The ettringite (AFt) formation and damage process are investigated for cement paste subjected to external sulfate attack with and without heating treatment during curing. Slice and disc specimens were exposed to 10 g/L Na2SO4 + pH of 13. The evolutions of AFt formation, pore structure, and expansion on slice specimens were characterized through 27Al NMR, MIP, and micrometer. The surface cracks on disc specimens were observed through an optical microscope. The experimental results show that: (1) heating treatment dissolves part of AFt and accelerates the ettringite formation and material expansion; and (2) heating treatment initiates the microcracks, thus promoting the material cracking.

This paper presents a numerical model developed to reproduce the behaviour of French simplified Bituminized Waste Products (BWP) during a leaching test. The model is calibrated on experimental data sets. BWP were mainly produced during industrial reprocessing of nuclear spent fuel and are classified as low or intermediate activity long lived radioactive waste. Geological disposal is the reference solution for intermediate level long-lived BWP. Under geological disposal facility conditions, and after a long period of time, BWP will undergo water re-saturation from the host rock. A chemo-hydro-mechanical numerical model has been implemented with a finite element scheme to model BWP behaviour under such conditions. The constitutive model takes into account the impact of dissolution, permeation, diffusion and osmosis. Original evolution laws of diffusion coefficient and permeability as a function of the porosity are proposed. Specific mechanical model is proposed including Mori-Tanaka homogenization law. To simulate the hydration of the material, an original and simple method is proposed, avoiding costly two-phase flow resolution and complex calibration of the related parameters. This model was mainly used to reproduce the evolution of the amount of both water absorbed and salt leached by the sample during unconfined water up-taking tests. The calibration is based on experimental data obtained on French simplified BWP containing one highly soluble salt. Water uptake could generate swelling mainly due to osmosis.

This paper presents a poromechanical model for drying of unsaturated porous media valid for a large range of relative humidity. Using the proper laws of thermodynamics, this model is derived and permits to account for different effects that contribute to the effective stress development: the average pore pressure effect, the energy of the interfaces effect, the surface adsorption effect and the Shuttleworth effect. The majority of the input parameters of this model are simply assessed by using two commonly known techniques for the characterization of pores structures applied on experimental desorption isotherms: the B.E.T theory (Brunauer et al., 1938) and the BJH technique (Barrett et al., 1951). Another input parameter (linked to the Shuttleworth effect) is fitted on experimental drying shrinkage strains. This model is tested and validated with experimental data for different porous materials - hardened ordinary cement paste, high-performance concrete and Vycor glass - found in the literature. The obtained results show a satisfactory evaluation of the drying shrinkage strains for all three tested materials, with the possibility of considering zero fitting parameter. Compared to other poromechanical models found in the literature such as the classical Biot-Bishop (Biot, 1941) model and the (Coussy et al., 2003) model, our model appears to be capable of displaying the transition at a certain relative humidity between the capillary pressure effects and the surface adsorption effect, which manifests itself by a plateau in the drying shrinkage strains curve at this value of relative humidity. (C) 2020 Elsevier Ltd. All rights reserved.

Modelling the mechanical response of gas hydrate bearing sediments has become a crucial issue in the environmental field. Insufficient experimental data due to the instability of the gas hydrates make it difficult to develop accurate mechanical constitutive models. This paper suggests a numerical homogenisation method to simulate the response of composite materials like gas hydrate bearing soils. The homogenisation technique is based on fast Fourier transform and thus can be used with real images of soils. This method is applied here to both granular and fine soils for various volume fractions and types of gas hydrate inclusions.

