In-operando X-ray scattering characterization of smectite swelling experiments

Swelling capacity of smectite was studied over decades regarding its application as barrier in disposal of nuclear wastes in geological repositories as well as the induced volume change potential in soils according to moisture. In order to improve our knowledge in the swelling capacity of smectite, a miniaturized oedometer was developed to combine swelling pressure measurement with wide angle X-ray scattering (WAXS) characterization in real time during hydration of smectite. This coupled set up allowed studying hydration of smectite up to saturation under confined condions and linking crystalline swelling to pressure at various densities. The modeling of the WAXS patterns gave also quantitative information about the relative proportion of the different interlayer water types at saturation. In situ and operando data were acquired for homo-ionic Na +-and Ca 2+-exchanged smectite at two different densities (1.5 and 1.8 g/cm 3). The results showed that the swelling pressure rise was correlated to a sequence of water layer type with the transition from 0W to interstratification of 2W/3W layers, depending on the density. The cation valency controlled the rate of hydration with faster hydration in the case of divalent exchanged smectite. At saturation, with increasing density, the amount of 3W layers decreased to the gain of 1W and 0W layers. Results also confirmed that at saturation and a density of 1.8 g/cm 3 , the interlayer porosity represented the total one. Finally, this development provided opportunity to improve our knowledge in the swelling mechanism of compacted swelling clay materials upon hydration

Concrete perturbation in a 13-year in situ concrete/bentonite interaction from FEBEX experiments. New insight of 2:1 Mg phyllosilicate precipitation at the interface

The Engineered Barrier System (EBS) implemented in full-scale experiments are designed to provide an understanding of the long-term performance of Deep Geological Repositories (DGR) as nuclear waste repositories. The existing interfaces between the engineered barrier materials, such as concrete and bentonite, constitute reactive surfaces on which the thickness and intensity of degradation of the material, which may affect the confinement properties, are under investigation. This study focuses on a concrete projected on a saturated compacted bentonite from the in situ FEBEX experiment, emplaced in the Grimsel Test Site (GTS, Switzerland) and dismantled during 2015 after 13 years of functioning. Preserved sections crossing the interface have shown macroscopic heterogeneities in 1–2 cm of the shotcrete from the contact with bentonite that presumably affected the porosity distribution. In this area, the distribution of mineral and chemical components has been analyzed in detail, both in concrete that is distant from the interface and in contact with the bentonite. The information provided by detailed mineralogical mappings was consistent with quantitative chemical analysis. Chemical mappings are used to explain the distribution, nature and evolution of the phases in the concrete at the interface with clay. The role of porosity, presumably affected by the initial application of the shotcrete, has influenced the characteristic geochemical reactions in the bentonite-concrete interaction. The chemical composition of di- and tri-octahedral Al–Mg smectites, in the mixing trend of high-charge beidellite-saponite, were identified in the concrete in the degraded area at the interface

Petrophysical Characterization of Fractured Limestone from Beauce Aquifer Vadose Zone (O-ZNS Observatory, France)

International audienceIn recent years, water needs increased, driven by climate change and world population growth. In this context, we study groundwater flow in the vadose zone of Beauce aquifer (O-ZNS site, France). This lacustrine limestone vadose zone is characterized by multi-scale heterogeneities. They are defined by strongly various pore structures. This leads to uncertainties for reservoir properties prediction using geophysical methods which impacts reservoir models for flow simulations.In this study, we combined microstructure description and petrophysical analysis in order to model and predict reservoir properties based on different limestones facies and to infer the influence of weathering/fracturing on both acoustics and electrical properties.A total of 16 samples from these facies were cored and characterized by their porosity, permeability, acoustic velocities, complex electrical properties and microstructure analysis.Based on our multi-method approach, we demonstrated the influence of rock structure on reservoir properties prediction and modelling. Petrophysical and microstructure characterization have highlighted two main facies (microporous and homogenous facies and macroporous and heterogenous facies) which can be used to improve reservoir and flow models. However, further development is needed in order to quantify macropores and their link with weathering and to assess permeability models using electrical properties

Experimental and modelling study of the interaction of bentonite with alkaline water

Compacted bentonite is planned to be used as buffer and backfill materials for the containment of radioactive waste in underground repositories. The performance of these barriers depends on the swelling capacity of bentonite upon hydration. Prolonged interaction between bentonite and alkaline fluids from neighbouring concrete structures can impair the swelling capacity due to profound changes in the chemical composition of bentonite. The coupled hydro-chemo-mechanical behaviour of bentonite under such conditions is at present not well understood. This paper presents for the first time a combined experimental and modelling study that addresses this coupled behaviour with the aim of understanding the key mechanisms leading to swelling pressure loss. Two experiments are presented in which compacted Wyoming bentonite was saturated with either clay or cementitious water, leading to different initial swelling capacities. The samples were subsequently subject to a flow of a KOH-rich cementitious water leading to a slow but sustained decrease in swelling pressure in both tests. The main novelty is the application of a recently developed hydro-chemo-mechanical model for bentonite for interpretation of the experiments. The model accounts for the impact of montmorillonite dissolution, cation exchange reactions, and changes in salinity on the swelling capacity of bentonite. The model results show a relatively good agreement with experimental measurements and suggest that the decrease in swelling capacity of bentonite is driven primarily by an increase in potassium fraction in the interlayer water and by montmorillonite dissolution

