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International audience; Concretes can be exposed to a magnesium attack in several environments leading to the for-mation 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 consump-tion 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 distribu-tion 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.

An optical fiber sensor based on a fluorescent molecular probe has been developed to measure the pH of cementitious materials, from the early stages of hydration to advanced ageing of the material. A fluorescent molecular probe, Naphth-AlkyneOMe was entrapped in a cross-linked polyvinyl alcohol-glutaraldehyde matrix to generate thin films with well-defined physicochemical and photophysical properties. The obtained sensing films, with an average thickness of 150 μm, have showed reversible and reliable responses to pH changes. The developed optode has the required characteristics for the study of concrete, with a response time of 100 s and a precision of ±0.1 pH units. Measurements of pH were performed on the surface of low-pH cement paste samples after 4, 6, 7 and 8 days of hydration.

International audience; This innovative study presents a local diagnosis approach based on nanoSIMS technique to determine the biotic/abiotic origin of iron sulphides of (sub)micron size heterogeneously located in the corrosion product layers of iron. For iron coupons corroded in controlled conditions, negative δ34Ssulphides-sulphates were obtained in biotic conditions contrary to δ34Ssulphides-sulphates obtained in abiotic ones. These results underline that nanoSIMS is a reliable local technique to detect the formation origin of (sub)micron iron sulphides. This successful methodology was then applied to a complex system representative of an iron corroded in the conditions similar to those of radioactive wastes storage in geological medium.

International audience; Most 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.

Strontium can be found in low/intermediate level radioactive waste streams, often solidified by Ordinary Portland Cement (OPC) matrices. Amorphous silica is added to improve wasteform performance, leading to low-pH systems. Our aim is to identify the phases controlling strontium immobilization in OPC matrices. The novelty of this work resides in combination between two approaches: 1) Comparing OPC systems to model systems based on alite, yielding matrices containing only major phases, emphasizing the relative role of minor phases; 2) Comparing high- & low-pH pastes. Pastes were characterized using calorimetry, XRD, DTG, SEM/EDS and porosimetry. Leaching experiments were conducted to study Sr retention and diffusivity. Strontium diffusivities in low-pH pastes are lower with respect to high-pH equivalents, despite their higher porosities, due to preferred Sr sorption to low Ca/Si C-S-H. OPC pastes show better Sr retention relative to model systems, suggesting that minor phases (like ettringite and AFm) improve Sr retention.

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 Ltd

International audience; Radioactive 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.

International audience; Model synthetic pastes were designed to obtain a simplified silica system that could be used in durability studies on materials with low calcium-to-silica ratios. The synthetic pastes made use of the well-known pozzolanic reaction between tricalcium silicate (C$_3$S) and nanosilica, covering calcium-to-silica ratios (C/S) from 0.8 to 3.0. The pure C$_3$S paste (C/S = 3.0) contained C-S-H and portlandite, while pastes with C/S < 1.4 only included C-S-H. The pore solution pH directly correlated with the C/S ratio. The synthetic pastes demonstrated a mineralogy, a C-S-H structure, a mean chain length and pore network features that were similar to ordinary Portland cement (OPC) and low-pH materials, depending on their C/S ratios. The main shortcomings were: (1) high water-to-binder ratio and high superplasticizer dosage to maintain the workability of the synthetic pastes of low C/S ratios and (2) significant cracking of the pastes of low C/S ratios.

International audience; In this study, the structure of synthetic magnesium silicate hydrated phases (M-S-H) and the distribution of wateras a function of the Mg/Si ratio were investigated. Different analytical techniques indicated that M-S-H phasesare nano-crystallite hydrated phyllosilicates with relatively short coherence length (1.2 nm) and small particlesize (> 200m2/g). Zeta potential and cations exchange capacity measurements showed a negative surfacecharge on M-S-H from the deprotonation of the silanol groups. The negative surface charge is compensated byexchangeable magnesium or others cations. Combined analyses were used to characterize the water distributionin M-S-H revealing the following: i) confined water; its amount depends mainly on the relative humidity ii) veryconfined water (or possibly water coordinated to magnesium), and iii) hydroxyl groups bound to magnesium andto silicon.