The hydro-mechanical behaviour of a high-density bentonite pellet, potential candidate for engineered barriers in high-level radioactive waste disposal, is investigated through laboratory tests. Water content and volumetric strain are first determined at various suctions (ranging from 9 to 89 MPa) during partial hydration from its initial state. Afterwards, compression tests allow Young's modulus and strength to be determined at various suctions. The experimental results are then interpreted by using an existing model describing the hydro-mechanical behaviour of an aggregate in compacted expansive clay. The analyses show that a single set of parameters is sufficient to predict the suction dependency of volumetric strain, Young's modulus and compressive strengths. These findings would be helpful for further numerical investigations on the hydro-mechanical behaviour of granular bentonite-based engineered barriers by using both finite- and discrete-element methods.

from teaching and research institutions in France or abroad, or from public or private research centers.L'archive ouverte pluridisciplinaire

The permeability of coal seams is pore pressure-dependent. A number of analytical models have been proposed to investigate this problem, but many disregard a crucial factor: the anisotropy of coal. This paper is devoted to investigating the role of anisotropy in modeling the change of horizontal permeability with pressure. Analysis is conducted using a fully anisotropic model that incorporates both the anisotropies of mechanical properties and of the permeability dependence on stresses. Analytical expressions of the pressure-permeability relationship are derived in oedometric and isochoric geomechanical conditions, and validations are conducted against both laboratory and field data. Then, the roles of the anisotropy of stiffness and of permeability dependence on stresses in the permeability-change model are explored. We demonstrate that the mechanical anisotropy can be simplified to an isotropic model without introducing significant errors in prediction of pressure-permeability relationship, while neglecting the anisotropy of permeability dependence on stresses leads to considerable errors. When both anisotropy sources are disregarded, the pressure-permeability curve can be exactly reproduced by a totally isotropic material in both oedometric and isotropic conditions. However, the material properties (e.g., bulk modulus) are skewed; moreover, such an equivalent material might lead to significant errors in other geomechanical conditions. Finally, the permeability change is investigated at the reservoir scale, and the reservoir simulation results confirm the conclusions obtained from the analytical analysis.

Proceedings of the Sixth Biot Conference on Poromechanics, held in Paris, France, July 9–13, 2017. Sponsored by Ecole des Ponts ParisTech; Instutut Francais des Sciences et Technologies des Transports, de l’Amenagement, et des Reseaux (IFSTTAR); Centre National de la Recherche Scientifique (CNRS); Agence Nationale de la Recherche (ANR); and the Engineering Mechanics Institute of ASCE. This collection contains 254 peer-reviewed papers on advances in poromechanics. Applications of poromechanics in fields as varied as civil and building engineering, biomechanics, wood science, geomechanics, and petroleum engineering were presented. Topics include: hydromechanical couplings in saturated materials; partially saturated porous materials, surface effects, and adsorption; multiphysical couplings; characterization of materials and properties; creep and plasticity; transport; relation between microstructure and properties; dynamic phenomena; instabilities and strain localization; and fracture propagation and petroleum engineering. Civil engineers, physicists, mechanicians, and physical chemists will find these proceedings a useful reference on poromechanics.

Proceedings of the Sixth Biot Conference on Poromechanics, held in Paris, France, July 9–13, 2017. Sponsored by Ecole des Ponts ParisTech; Instutut Francais des Sciences et Technologies des Transports, de l’Amenagement, et des Reseaux (IFSTTAR); Centre National de la Recherche Scientifique (CNRS); Agence Nationale de la Recherche (ANR); and the Engineering Mechanics Institute of ASCE. This collection contains 254 peer-reviewed papers on advances in poromechanics. Applications of poromechanics in fields as varied as civil and building engineering, biomechanics, wood science, geomechanics, and petroleum engineering were presented. Topics include: hydromechanical couplings in saturated materials; partially saturated porous materials, surface effects, and adsorption; multiphysical couplings; characterization of materials and properties; creep and plasticity; transport; relation between microstructure and properties; dynamic phenomena; instabilities and strain localization; and fracture propagation and petroleum engineering. Civil engineers, physicists, mechanicians, and physical chemists will find these proceedings a useful reference on poromechanics.

