ultimately prevent oxygen entry into the structure, which could have an im- pact on the dimensioning of carbon steel components and disposal concepts. The proposed optimization methodology could be used to quantify diffusion and reaction kinetics in other contexts where pyrite reactivity and/or oxygen diffusion in water unsaturated media play a key role, such as the leaching of mining ores and acid mine drainage

Effet d'une eau contenant du magnésium sur les propriétés chimiques et mécaniques d'une pâte de ciment à faible rapport C/S

International audienceDans le contexte d'une installation de stockage géologique profond de déchets radioactifs, un enrichissement en magnésium peut se produire dans la matrice cimentaire à l'interface avec la roche argileuse, dont l'eau interstitielle contient du Mg. Cet enrichissement conduit à la formation de silicate de magnésium hydratés (M-S-H) et/ou de brucite (MH). La formation de M-S-H est susceptible d'altérer les propriétés de la matrice cimentaire car elle est liée à la décalcification du C-S-H. Cependant, relativement peu de données existent sur les M-S-H dans la littérature. Afin de caractériser les conséquences de l'attaque magnésienne, une pâte cimentaire modèle, avec un faible rapport atomique Ca/Si, comme dans les liants en contact avec la roche argileuse, a été fabriquée et immergée dans une solution magnésienne à 5 mmol/L de MgCl2. Une analyse EDS couplée à de la DRX montre un enrichissement en Mg de la pâte se traduisant par la formation de M-S-H, associé à une décalcification. La zone enrichie en Mg possède des propriétés microstructurales similaires à celles déterminées précédemment sur des pâtes de M-S-H pures et montre un module de Young résiduel plus faible

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

Chemical and Microstructural Properties of Designed Cohesive M-S-H Pastes

International audienceConcretes can be exposed to a magnesium attack in several environments leading to the formation of magnesium silicate hydrates (M-S-H) and brucite (MH). The formation of M-S-H is likely to alter the properties of the cement matrix because it is linked to the decalcification of C-S-H. However, relatively few data on M-S-H exist in the literature. In order to characterize, physically and mechanically, the M-S-H phase, pure M-S-H cohesive pastes are needed. This work studies the formation of cohesive M-S-H pastes made with MgO-to-SiO2 atomic ratios of 0.78, 1 and 1.3, from two types of silica (silica fume or colloidal silica) and under 20 °C and 50 °C thermal curing. X-ray diffraction and thermogravimetric analyses confirmed that the consumption of brucite and the formation of M-S-H were quicker with a 50 °C curing. Energy-dispersive X-ray spectroscopy and microtomography showed that colloidal silica enabled a better distribution of the particles than silica fume. Microstructural characterizations were conducted under the protocol with colloidal silica and 50 °C thermal curing. Porosity investigations allowed to describe the M-S-H pastes as highly porous materials with a low content of micropores in comparison with mesopores. The type of mixing influenced the mesopore size distribution

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