International audience; A Portland cement (CEM I) paste was poured onto an argillite disk in diffusion cells with reservoirs filled by alkaline water and argillite pore water. The system evolution was followed over the course of 415 days. The imposed temperature of 70 °C affected the mineralogy (precipitation of crystallized C-S-H) and mechanical strength of the interface that became brittle over time. The interface consisted of a calcite/tobermorite/C-A-S-H layer, whose thickness increased at a growth rate of 0.3 μm/d. Contrary to calcite crusts that formed in immersion tests or when hardened cement was placed in contact with argillite, this layer had no significant effect on the diffusion properties during the one-year duration of the experiment due to its microporous structure and rather small thickness (100 μm). The argillite mineralogy was altered over 100 μm. In the cement paste, the total porosity decreased because carbonation was enhanced with temperature, which counterbalanced the effect of decalcification over 400 μm.

International audience; Large 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.

International audience; The 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.

Calcium silicate hydrate (C-S-H) and magnesium silicate hydrate (M-S-H) have been described as two separate phases. This work investigates their stability domains and the possible uptake of small amounts of magnesium by C-S-H or, reversely, the uptake of small amounts of calcium by M-S-H. The phases, synthesized by co-precipitation, are characterized using a large panel of techniques (thermogravimetry analysis, X-ray diffraction, Xray pair distribution function analysis, 29Si MAS-NMR spectroscopy, measurement of zeta potential and cation exchange capacity) while the compositions of the solutions at equilibrium are determined by ion chromatography and pH measurements. Syntheses of C-S-H samples in the presence of magnesium ((Ca + Mg)/Si = 0.80 and Mg/Si = 0.05 or 0.10) yield two separate phases: C-S-H and M-S-H. There is no experimental evidence of any uptake of magnesium by C-S-H. On the contrary, when M-S-H samples are synthesized in the presence of calcium ((Ca + Mg)/Si = 0.80 and Ca/Si = 0.05 or 0.10), the low pH of the suspension (9–10) prevents the formation of C-S-H but favors the precipitation of M-S-H with small amounts of calcium. This latter may be sorbed onto the surface of M-S-H to outbalance its negative charges and/or incorporated into the interlayer, as suggested by small structural changes.

The addition of MgCl2 to synthetic calcium silicate hydrate (C-S-H) with Ca/Si = 0.8 lowers the pH values below 10. Experimental observations and thermodynamic calculations show the destabilization of C-S-H and the precipitation of magnesium silicate hydrate (M-S-H). In contrast, the addition of MgO to C-S-H increases the pH values up to 11.5 and the Ca/Si in C-S-H from 0.8 to ~ 1.0. At high pH, the formation of M-S-H and brucite is observed experimentally. Less brucite is present at 50 than at 20 °C indicating a slow dissolution kinetic of brucite at 20 °C. Thermodynamic modelling is in good agreement with the changes in C-S-H and M-S-H formation in the presence of MgO although no brucite is predicted. M-S-H is observed in the presence of both MgCl2 and MgO, is stable from pH 7.5 to 11.5 and has a structure comparable to the structure of M-S-H prepared from MgO and SiO2 only.

International audience; This paper describes a multi-scale approach for the modelling of the degradation of model cement pastesusing reactive transport. It specifically aims at incorporating chemistry-transport feedback results from apore-scale approach into a continuum description. Starting from a numerical representative elementaryvolume of the model cement paste, which was built according to extensive experimental dedicatedchacarterizations, this paper provides three separate descriptions of two different degradations: leachingand carbonation. First, 2D pore-scale simulations are performed and predict degradation depths in verygood agreement with experiments. Second, 3D pore scale descriptions of how the microstructre evolvesprovides accurate description of the evolution of transport properties through degradation. Finally, thoselatter results are incorporated as a feedback law between porosity and effective diffusion coefficient intoa 1D continuum approach of reactive transport. This paper provides pore-scale explanations of whyreactive transport modelling has encountered mitigated success when applied to cementitious materials,especially during carbonation or degradations consisting of precipitation reactions. In addition to that,different degradation modellings are in very good agreement with experimental observations.