The understanding of transport and mechanical processes in rocks with a tight matrix, such as coal, is important to assess unconventional hydrocarbon-bearing geological formations. Whenever coal cores cannot be recovered to be studied in the laboratory, reconstituted coal samples from crushed granular coal can help estimate the coal matrix transport and adsorptive-mechanical properties. In fact, the use of crushed granular coal is advantageous to measure coal adsorption isotherms quickly. Yet, reconstituted coal specimens have an intergranular porosity system and a mechanical behavior that significantly differ from the in-situ coal seam. We apply a double porosity poromechanical model that captures the influence of the coal matrix adsorptive-mechanical properties on the overall response of reconstituted granular coal specimens. Two laboratory examples provide evidence for (1) adsorption-induced swelling stress during constrained CO2 adsorption and (2) desorption upon mechanical loading at constant CO2 pressure. Results show that the magnitude of adsorption-mechanical couplings in reconstituted granular coal specimens is extremely sensitive to the compaction of the granular specimen. High initial compaction, high effective stress during testing, and relatively low fluid pressure facilitate identifying the impact of the coal matrix adsorption and swelling on the overall specimen response. Yet, creep and elastic nonlinearities may hinder sorption-induced strains and stresses. Well designed experiments and application of a double porosity model are critical to obtain a robust interpretation and understanding of the adsorptive-mechanical properties of reconstituted granular coal. Conclusions from our findings, however, discourage the use of reconstituted granular coal to quantify swelling strains and stresses and their potential impact on reservoir permeability.

The present study aims to investigate the influence of microstructure, particularly the effect of aggregates content and interfacial transition zone (ITZ), on tritiated water (HTO) diffusivity in mortars.To this end, three different series of mortars were prepared and HTO diffusion tests were conducted. Variables are water-to-cement ratio, sand volume fractions, and particle size distribution. In parallel, the microstructure of these materials was characterized by water porosimetry, mercury porosimetry, and by backscattered electron microscopy associated to images analysis.It was observed that at low sand content (0%–50%), diffusion properties of mortars are dominated by aggregates dilution effect. Beyond 50% of standard sand, other effects related to the large number of sand grains appear, such as air voids and porous areas mainly due to the difficulty of obtaining well-compacted materials.

Cement is a huge carbon dioxide producer. Supplementary cementitious materials can help reduce this outcome. However, carbonation of these blended cements remains an active subject of research. Accelerated carbonation tests (10% CO2, 25°C and 62% RH) are performed on fly ash blended cement pastes. Experiments are performed at varying ages of carbonation (1 to 16weeks) to measure the evolution of the carbonation depth over time and to quantify key parameters: thermogravimetric analysis (TGA), mercury intrusion porosimetry (MIP) and gamma ray attenuation method (GRAM). The total porosity decreases with a rearrangement of the microstructure due to carbonation and the creation of big capillary pores for the paste with the highest contents of fly ash (60vol.%). The C-S-H molar volume evolution during fly ash-blended cement carbonation is calculated using a method combining MIP, TGA and GRAM formerly successfully applied to OPC paste in a paper published in the same journal.

The recovery of natural gas from coal bed seams is usually accompanied by a significant increase of permeability induced by coal matrix shrinkage and stress relaxation upon gas desorption. This advantageous increase in permeability may be impaired sometimes by mechanical failure of the reservoir rock and ensuing production of coal fines. Near-wellbore stress concentration and reduction of lateral stresses are known to promote shear failure during depletion in oil and gas reservoir formations. Yet, conventional analyses have shown limited success in predicting coal failure, since other chemo-physical mechanisms may be responsible in enhancing the conditions towards mechanical failure in the coal bed reservoir rock. We show a set of triaxial experiments involving gas desorption from coal cores under zero-lateral strain condition (radial stress measured and controlled) and constant total vertical stress meant to simulate the stress path during production far from the wellbore. CO2 is used as surrogate fluid for CH4. The experimental data indicates that desorption can significantly help reduce lateral stress (and increasing deviatoric stress) until shear failure occurs. The results suggest that depletion-induced shear failure is much more likely to occur in coal seams than in conventional non-sorbing reservoir rocks. The adsorptive-mechanical coupling turns out to be a key phenomenon in the process. Numerical simulations at the representative elementary volume scale adopting a double-porosity poromechanical model support the experimental findings and permit calculating a critical gas pressure for shear failure to happen. This emergent phenomenon is comparable to the outcome of other situations such as mineral dissolution or thermal contraction, where shrinkage relaxes lateral stress and acts as an intensifying driver for promoting shear failure within the reservoir rock. Coupled numerical simulation is needed to include near-wellbore effects and validate our findings with actual field observations. A thorough understanding of the coupled response of coal seams is necessary to enhance reservoir management and mitigate the effects of coal failure on fines production.

