Effect of Water Activity on Reaction Kinetics and Intergranular Transport: Insights from the Ca(OH) 2 + MgCO 3 → CaCO 3 + Mg(OH) 2 Reaction at 1·8 GPa

The kinetics of the irreversible reaction Ca(OH)2 + MgCO3 → CaCO3 + Mg(OH)2 were investigated at high pressures and temperatures relevant to metamorphic petrology, using both in situ synchrotron X-ray diffraction and post-mortem analysis of reaction rim growth on recovered samples. Reaction kinetics are found to strongly depend on water content; comparable bulk-reaction kinetics are obtained under water-saturated (excess water, c. 10 wt %) and under intermediate (0·1–1 wt % water) conditions when temperature is increased by c. 300 K. In contrast, similar reaction kinetics were observed at ∼673 K and 823 K between intermediate and dry experiments, respectively, where dry refers to a set of experiments with water activity below 1·0 (no free water), as buffered by the CaO–Ca(OH)2 assemblage. Given the activation energies at play, this gap—corresponding to the loss of no more than 1 wt % of water by the assemblage—leads to a difference of several orders of magnitude in reaction kinetics at a given temperature. Further analysis, at the microscopic scale, of the intermediate and dry condition samples, shows that intergranular transport of calcium controls the reaction progress. Grain boundary diffusivities could be retrieved from the classic treatment of reaction rim growth rate. In turn, once modeled, this rate was used to fit the bulk kinetic data derived from X-ray powder diffraction, offering an alternative means to derive calcium diffusivity data. Based on a comparison with effective grain boundary data for Ca and Mg from the literature, it is inferred that both dry and intermediate datasets are consistent with a water-saturated intergranular medium with different levels of connectivity. The very high diffusivity of Ca in the CaCO3 + Mg(OH)2 rims, in comparison with that of Mg in enstatite rims found by earlier workers, emphasizes the prominent role of the interactions between diffusing species and mineral surfaces in diffusion kinetics. Furthermore, we show that the addition of water is likely to change the relative diffusivity of Mg and Ca in carbonate aggregates. From a qualitative point of view, we confirm, in a carbonate-bearing system, that small water content variations within the 0–1 wt % range have tremendous effects on both intergranular transport mechanisms and kinetics. We also propose that the water content dependent diffusivity of major species (Mg, Ca) in low-porosity metamorphic rocks is strongly dependent on the interaction between diffusing species and mineral surfaces. This parameter, which will vary from one rock-type to another, needs also to considered when extrapolating (P, T, t, xH2O) laboratory diffusion data to metamorphic processes

Evidence of a Louse-Borne Outbreak Involving Typhus in Douai, 1710-1712 during the War of Spanish Succession

Background: The new field of paleomicrobiology allows past outbreaks to be identified by testing dental pulp of human remains with PCR. Methods: We identified a mass grave in Douai, France dating from the early XVIII th century. This city was besieged during the European war of Spanish succession. We tested dental pulp from 1192 teeth (including 40 from Douai) by quantitative PCR (qPCR) for R. prowazekii and B. quintana. We also used ultra-sensitive suicide PCR to detect R. prowazekii and genotyped positive samples. Results and Discussion: In the Douai remains, we identified one case of B. quintana infection (by qPCR) and R. prowazekii (by suicide PCR) in 6/21 individuals (29%). The R. prowazekii was genotype B, a genotype previously found in a Spanish isolate obtained in the first part of the XX th century. Conclusion: Louse-borne outbreaks were raging during the XVIII th century; our results support the hypothesis that typhus was imported into Europe by Spanish soldiers from America

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

Modélisation chimio-mécanique du gonflement et de la fissuration de matériaux cimentaires soumis à une attaque sulfatique externe modérée

International audienceDurability of concrete exposed to an external sulfate attack is a great concern for long-time reliability of nuclear waste containment. Concrete can be subjected to moderate external sulfate attacks, leading to the precipitation of gypsum and ettringite in the porous media. The formation of these minerals generates internal swelling pressures, leading to cracking and expansion of the material, reducing its containment properties. This paper presents experimental results obtained on cement paste samples subjected to a low concentration (30.10-3 mol/l) external sulfate attack during 6 months in realistic service conditions. Samples were characterized using multiple experimental characterization tools such as XRD, SEM-EDS, and nanoindentation to follow the evolution of chemical, mineralogical, microstructural and mechanical properties of the material. Cement paste samples with different C3A ratios were used to boost either ettringite or gypsum formation during the chemical attack in order to assess the impact of each mineral on the degradation. Results showed apparition of cracks parallel to the attacked surface, located in the gypsum formation and portlandite dissolution area. This observation suggested an active participation of gypsum formation in the expansion mechanism. Numerical simulations of the chemical degradation were performed using the reactive transport code HYTEC in order to enrich the interpretation of experimental results. Based on simulation results, an analytical homogenization scheme was applied to estimate Young Modulus values in the degraded area. Depth values of degradation fronts and calculated elastic properties were in good agreement with experimental results

Modélisation chimio-mécanique du gonflement et de la fissuration de matériaux cimentaires soumis à une attaque sulfatique externe modérée

International audienceDurability of concrete exposed to an external sulfate attack is a great concern for long-time reliability of nuclear waste containment. Concrete can be subjected to moderate external sulfate attacks, leading to the precipitation of gypsum and ettringite in the porous media. The formation of these minerals generates internal swelling pressures, leading to cracking and expansion of the material, reducing its containment properties. This paper presents experimental results obtained on cement paste samples subjected to a low concentration (30.10-3 mol/l) external sulfate attack during 6 months in realistic service conditions. Samples were characterized using multiple experimental characterization tools such as XRD, SEM-EDS, and nanoindentation to follow the evolution of chemical, mineralogical, microstructural and mechanical properties of the material. Cement paste samples with different C3A ratios were used to boost either ettringite or gypsum formation during the chemical attack in order to assess the impact of each mineral on the degradation. Results showed apparition of cracks parallel to the attacked surface, located in the gypsum formation and portlandite dissolution area. This observation suggested an active participation of gypsum formation in the expansion mechanism. Numerical simulations of the chemical degradation were performed using the reactive transport code HYTEC in order to enrich the interpretation of experimental results. Based on simulation results, an analytical homogenization scheme was applied to estimate Young Modulus values in the degraded area. Depth values of degradation fronts and calculated elastic properties were in good agreement with experimental results

Nanostructure and Physical Properties Control of Indium Tin Oxide Films Prepared at Room Temperature through Ion Beam Sputtering Deposition at Oblique Angles

International audienceIn this paper, we report the fabrication of porous and crystalline tin-doped indium oxide (ITO) thin films at room temperature by ion beam sputtering deposition at oblique angles using either argon or xenon ions. Deep insights into these systems are provided by coupling nanostructural (scanning and transmission electron microscopies, X-ray diffraction) and optical (spectroscopic ellipsometry, spectral reflectometry) characterizations. This original approach allows extracting important features of the films (porosity, refractive indexes, in-grain carrier densities, and mobilities) not easy to reach locally by other techniques. We propose a model decomposing the complex film’s nanostructure into two layers presenting different electro-optical properties, which are attributed to the shadowing effect, but also to the presence of growth defects and impurities due to the atomic peening. In particular, we demonstrate that ITO films deposited with Xe present a better crystallinity and larger porosity, providing superior in-grain carrier transport and offering more flexibility to design broad-band low-reflectivity surfaces. These results widen the possibilities to engineer transparent and conductive thin films at room temperature with enhanced properties, especially in the near-infrared range where oblique angle deposition allows a reduction of reflectivity even at high doping