The formation of magnesium silicate hydrates (M-S-H) with MgO to SiO 2 ratios from 0.7 to 1.6 has been studied at 20, 50 and 70 °C. TGA and XRD data reveal that initially brucite and M-S-H are formed while amorphous silica is still present as indicated by FT-IR and 29 Si MAS NMR experiments. In this first step M-S-H with Mg/Si ∼1 with pH values ∼9.4 is formed independently of the total Mg/Si. Investigations by FT-IR and 29 Si MAS NMR detail that the structure of that initial M-S-H evolved with time and M-S-H formed with Mg/Si ranging from ∼0.8 to ∼1.3 after 2 years at 20 °C and after 1 year at 50 and 70 °C. This implies that the composition of synthetic M-S-H depends strongly on temperature and equilibration time. At 50 and 70 °C the M-S-H formation occurs faster although it is thermodynamically slightly less stable. The solubility of M-S-H, talc and antigorite after 1 year or longer shows comparable trends.

International audience; Characterisation and determination of the bio-origin of iron sulphides in anoxiccorrosion systems thanks to isotopic analyses by nanoSIMS. This study is devoted to the determinationof the bacterial or inorganic origin of iron sulphides formed during anoxic corrosion processesby sulfur isotopic composition analyses. Broadly, these analyses are realized by IRMS on dissolved sulphidesand sulphates. Yet in this study, surface local isotopic analyses are realized by nanoSIMS in orderto preserve the information about the corrosion products’ localization. Two kinds of samples are studied:a sample (CBCC) corroded during 13 months in controlled laboratory conditions; and one archeologicalsample corroded during about 2000 years in not very well known conditions. When the corrosion system isperfectly well known, as for the CBCC sample, the local isotopic analyses by nanoSIMS actually enable toconclude on the (bio-)origin of the iron sulphides observed. However, when the isotopic composition of thesulphate’s source is unknown, as for the archeological sample, the origin of iron sulphides cannot alwaysbe determined.; Cette étude est consacrée à la détermination de l'origine biotique ou abiotique de sulfures de fer formés au cours de processus de corrosion anoxiques par l'analyse de la composition isotopique du soufre. Usuellement ces analyses sont réalisées par IRMS (Isotopic Ratio Mass Spectrometry) sur des sulfures et des sulfates préalablement dissous. Dans cette étude, en revanche, une nouvelle approche est utilisée, basée sur des analyses isotopiques locales de surface réalisées en nanoSIMS afin de conserver l'information sur la localisation des produits de corrosion. Deux types d'échantillons ont été étudiés : un échantillon (CBCC) corrodé pendant 13 mois en conditions contrôlées, et un échantillon archéologique corrodé pendant environ 2000 ans dans des conditions mal connues. Lorsque le système de corrosion est parfaitement connu, tel que le cas de l'échantillon CBCC, l'analyse isotopique locale par nanoSIMS permet effectivement de conclure sur la (bio-)origine des liserés de sulfures de fer formés. En revanche, lorsque la composition isotopique de la source de sulfates est inconnue, comme dans le cas de l'échantillon archéologique, il n'est pas toujours possible de conclure sur l'origine des sulfures de fer observés.

This study focuses on a subaqueous mill tailings disposal site located in France (Bois Noirs) where 1.3 million tons of uranium mill sludge (fine tailings fraction < 50μm) have been disposed since the 60's in a man-made pond below 4 meters of water maintained artificially by a rock-fill dam. A significant attenuation of aqueous 226Ra activity is observed in ground waters. This paper presents the preliminary modeling work performed for evaluating the role of water-rock interactions on aqueous 226Ra attenuation through the dam. This modeling attempt, assuming thermodynamic equilibrium, aims at checking the hydrochemical conceptual model developed in a previous study, in which Ra retention through the dam was assumed to most likely result from sorption onto metallic oxide-hydroxides. A 2D coupled reactive transport model was conceived to test this hypothesis over time and identify the measurements required to verify its consistency over the long term.

International audience; The 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.

International audience

In situ interaction experiments over periods of 2, 6, and 10 years between Toarcian argillite and carbon steel discs were carried out in the Tournemire Underground Research Laboratory (URL), yielding a dataset of the materials’ geochemical evolution under conditions representative of the future geological disposal of high-level long-lived radioactive wastes. The carbon steel discs were exposed to corrosion due to trapped oxygen. The corrosion rates indicate that the oxidizing transient lasted between 2 and 6 years. A systematic dissolution of calcium phases (Ca- smectite sheets in I/S and calcite) was observed in the iron diffusion halos. The iron release induced mineralogical dissolution and precipitation reactions, which partly clogged the argillite porosity.