The reservoir response of unmineable coal seams to primary and enhanced natural gas recovery is strongly affected by gas sorption and the swelling properties of the coal reservoir rock. In-depth understanding of the process of gas sorption/desorption in the coal matrix, induced deformation and measurement of relevant physical parameters are critical for predictive reservoir management. Models used in industry practice are based on swelling strains measured in “free” swelling coal or on empirical correlations between strain and adsorption, and predict permeability changes based on changes of porosity or stress calculated assuming an analogy with thermoelasticity. However, not only coal seams are subjected to in-situ stresses and geometrical boundary conditions but also sorption and strain are strongly coupled. Representative experiments and a truly coupled model for coal seams are needed in challenging applications. We present a set of triaxial testing measurements on 38mm diameter fractured sub-bituminous/bituminous coal cores exposed to CO2. Testing includes the measurement of fluid uptake, adsorption-induced strains and stresses, and the impact on simultaneously measured permeability. Noteworthy, we measured increases in effective stress of up to 29MPa when injecting CO2 at 5MPa and preventing the coal core to swell. The results are analyzed with a poromechanical model in which coal matrix microporosity and adsorption-induced phenomena are embedded into a fractured reservoir rock with transverse isotropic properties. The adsorptive–mechanical coupling in the coal matrix is integrated through an adsorption stress function and fractured coal permeability is estimated as a function of Terzaghi's effective stresses (parallel and perpendicular to the bedding plane). The experimental results and model predictions help identify the characteristic response of coal microporosity and cleat macroporosity on the poromechanical response of coal cores, and suggest that order of magnitude changes of reservoir permeability observed in the field are linked to sorption-induced change on Terzaghi's effective horizontal stress under laterally constrained displacement condition. Together, the modeling and experimental characterization offer unprecedented insights into the mechanics of coal.

The purpose of this article is to investigate the carbonation mechanism of CH and C-S-H within type-I cement-based materials in terms of kinetics, microstructure changes and water released from hydrates during carbonation. Carbonation tests were performed under accelerated conditions (10% CO2, 25 °C and 65 ± 5% RH). Carbonation profiles were assessed by destructive and non-destructive methods such as phenolphthalein spray test, thermogravimetric analysis, and mercury intrusion porosimetry (destructive), as well as gamma-ray attenuation (non-destructive). Carbonation penetration was carried out at different ages from 1 to 16 weeks of CO2 exposure on cement pastes of 0.45 and 0.6 w/c, as well as on mortar specimens (w/c = 0.50 and s/c = 2). Combining experimental results allowed us to improve the understanding of C-S-H and CH carbonation mechanism. The variation of molar volume of C-S-H during carbonation was identified and a quantification of the amount of water released during CH and C-S-H carbonation was performed.

Ions and radionuclide diffusivity in concrete is one of the most important factors that determine service life and safety assessment of cement based structures in nuclear power plants and radioactive-waste repositories. Apart from the influence of cement paste microstructure, the presence of aggregates may have an impact on transport properties of the material. The well-known interfacial transition zone, denoted by ITZ, is created near the aggregates and characterized by a greater porosity. The goal of this study is to investigate the competition between the more diffusing ITZ zone and the less diffusing aggregates. To this end, several series of tritiated water diffusion tests are conducted on mortars characterized by different water-to-cement ratios and sand volume fractions. In parallel, microstructure of these materials is explored by mercury and water porosimetry. It was observed that at low sand content (0% -50%), diffusion properties of mortars are dominated by aggregates dilution effect. At 60% sand, diffusion increases significantly suggesting that percolation's pores threshold has been reached. Results indicate also that sand particle size distribution has a great impact on the diffusivity of mortars.

