Search

This study concerns the project of high and intermediate level radioactive waste disposal (Cigéo project for Centre Industriel de stockage GEOlogique). Millions of tons of argillite will be excavated during the construction of the site. Therefore, the aim of this work is to reuse this argillite in the formulation of an injectable geopolymer grout and study its evolution in different storage conditions (T = 20 and 80 °C). To achieve this challenge, a metakaolin and calcined Callovo-Oxfordian (COx) argillite were used with potassium silicate solutions. The grouts were successfully obtained from both metakaolin and COx argillite in both temperatures 20 and 80 °C. The range of viscosity of several mixtures respected the required specifications (viscosity ≤5 Pa s and setting time superior to 6 h). The metakaolin-based geopolymers didn't display any major changes no matter the storage temperature, except for the pore size, a coalescence phenomenon was observed with the increase of temperature. In contrast, for argillite or argillite-metakaolin based samples, a decrease of pore size as well as in pH values were observed with the increase of temperature. This behaviour was attributed to the dissolution of argillite's impurities such as illite, providing more oligomers to allow the formation of different geopolymer networks. [ABSTRACT FROM AUTHOR]

Corrosion of steel reinforcing bars (rebars) in reinforced concrete structures can be addressed by using corrosion-resistant materials like Fiber-Reinforced Polymers (FRP). However, in France, FRP rebars are primarily restricted to temporary applications due to lack of comprehensive durability data. While numerous studies have explored FRP durability in alkaline environments, understanding the combined effect of alkali ions and sustained loading remains limited. Furthermore, the impact of incomplete matrix curing on the long-term performance of FRP rebars is limited. Accordingly, this paper investigates the durability of Glass FRP (GFRP) rebars exposed to alkali environments at varying temperatures. A batch of partially cured rebars was selected for this study, with part of them undergoing post-curing to investigate the effect of curing state on the ageing behaviour. Additionally, some partially cured specimens were subjected to combined sustained tensile loading at 20 % and 40 % of ultimate strength. Mechanical, physico-chemical and microstructural characterizations were conducted after various exposure durations. Results revealed that incomplete curing minimally impacted long-term tensile properties but significantly affected the interlaminar shear strength of aged rebars. Moreover, applying a combined load had little influence on residual properties at 20°C but led to rapid reduction in GFRP residual tensile strength at higher ageing temperatures (40 and 60°C). • Incomplete curing of GFRPs did not affect residual tensile properties after 90 days of direct alkaline aging. • For partially cured GFRP rebars, both post-curing and degradation occur during aging. • Sustained load significantly impacts the durability of GFRPs in alkaline environments. • This load effect exhibits high dependence on temperature. • Partial curing of the GFRPs may amplify the effect of load on long term performance. [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes to compare several experimental ways of obtaining the chloride diffusion coefficient through saturated porous materials. The first method is based on the application of the Nernst–Einstein equation for which the conductivity of the saturated sample is measured by impedance spectroscopy for two kinds of materials: inert samples of TiO2, and concretes based on type I and type V cements. The second method is a migration test in which the flux of chloride measured upstream allows calculating the diffusion coefficient by means of the Nernst–Planck equation. In the third case, the diffusion coefficient is calculated by current measurements in an equivalent configuration as the second method. It is shown that the formation factor does not vary neither with the ionic strength of the saturation solution, nor with the change in the pore solution constituents when the material is without mineral additions. The CEM-V concrete exhibits a specific behavior with a strong influence of the addition of chloride in its pore solution on the formation factor. The diffusion coefficients calculated with the three methods are in good agreement provided the metrology of the experiments is carefully controlled. [Copyright &y& Elsevier]

Abstract: Cement based materials play an important role in the design and performance of future deep radioactive waste disposals. The type of materials selected for engineered barrier systems (EBS) is an important issue: protection of waste packages against corrosion and chemical degradation may be greatly enhanced by a proper choice and dimensioning. The long-term geochemical performance of two concrete types, CEM-I (pure Portland cement) and CEM-V (blended Portland, fly ash, blast furnace slag cement) was evaluated using reactive transport simulations of the interactions between the Callovo-Oxfordian mudrock (argillite) and concrete EBS over periods up to 106 years at 25°C. Simulations have been run with cylindrical geometry using the modeling tool Hytec 3.5 (ENSMP/CIG). Diffusion of solutes in the porewater of the solid media was considered to be the dominant transport process. Different scenarios were studied, including the case of sulfate attack. Various assumptions on the transport properties of the EBS and the argillite medium have been considered and compared. The interactions between concrete, CEM-I or CEM-V, and mudrock are in both cases predicted to lead to a significant clogging in the mudrock, in the vicinity of the interface with the concrete EBS. However, the nature and extent of the EBS/mudrock interface transformations greatly differ for the two materials considered. CEM-V, due to lower diffusion coefficients, keeps longer internal high pH conditions which are favorable to the waste packages from a chemical point of view. In addition, simulations predict that CEM-V material should have a better resistance to sulfate attack than CEM-I based barriers. In the case of CEM-I, the altered clay zone has a greater extent (up to 1.3m in 400,000y) and the drop of porosity is mainly due to zeolites and reprecipitated illite, quartz and calcite. For CEM-V, the altered clay zone is much smaller (less than 0.2m in 400,000y) and secondary smectite and calcite precipitation are predicted to cause porosity reduction. [Copyright &y& Elsevier]