International audience; The interactions between cementitious materials and a clayey deep formation were investigated by studying the specific in situ context of the Tournemire Underground Research Laboratory (URL) of the French Institute for Radioprotection and Nuclear Safety and by reactive transport modelling using the HYTEC code. The study forms part of the safety assessment framework for the deep geological disposal of high to intermediate level long-lived radioactive waste. The in situ context investigated in the Tournemire URL corresponds to an engineered cemented borehole crosscutting the Toarcian argillite formation. The argillite/CEM II cement paste contacts have been in place over 18 a and were sampled in a saturated context outside the excavated disturbed zone (EDZ). Studies of the mineralogy (XRD, carbonatometry, SEM and TEM), petrophysical properties (BET) and geochemistry (TOC, Sr contents, C, O and Sr isotopes, EDS analyses) were carried out both on the argillite and on the cement paste in contact. Alteration of the cement paste is clearly expressed by decalcification and the opening of macroporosity. These modifications are mainly due to the dissolution of portlandite. The neoformation of C-S-H phases was identified in the first few micrometre next to the argillite interface, along with secondary carbonates at the outermost contact. Geochemical measurements argue for the introduction of a sedimentary fluid into the macroporosity of the cement paste to explain the formation of part of these secondary phases. This hypothesis is considered and tested using the HYTEC code, which indicates that such transport could have occurred near the argillite/cement paste contact at a very early stage. After this stage, the transport was reversed and 'cementitious' fluids flowed from the cement paste to the argillite. The changes brought about by these fluids are observed over a thickness of 11-13 mm in a so-called 'black rim', in which carbonates and C-S-H secondary phases are identified in the matrix of the sediment. An illitization process may also be observed in this altered rim, reaching its maximum development towards the inner part. Geochemical analyses show that the argillite disturbances are strictly confined to the black rim. Theoretical mineralogical profiles based on thermodynamic equilibria defined by the HYTEC code are in good agreement with the observations, and are used to achieve a better understanding of transport processes.

The purpose of this work is to study the diffusion properties in an argillaceous formation and how these properties change after the excavation of galleries, over a long period. The Tournemire Underground Research Laboratory (URL) located in Aveyron (France) offers the unique opportunity to investigate different stages of the evolution of the so-called Excavation Damaged Zone (EDZ) over a period of about 124 a. Four horizontal boreholes parallel to the bedding and one vertical borehole were drilled in order to study diffusion transport in the EDZ in both directions in galleries excavated 124, 13, 6, and 1 a ago, respectively. Noble gases dissolved in the pore water of 47 core samples were extracted in order to establish precise depth profiles. Mainly produced in situ by decay of U and Th, radiogenic He accumulates in the pore water of rocks and escapes toward the gallery, allowing a characterization of the diffusive transport in the EDZ and its evolution with time. For the oldest gallery (1882–1888), core mapping reveals an EDZ well developed with fractures up to a depth of 1.8 m. In this zone, all radiogenic He has been lost and at the same time heavy noble gases are trapped in the rock with an enrichment of Xe relative to Kr and of Kr relative to Ar. From 1.8 m, He concentration increases with depth to reach a plateau at about 3.5 m corresponding to a concentration normally found in the unaltered zone. Considering diffusive transport of the radiogenic He to the EDZ, model calculations allowed proposing transport properties in the unfractured and undisturbed zone (UZ) with a pore diffusion coefficient Dp of 1 × 10 −10 m 2 /s deduced from the best-fit curve to experimental data and a mean porosity of 0.1 obtained form petrophysical measurements. The most recent gallery (2008) did not show any evidence of EDZ at depths greater than 7.5 cm. A fracture located around 0.75 m, possibly of tectonic origin, induced large advection and diffusion losses of He on both sides in the next 50 cm. Excesses of Ar, Kr and Xe are found, suggesting diffusion of air from the gallery associated with He losses. The 6-a-old gallery is clearly influenced by fractures of tectonic origin increasing the EDZ thickness of 0.7 m up to a Damaged Zone of 1.8 m. Based on the He concentration profile, the 13-a-old gallery (1996) has a net EDZ of 1.2 m, larger than the 0.7 m deduced from the structural analysis by core mapping, followed by a disturbed zone of up to 3.5 m that could represent an intermediate stage before the development of fractures in the EDZ. The vertical borehole drilled in the same gallery shows a less extensive damaged zone (1.4 m) and allows the determination of a Dp of 3 × 10 −11 m 2 /s taking into account the anisotropy relative to the stratification plane. The rare gas analysis has, therefore, enabled confirmation and better description of the net time-dependent evolution of the EDZ around drifts in the argillaceous Tournemire site: no obvious EDZ for the 1-a gallery (2008), 1.2 m width for the 13-a-old gallery (1996), and 2 m width for the century-old tunnel. The occurrence of a disturbed and unfractured zone (dZ) around the 13-a-old gallery undetected by structural studies and geophysical methods clearly demonstrates the power of radiogenic He in assessing the transport properties around drifts excavated at different ages in compacted clay rocks.

