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

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

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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

Temperature effect on CEM-I and low-pH hydrated cement pastes in a clayey environment

International audienceThe CEMTEX project in the Tournemire URL studied the temperature effect on the in situ evolution of CEM-I and low-pH cement pastes in contact with argillite after 1, 2 and 5 years, in the context of radioactive waste disposals. Reactive transport modeling was used as a tool to support the most significant experimental results, e.g. occurrence of zeolite and well-crystallized C-S-H at the interface and reduction of cement alteration depth with temperature for CEM-I, but M-S-H precipitation and increase of alteration depth with temperature for low-pH cement

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

Geochemical evolution of cementitious materials in contact with a clayey rock at 70 C in the Tournemire underground research laboratory

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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

Tracing interactions between natural argillites and hyper-alkaline fluids from engineered cement paste and concrete: Chemical and isotopic monitoring of a 15-years old deep-disposal analogue

International audienceSamples of Toarcian argillite were collected both next to and far from a CEM II cement paste and a CEM II concrete, within the specific context of a 15-a old borehole located in the Tournemire Experimental Platform (Aveyron, France). The objectives were evaluation of the mineralogical and geochemical changes of the claystone at the contact with the cementitious materials and determination of the spatial extent of the interactions. The approach includes the examination of the mineralogical (XRD, SEM, TEM), chemical (major, trace, rare earth elements) and isotopic (Sr, C, O) compositions of argillite whole-rocks and of various soluble phases, at two scales: in the rock matrix (P1 scale) and along micro-cracks (P2 scale). The two study scales outline nearly similar mineralogical modifications, shown by the presence of Ca silicate hydrates (C–S–H) and newly-formed CaCO3 within 10–15 mm of the cement paste and concrete. Chemical data from whole-rock argillites indicate few changes in a slightly thicker zone (18–20 mm), mainly consisting of an increase in the CaO wt.%, and a decrease in Sr contents. The other elementary contents remained quite constant except for MgO, which suggests redistribution with precipitation of a Mg-rich mineral phase at 20 mm from cement paste/concrete interface. Acetic acid leachates had more pronounced variations, including a decrease of the total elementary content in the same ‘geochemical disturbed zone’ (GDZ), together with a significant increase of the Sr isotopic ratios. A combination of Sr and C/O isotopic patterns was used to distinguish the behavior of secondary cementitious phases in the clay-rich rock: (i) calcite dissolution and re-precipitation is supported by C/O isotopic data and (ii) C–S–H neoformation is evidenced by the 87Sr/86Sr ratios; this tool also contributes to determine the origin of the fluids. The proportion of newly-formed C–S–H in the matrix and in the micro-cracks of the argillite is modeled