Abstract The impact of temperature on carbonation was investigated in laboratory conditions using a device developed for this purpose. Two hardened cement pastes (CEM I and CEM V/A) were tested between 20 °C and 80 °C at different levels of relative humidity (RH). The carbonation rate of the CEM I increased with temperature, whereas that of CEM V/A reached a maximum at around 50 °C. [ABSTRACT FROM AUTHOR]

The consequences of accelerated carbonation at 3% CO 2 were compared with those of natural carbonation (0.04%). Cement pastes (CEM I and CEM V/A) as well as the three major constitutive phases (C-S-H of different C/S ratios, portlandite and ettringite) were used and changes in the mineralogy, microstructure, water retention and cracking were investigated. The main conclusion was that accelerated carbonation at 3% CO 2 was representative of natural carbonation although it promoted the precipitation of metastable calcium carbonate (aragonite and vaterite) in place of calcite. The results also showed that the presence of aragonite and vaterite were characteristic of the carbonation of ettringite and C-S-H respectively. [ABSTRACT FROM AUTHOR]

In this study, an experimental study is first performed. Two complementary tests are realized on a typical reinforced concrete component sample composed of cement paste cylinders with inserted steel discs. In the first test, the deformation of sample due to the corrosion expansion of steel discs is measured. In the second test, the deformation is restricted while the reaction force due to steel corrosion expansion is identified. Based on the experimental data, a novel numerical approach is developed for the analysis of mechanical response and damage process of the tested samples. A simple but realistic homogenization scheme is proposed for the estimation of effective elastic properties of corroded steel. The evolution of corrosion-induced damage is described by using a phase-field method. The elastic properties of corroded steel are estimated. The predicted values of reaction force are compared with the experimental data. The amplitude of damage and the extent of damaged zone are evaluated and analyzed in terms of corrosion-related expansion ratio. • An original experimental device is designed and carried out for the corrosion study. • The deformation induced by corrosion under a constant prescribed force is measured. • The reaction force under a prescribed displacement due to corrosion is described. • A phase-field method is developed for modeling the corrosion process. • The numerical results are in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Concrete creep models have to consider several important phenomena (non-linearity, multi-axiality, hydration, and thermal and drying effects) to be relevant in structural applications. A selection of experimental results of creep tests found in the scientific literature are used to highlight these phenomena. Firstly, regarding the creep rate in different directions of a specimen under various loads, it is shown that creep rate under moderate loading can derive from elastic strains. Secondly, the reason why a Drucker Prager criterion can be chosen to model non-linear creep is discussed. Thirdly the interest of resorting to a creep theory able to decouple ageing (or hydration) effects and consolidation effects is explained. Moreover, interest using a poro-mechanical formulation, in which Biot coefficient depends on stress state, to model drying creep and shrinkage is discussed in the light of short meso-scopic analysis. The effect of temperature on creep is also addressed. The numerical implementation of the proposed modelling is briefly exposed and the model responses are confronted with experimental results. [ABSTRACT FROM AUTHOR]

The aim of this study is to determine the Representative Volume Element (RVE) size related to the desorption isotherm on concrete. Slices of three thicknesses taken from cast cylinders of an OPC concrete were desorbed from fully saturated condition to the dry state. The slice thickness had no clear influence on the water content once mass stabilization was reached. A method is being proposed to determine the RVE based on a statistical analysis of the water contents (or water saturation degrees) of the slices. The results show that a sample with the volume 7 d 3 may be considered as the RVE of an OPC concrete ( d is the maximum aggregate size). [ABSTRACT FROM AUTHOR]

In unsaturated conditions, the durability of concrete structures is strongly dependent on the evolution of the amount of free water within concrete porosity. Reliable durability assessment of concrete structures in relation to their environment thus requires accurate unsaturated water transport description as well as reliable input data. The effect of carbonation on water transport remains poorly studied and data are lacking. It was then the purpose of this article to acquire all the data needed to describe unsaturated water transport in carbonated cementitious materials (porosity, water retention and unsaturated permeability). Four hardened pastes made with four different binders were carbonated at 3% CO 2 to ensure representativeness with natural carbonation. Beyond the modification of the water retention curve and porosity clogging, significant microcracking due to carbonation shrinkage was observed. The consequence on permeability highlighted a competition between porosity clogging and microcracking that was dependent on the initial mineralogical composition. [ABSTRACT FROM AUTHOR]