International audience; Samples 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.

The various options to store radioactive wastes in deep geological strata considered in France or Switzerland include the use of large volumes of cementitious materials for infrastructure in contact with argillaceous rocks. So-called low-pH binders were developed to minimize disruption to the surrounding rock by the alkaline plume. Studies conducted on the interaction zone between concrete and clay systematically highlighted the formation of magnesium silicate phases including magnesium silicate hydrate (M-S-H) at the interfaces, which can presently be modeled only partially due to incomplete thermodynamic data. The purpose of this study was to characterize these phases in temperature, aluminum, calcium, and alkali conditions in order to provide the thermodynamic data and improve the calculations on physicochemical evolutions of low-pH concretes and possibly Portland concretes.M-S-H phases were synthesized from magnesium oxide and silica fume in batch experiments at different temperatures, for various times and varying Mg/Si. A large number of different techniques such as chemical solid characterizations coupled with suspension investigations and liquid analyses were used to characterize the phases synthesized. Initially a M-S-H phase with Mg/Si equal to 1 was precipitated in addition to amorphous silica and brucite whatever the total Mg/Si used for the synthesis. After long equilibration times, 2 to 3 years at 20°C or 1 year at 50 and 70°C, the Mg/Si in M-S-H ranged from ~0.8 to ~1.4. The temperature had little influence on the M-S-H formed even if the M-S-H formation occurred faster and M-S-H was thermodynamically slightly less stable when the temperature was increased. At or near to equilibrium, M-S-H phases were characterized with ill-defined structure comparable to nano-crystallite, hydrated phyllosilicates with a surface area greater than 200 m2/g. A M-S-H solid-solution model was calculated and implemented in the thermodynamic database.It was observed that M-S-H also form from calcium silicate hydrate (C-S-H) with a Ca/Si = 0.8 in the presence of additional magnesium. In batch experiments, a low pH of the suspensions (pH ≤ 10) destabilized C-S-H or prevented its formation and favored the precipitation of M-S-H. Detailed investigations showed that small amounts of calcium could be incorporated in M-S-H (Ca/Si ≤0.10), such that also calcium containing end-members were added to the M-S-H solid-solution. At pH ≥ 10-10.5, two separate silicate phases coexist: C-S-H and M-S-H. The interface between a simplified “low-pH” binder mimicked by C-S-H with Ca/Si = 0.8 and a magnesium-rich environment mimicked by M-S-H with Mg/Si = 0.8 confirmed these phenomena. SEM-EDS observations and reactive transport modelling using the thermodynamic data derived in the batch experiments showed the fast deterioration of the C-S-H and the precipitation of M-S-H in the C-S-H disk at the interface and a homogeneous uptake of calcium in the M-S-H disk.The increase of pH favors the sorption. M-S-H with a sodium uptake up to Na/Si ~ 0.20 and without brucite formation were observed at high pH (12.5). The sorption on M-S-H was favored in the order Na+ < Mg2+ < Ca2+. Finally, aluminum was incorporated into M-S-H to form magnesium alumino-silicate hydrate (M-A-S-H). An Al/Si ratio up to 0.2 was observed in presence of sodium aluminate or metakaolin. 