5-year chemico-physical evolution of concrete–claystone interfaces, Mont Terri rock laboratory (Switzerland)

The Cement–Opalinus Clay Interaction (CI) Experiment at the Mont Terri rock laboratory is a long-term passive diffusion–reaction experiment between contrasting materials of relevance to engineered barrier systems/near-field for deep disposal of radioactive waste in claystone (Opalinus Clay). Reaction zones at interfaces of Opalinus Clay with two different types of concrete (OPC and “low-pH”/ESDRED) were examined by sampling after 2.2 and 4.9 years. Analytical methods included element mapping (SEM, EPMA), select spot analysis (EDAX), 14C-MMA impregnation for radiography, and powder methods (IR, XRD, clay-exchanger characterisation) on carefully extracted miniature samples (mm). The presence of aggregate grains in concrete made the application of all methods difficult. Common features are a very limited extent of reaction within claystone, and a distinct and regularly zoned reaction zone within the cement matrix that is more extensive in the low-alkali cement (ESDRED). Both interfaces feature a de-calcification zone and overprinted a carbonate alteration zone thought to be mainly responsible for the observed porosity reduction. While OPC shows a distinct sulphate enrichment zone (indicative of ingress from Opalinus Clay), ESDRED displays a wide Mg-enriched zone, also with claystone pore-water as a source. A conclusion is that substitution of OPC by low-alkali cementitious products is not advantageous or necessary solely for the purpose of minimizing the extent of reaction between claystone and cementitious materials. Implications for reactive transport modelling are discussed

Characterization of Cebama low-pH reference concrete and assessment of its alteration with representative waters in radioactive waste repositories

Concretes, mortars and grouts are used for structural and isolation purposes in radioactive and nuclear waste repositories. For example, concrete is used for deposition tunnel end plugs, engineered barriers, mortars for rock bolting and injection grouts for fissure sealing. Despite of the materials anticipated functionality, it is extremely important to understand the long-term material behaviour in repository environments. A reference concrete and mortar for the Cebama project based on a cement, silica and blast furnace slag ternary blend were designed and characterized in different laboratories with multiple experimental methods (XRD, XAS at the Fe and Cl K-edges, SEM-EDX, 29Si and 27Al MAS-NMR, TG-DSC, MIP and Kerosene porosimetry) and techniques (punch strength tests). The reference concrete enabled comparison of results from different institutes and experimental techniques, unifying the individual results to more comprehensive body. The Cebama reference concrete and mortar were designed to have high durability and compatible formulation with respect to an engineered barrier system in clay or crystalline host-rocks, having pore solution pH significantly lower than traditional concretes. This work presents main results regarding their characterization and alteration in contact with representative waters present in radioactive waste repositories. Pore solution pH of the matured reference concrete was 11.4–11.6. The main hydrated phases were C–S–H and C-A-S-H gels with a Ca:Si ratio between 0.5 and 0.7 and an Al:Si ratio of 0.05. Minor phases were ettringite and hydrotalcite. Iron(III) could be in the C–S–H phases and no Cl-bearing solid phases were identified. Connected porosity and pore size distribution was characterized by MIP observing that, as expected, the size of the pores in the hydrated cement phases varies from the micro-to the nanoscale. Connected porosity of both materials were low. Compressive strength of the concrete was 115 MPa, corresponding to traditional high-performance concrete. Degradation of these materials in contact with different waters mainly produce their decalcification and enrichment in Mg for waters containing high amount of this element, like the clay waters.•The performance of “low-pH” mix designs containing slag exceeds the performance of traditional Portland cement concretes.•That slag is able to replace fly ash when producing ”low-pH” mixtures, if needed due to material supply or quality problems.•Eight experimental techniques have provided mechanical, hydraulic and geochemical properties of new designed materials.•Micro-mortar in contact with clay and bentonite waters, shows a Mg enrichment and calcium depletion at the reaction front

Structure of Water Adsorbed on Nanocrystalline Calcium Silicate Hydrate Determined from Neutron Scattering and Molecular Dynamics Simulations

International audienc

Characterization of Cebama low-pH reference concrete and assessment of its alteration with representative waters in radioactive waste repositories

