The petrography of weathering processes: facts and outlooks

International audienceAbstract Rock weathering has been investigated from atomic to global scales through the different but complementary approaches of mineralogy, petrography, geomorphology and geochemistry. The sequences of mineral reactions involved in the alteration process are now well known. They explain the global trend of weathering phenomena but do not account for the actual rock transformation dynamics. In particular, they ignore the intimate relation of the mineral reaction progress with the increase in connected porosity. At the hand specimen scale, heterogeneity is the rule: mineral reactions are controlled by local physicochemical conditions. Alteration processes depend largely on the rock microstructure properties. They proceed through nearly-closed, semi- and completely open microsystems which are interconnected by fractures or pores. Before being leached out by the solutions which flow in the large fractures (flux), the soluble elements migrate inside the connected porosity through chemical diffusion. The dissolution of the primary minerals is mediated through local gradients of chemical potential. With increasing alteration, the rock porosity increases, as does the length of the fluid passageways and their constrictivity and tortuosity. Consequently, the apparent diffusion coefficient for the most soluble elements decreases. The amplitude of the chemical potential gradients for the most soluble elements is reduced by the progressive coating of the reactive surfaces by clays and Fe oxyhydroxides. The residence time of these elements inside the weathered rock increases as alteration progresses; an effect enhanced by their temporary adsorption on the exchangeable sites of clays and Fe oxyhydroxides. Consequently, the weathering rate decreases with time. A possible new way to calculate weathering rates could be to measure the residence time of soluble elements inside the different microsystems during their migration towards the diluted solution which occurs in the large fractures

15 years of in situ cement - argillite interaction from Tournemire URL: Characterisation of the multi-scale spatial heterogeneities of pore space evolution

International audienceThe solution selected by some countries to isolate radioactive wastes from the biosphere for up to one million years in deep geological repositories includes a multi-barrier disposal design, with steel canister, bentonite and cement materials. The geochemical contrast between such materials and the host rock formation creates perturbations potentially altering the confinement properties of the formation. In this context, the French Institute for the Radiological protection and Nuclear Safety (IRSN) have developed an in situ experimental programme based on the study of cement/argillaceous formation interfaces in their Underground Research Laboratory at Tournemire (Aveyron, France). An in situ engineered analogue of a cement/clay-rock interface which has undergone 15 years of interaction has been characterised. Such important interaction time for an in situ engineered analogue provides a bridge between laboratory-derived data and the long time scale of safety assessment modelling. As the mineralogical and petrological investigations have already been published, this work presents for the first time a quantitative characterisation of the spatial distribution of the porosity in the cement and the clay-rock in terms of time scale and design. Interfaces have been characterised using an autoradiography technique in addition to petrophysical measurements. This technique enables visualisation and quantification of the spatial distribution of the porosity using 2D mapping of decimetric-scale specimens. Thus autoradiographs allow highlighting the relationship between the field heterogeneities and the pore space evolution in each material in contact. Moreover, the porosity measurements show a clogging of the porosity in the clay-rock while the porosity increases in the cement. The extension of the porosity evolution extends to a centimetre on both sides of the interface but is heterogeneously distributed in space as a function of the fissure network and interface geometries. The connected fissure network visualised using autoradiography in the clogged area could permit solute (e.g. radionuclide) transport and may also be interpreted as an evolution of the mechanical properties of the clay-rock formation upon alkaline perturbation. This set of data, with the spatial quantification of the porosity in both cement and clay materials will be useful to constrain reactive transport modelling and thus to predict long term evolution of an engineered barrier

Heterogeneity and variability of clay rock microstructure in a hydro-mechanical double scale FEMxFEM analysis

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Corrosion at the carbon steel-clay compact interface at 90°C: Insight into short- and long-term corrosion aspects

International audienceRods of low alloy carbon steel were corroded under anoxic conditions in compact clay at 90 °C. Gravimetrically-assessed corrosion damage equalled 8.5 μm at 7 months, then slowly increased to 29.5 μm at 76 months. The corrosion damage was heterogeneous at 7 months, and two distinct aspects coexisted: steel was replaced by (Fe,Si,O) corrosion products in some areas, and by Si-poor hydroxide and covered by akaganeite (FeO(OH,Cl)) in others, indicating corrosion in suboxic conditions. A magnetite fringe contacted the steel surface. For greater reaction times, akaganeite disappeared, and only (Fe,Si,O) corrosion products containing Fe sulfide and chukanovite were detected

Distribution of Water in Synthetic Calcium Silicate Hydrates

International audienceUnderstanding calcium silicate hydrates (CSHs) is of paramount importance for understanding the behavior of cement materials because they control most of the properties of these man-made materials. The atomic scale water content and structure have a major influence on their properties, as is analogous with clay minerals, and we should assess these. Here, we used a multiple analytical approach to quantify water distribution in CSH samples and to determine the relative proportions of water sorbed on external and internal (interlayer) surfaces. Water vapor isotherms were used to explain the water distribution in the CSH microstructure. As with many layered compounds, CSHs have external and internal (interlayer) surfaces displaying multilayer adsorption of water molecules on external surfaces owing to the hydrophilic surfaces. Interlayer water was also quantified from water vapor isotherm, X-ray diffraction (XRD), and thermal gravimetric analyses (TGA) data, displaying nonreversible swelling/shrinkage behavior in response to drying/rewetting cycles. From this quantification and balance of water distribution, we were able to explain most of the widely dispersed data already published according to the various relative humidity (RH) conditions and measurement techniques. Stoichiometric formulas were proposed for the different CSH samples analyzed (0.6 < Ca/Si < 1.6), considering the interlayer water contribution

Ceramic technology: how to recognize clay processing

The issue of clay processing concerns both provenance and techno-functional ceramic investigations. In the former, the compositional/textural modification of clay alters the petrofacies expressed by the ceramic body and causes a change from the raw material in terms of bulk chemical and mineralogical composition and petrographical features as well. In the latter, identifying the signs of clay processing will provide information on the steps of the chaîne opératoire and on the technological choices made to adjust paste plasticity and to avoid failures in the following stages of manufacture. Several examples of clay processing were considered, encompassing deliberate addition of natural and artificial temper and clay mixing, other than fractioning and homogenisation to prepare the forming stage. The expected effects ofmineral, vegetal and animal tempers on the paste and on the fired body were outlined. Finally, some analytical guidelines are provided to identify clay processing, using the most common analytical methods. Optical microscopy and electron microscopy provide the main contribution to identifying most of the processing practices on the clay, whereas bulk methods provide indirect evidence that may alone be insufficient to prove the occurrence of a specific transformation, as well as to detect homogenisation features. However, only a careful and multidisciplinary investigation of the ceramic body will help reveal the actions of the chaîne opératoire and to test archaeologicalmodels in a sound bottom-up perspective