27Al MAS NMR data showed that aluminum was present in both tetrahedral and octahedral sites of M-(A-)S-H. The M-(A-)S-H formed had a similar structure as M-S-H with a comparable polymerization degree of the tetrahedral silicates and a similar surface charge., Les différentes options envisagées par la France et la Suisse pour le stockage de déchets radioactifs en couches géologiques profondes argileuses prévoient l’utilisation d’importants volumes de matériaux cimentaires. Les liants dits bas-pH ont été développés afin de limiter la perturbation de la roche encaissante par le panache alcalin. Les études expérimentales menées sur les interfaces béton bas-pH-argile mettent systématiquement en évidence la formation de phases silico-magnésiennes, potentiellement de silicate de magnésium hydraté (M-S-H), mal modélisées à cause de données thermodynamiques limitées. Cette étude a pour objectif de caractériser ces phases en température, en présence d’aluminium, calcium et d’alcalins pour alimenter les bases de données thermodynamiques et améliorer les calculs sur les évolutions physico-chimiques des bétons bas pH et éventuellement des bétons de Portland. Des suspensions de M-S-H ont été synthétisées à partir d'oxyde de magnésium et de fumée de silice à différentes températures, à différents temps de réaction et différents rapports Mg/Si. Un panel de techniques d’analyses de chimie du solide et des mesures en suspensions couplées à des analyses des phases liquides a été utilisé pour caractériser les phases synthétisées. Initialement, et quel que soit le Mg/Si total choisi pour la synthèse, un M-S-H avec un rapport Mg/Si ~1 précipite en présence de brucite et de silice amorphe. Lorsque l’équilibre du système est atteint, 2 à 3 ans à 20 °C ou 1 an à 50 et 70 °C, le Mg/Si varie de ~0,8 à ~1,4. La température a peu d'influence sur le M-S-H formé même si le M-S-H se forme plus rapidement et qu’il est légèrement moins stable thermodynamiquement lorsque la température augmente. A l'équilibre, sa structure mal définie est comparable à des nano-cristallites de phyllosilicates hydratés avec une surface spécifique supérieure à 200 m2/g. Un modèle de solution solide pour le M-S-H a été calculé et ajouté à la base de données.Dans un second temps, les travaux ont été focalisés sur la formation de M-S-H à partir de silicate de calcium hydraté (C-S-H) avec un faible Ca/Si (= 0,8) et de magnésium. Le C-S-H n’est pas stable à des pH avoisinant un pH = 10, ce qui favorise la précipitation de M-S-H. Des recherches détaillées montrent que du calcium peut être faiblement incorporé dans le M-S-H (Ca/Si ≤ 0,10), et des solutions solides contenant du calcium ont été ajoutés à la base de données. Pour des pH supérieurs à 10-10,5, les C-S-H et M-S-H coexistent. L’observation par MEB-EDS d’une interface en cellule de diffusion entre C-S-H (Ca/Si=0,8 représentant un liant bas pH) et M-S-H (Mg/Si=0,8), couplée à la modélisation de celle-ci en transport réactif, sur la base des nouvelles données thermodynamiques dérivées des expériences précédentes, montrent la détérioration rapide du C-S-H et la précipitation de M-S-H dans le disque C-S-H, ainsi qu’une absorption homogène du calcium dans le disque de M-S-H.L’augmentation du pH en solution favorise la sorption de cations. Des M-S-H présentant une sorption de sodium jusqu'à Na/Si ~ 0,20 en absence de brucite ont été observés à des pH avoisinants 12,5. La sorption sur le M-S-H est favorisée dans l'ordre Na+ < Mg2+ < Ca2+. Enfin, l'aluminium s’incorpore dans le M-S-H pour former du M-A-S-H. Un rapport Al/Si jusqu’à 0,2 est observé dans des suspensions synthétisées en présence d’aluminate de sodium ou de métakaolin. Les données de RMN de l’aluminium ont montré que celui-ci est présent dans les sites tétraédriques et octaédriques du M-A-S-H. La phase formée a une structure similaire à celle du M-S-H avec un degré de polymérisation des silicates et une charge effective de surface comparables.