One of the main issues encountered with CO2 storage in coal beds is the loss of permeability of the reservoir during the injection of carbon dioxide. Such a decrease of permeability originates from the closure of the cleat system in these reservoirs, which itself is a consequence of the swelling of the coal matrix during injection. In this study, we first perform a comparison of the numerical simulation based on the derived model, at the scale of a coal sample, with available permeability measurements of coal injected with carbon dioxide. The second objective of the paper is to capture the effect of the kinetics of transfer of fluid between cleats and coal matrix using the set of equations derived before. We then perform simulations at the scale of a Representative Elementary Volume, and we show that adsorption-induced variations of permeability depend significantly on the kinetics of transfer of fluid between cleats and coal matrix. At the scale of a reservoir, simulations of injection of carbone dioxide in a methane-free reservoir are performed. We discuss the effects of the kinetics of transfer of fluid between cleats and coal matrix on the rates of injection.

Understanding the adsorption‐induced swelling in coal is critical for predictable and enhanced coal bed methane production. The coal matrix is a natural anisotropic disordered microporous solid. We develop an elastic transverse isotropic poromechanical model for microporous solids which couples adsorption and strain through adsorption stress functions and expresses the adsorption isotherm as a multivariate function depending on fluid pressure and solid strains. Experimental data from the literature help invert the anisotropic adsorptive‐mechanical properties of Brzeszcze coal samples exposed to CO2. The main findings include the following: (1) adsorption‐induced swelling can be modeled by including fluid‐specific and pressure‐dependent adsorption stress functions into equilibrium equations, (2) modeling results suggest that swelling anisotropy is mostly caused by anisotropy of the solid mechanical properties, and (3) the total amount of adsorbed gas measured by immersing coal in the adsorbate overestimates adsorption amount compared to in situ conditions up to ∼20%. The developed fully coupled model can be upscaled to determine the coal seam permeability through permeability‐stress relationships.

Taking advantage of atmospheric carbonation of recycled concrete aggregates (RCA) seems particularly attractive to partially reabsorb the chemical part of CO2 emitted during limestone calcination. The purpose of this article is to investigate the carbonation mechanism of a heap of RCA. As a first approximation, the rate of CO2 absorption is studied on model materials made of sieved grains of cement paste made of CEM I. Carbonation penetration is measured by gamma-ray attenuation and by thermogravimetric analysis. A model is proposed and verified thanks to experiments. It is based on a dual-scale approach, associating CO2 diffusion through a granular bed and carbonation of the cementitious matrix. Information is provided concerning the influence of the characteristics of the cementitious phase attached to the original aggregates on the CO2 absorption rate. Moreover a study of the carbonatable amount of hydration products is performed according to the composition of the material and the CO2 concentration.

The present study aims to investigate the influence of mortar’s microstructure, particularly the effect of aggregates content and t he aggregate-paste ‘interfacial transition zone’ (ITZ), on tritiated water diffusivity of cemen t-based materials. To this end, three different series of mortars were prepared and tritia ted water diffusion tests are conducted. Variables are water-to-cement ratio, sand volume fra ctions and particle size distribution. In parallel, microstructure of these materials is char acterized by water porosimetry, mercury porosimetry and by backscattered electron microscop y associated to images analysis. It was observed that at low sand content (0% 50%), diffusion properties of mortars are dominated by aggregates dilution effect. At 60% san d, diffusion increases significantly suggesting that a pores percolation threshold has b een reached. Tests results indicate also that sand particle size distribution has a great im pact on the diffusivity of mortars. RÉSUMÉ. La présente étude consiste à étudier l’influence de la microstructure des mortiers sur la diffusion de l’eau de tritieé au sein de ces matériaux, en particulier, la concurrence entre l’effet diffusive des ITZs et l’effet des gra nul ts non diffusif. À cette fin, plusieurs séries de tests de diffusion de l'eau tritiée sont menées sur des mortiers ayant différents rapport eau sur ciment, différentes fractions volumique de sabl e et différentes granulométries. En parallèle, la microstructure est caractérisée par p orosimétrie au mercure, porosimétrie à l’eau et par l’analyse d’images MEB. Il a été obser vé que, à faible teneur en sable (0% 50%), les propriétés de diffusion des mortiers sont dominées par l’effet de dilution des granulats. À 60% de sable, la diffusion augmente co nsidérablement suggérant qu'un seuil de percolation de pores est atteint. Les résultats des es ais indiquent également que la granularité du sable a un impact important sur la d iffusivité des mortiers.