Concretes, mortars and grouts are used for structural and isolation purposes in radioactive and nuclear wasterepositories. For example, concrete is used for deposition tunnel end plugs, engineered barriers, mortars for rockbolting and injection grouts for fissure sealing. Despite of the materials anticipated functionality, it is extremelyimportant to understand the long-term material behaviour in repository environments. A reference concrete andmortar for the Cebama project based on a cement, silica and blast furnace slag ternary blend were designed andcharacterized in different laboratories with multiple experimental methods (XRD, XAS at the Fe and Cl K-edges,SEM-EDX, 29Si and 27Al MAS-NMR, TG-DSC, MIP and Kerosene porosimetry) and techniques (punch strengthtests). The reference concrete enabled comparison of results from different institutes and experimental techniques,unifying the individual results to more comprehensive body. The Cebama reference concrete and mortarwere designed to have high durability and compatible formulation with respect to an engineered barrier systemin clay or crystalline host-rocks, having pore solution pH significantly lower than traditional concretes. This workpresents main results regarding their characterization and alteration in contact with representative waterspresent in radioactive waste repositories. Pore solution pH of the matured reference concrete was 11.4–11.6. Themain hydrated phases were C–S–H and C-A-S-H gels with a Ca:Si ratio between 0.5 and 0.7 and an Al:Si ratio of0.05. Minor phases were ettringite and hydrotalcite. Iron(III) could be in the C–S–H phases and no Cl-bearingsolid phases were identified. Connected porosity and pore size distribution was characterized by MIPobserving that, as expected, the size of the pores in the hydrated cement phases varies from the micro-to thenanoscale. Connected porosity of both materials were low. Compressive strength of the concrete was 115 MPa,corresponding to traditional high-performance concrete. Degradation of these materials in contact with differentwaters mainly produce their decalcification and enrichment in Mg for waters containing high amount of thiselement, like the clay waters

Characterization of Cebama low-pH reference concrete and assessment of its alteration with representative waters in radioactive waste repositories

Concretes, mortars and grouts are used for structural and isolation purposes in radioactive and nuclear waste repositories. For example, concrete is used for deposition tunnel end plugs, engineered barriers, mortars for rock bolting and injection grouts for fissure sealing. Despite of the materials anticipated functionality, it is extremely important to understand the long-term material behaviour in repository environments. A reference concrete and mortar for the Cebama project based on a cement, silica and blast furnace slag ternary blend were designed and characterized in different laboratories with multiple experimental methods (XRD, XAS at the Fe and Cl K-edges, SEM-EDX, 29Si and 27Al MAS-NMR, TG-DSC, MIP and Kerosene porosimetry) and techniques (punch strength tests). The reference concrete enabled comparison of results from different institutes and experimental techniques, unifying the individual results to more comprehensive body. The Cebama reference concrete and mortar were designed to have high durability and compatible formulation with respect to an engineered barrier system in clay or crystalline host-rocks, having pore solution pH significantly lower than traditional concretes. This work presents main results regarding their characterization and alteration in contact with representative waters present in radioactive waste repositories. Pore solution pH of the matured reference concrete was 11.4–11.6. The main hydrated phases were C–S–H and C-A-S-H gels with a Ca:Si ratio between 0.5 and 0.7 and an Al:Si ratio of 0.05. Minor phases were ettringite and hydrotalcite. Iron(III) could be in the C–S–H phases and no Cl-bearing solid phases were identified. Connected porosity and pore size distribution was characterized by MIP observing that, as expected, the size of the pores in the hydrated cement phases varies from the micro-to the nanoscale. Connected porosity of both materials were low. Compressive strength of the concrete was 115 MPa, corresponding to traditional high-performance concrete. Degradation of these materials in contact with different waters mainly produce their decalcification and enrichment in Mg for waters containing high amount of this element, like the clay waters.</p

Water dynamics in calcium silicate hydrates probed by inelastic neutron scattering and molecular dynamics simulations

International audienceCalcium-silicate-hydrate (C-S-H) is a disordered, nanocrystalline material, acting as a primary binding phase in Portland cement. C-S-H and C-A-S-H (an Al-bearing substitute present in low-CO2 cement) contain thin films of water on solid surfaces and inside nanopores. Water controls multiple chemical and mechanical properties of C-S-H, including drying shrinkage, ion transport, creep, and thermal behavior. Therefore, obtaining a fundamental understanding of its properties is essential. We applied a combination of inelastic incoherent neutron scattering and molecular dynamics simulations to unravel water dynamics in synthetic C-(A)-S-H conditioned at five hydration states (from drier to more hydrated) and with three Ca/Si ratios (0.9, 1, and 1.3). Our results converge towards a picture where the evolution from thin layers of interfacial water to bulk-like capillary water is dampened by the structure of C-(A)-S-H. In particular, the hydrophilic Ca2+ sites organize the distribution of interfacial C-(A)-S-H water