Abstract: Relative permeabilities and water vapor diffusion reduction factor coefficients are essential data regarding the modeling of moisture transport and hence the prediction of structure lifetimes. The dependency of these parameters on the humidity history and the chemical evolutions of the material limits their experimental measurement since tests are long to carry out. For cement-based materials, the most usual approach for the calculation of relative permeabilities is the one developed for soils by Van Genuchten, requiring calibration on experimental water retention curves. Moreover, the model involves a pore network connectivity factor, the value of which varies from one author to another. The water vapor diffusion reduction factor is often calculated via Millington's semi-empirical model. This paper presents new methods for calculating transport factors based on physical and statistical considerations associated with a pore network model developed for the prediction of water sorption isotherms. [Copyright &y& Elsevier]

Abstract: The healing of cracks plays an important role with regard to the air- and water-tightness of concrete structures and the durability of cement based materials in general. This paper aims at further characterization and better comprehension of the healing phenomenon induced by the precipitation of calcite in localized cracks. The experimental program consisted in generating a localized crack in a cement paste or concrete specimen and healing the crack created by percolation tests with carbonated water or gas (CO2-air mixture). For tests with liquid, results show that the healing process depends on physical parameters like crack width, pressure gradient and carbonate content in the percolating fluid. For tests with gas, the supply of the crack with Ca2+ ions to form calcite depends on the moisture transport mode on the crack edges, conditioned by the relative humidity of the percolating gas mixture. A simplified model of the leakage rate evolution is proposed. It provides indications concerning the effect of each test parameter on the healing process and allows the experimental results to be reproduced. Application of the model to other tests from the literature shows its limits and gives guidance for future investigations. [Copyright &y& Elsevier]

Abstract: This experimental study aims at identifying the water retention properties of two industrial concretes to be used for long term underground nuclear waste storage structures. Together with water retention, gas transfer properties are identified at varying water saturation level, i.e. relative gas permeability is assessed directly as a function of water saturation level Sw. The influence of the initial de-sorption path and of the subsequent re-saturation are analysed both in terms of water retention and gas transfer properties. Also, the influence of concrete microstructure upon water retention and relative gas permeability is assessed, using porosity measurements, analysis of the BET theory from water retention properties, and MIP. Finally, a single relative gas permeability curve is proposed for each concrete, based on Van Genuchten–Mualem''s statistical model, to be used for continuous modelling approaches of concrete structures, both during drying and imbibition. [Copyright &y& Elsevier]

Abstract: This contribution aims at (1) determining experimentally the first desorption isotherms of two high performance concretes (based on CEM I and CEM V cements), at three different temperature levels T =20, 50 and 80°C, and (2) showing whether these may be simply described by adsorption and capillarity. Results at 20°C compare well with recently published results; in particular, an important desorption is observed at high RH (≥98%). A simple pore network model, based on Kelvin–Laplace''s capillary theory, is implemented, and shows the existence of two main de-saturation modes, one at high pore radii (capillary pores), the other for pore radii on the order of a few nanometers (C-S-H porosity), where desorption is shown to occur. A complementary drying mechanism is evidenced at 80°C, which is attributed to ettringite and/or C-S-H degradation. Finally, proper numerical fitting of all isotherm data is achieved using the well-known three-parameter BSB model. [Copyright &y& Elsevier]

Abstract: Investigations were carried out in order to formulate and characterize low-alkalinity and low-heat cements which would be compatible with an underground waste repository environment. Several systems comprising Ordinary Portland Cement (OPC), a fast-reacting pozzolan (silica fume (SF) or metakaolin (MK)) and, in some cases, a slow-reacting product (fly ash (FA) or blastfurnace slag (BFS)) were compared. Promising results were obtained with some binary mixtures of OPC and SF, and with some ternary blends of OPC, SF and FA or BFS: pH of water in equilibrium with the fully hydrated cements dropped below 11. Dependence of the properties of standard mortars on the high contents of FA and SF in the low-pH blends was examined. Combining SF and FA seemed attractive since SF compensated for the low reactivity of FA, while FA allowed to reduce the water demand, and dimensional variations of the mortars. Finally, low-heat (ΔT <20 °C under semi-adiabatic conditions) and high strength (≈70–80 MPa) concretes were prepared from two low-pH cements: a binary blend made from 60% of OPC and 40% of SF, and a ternary blend including 37.5% OPC, 32.5% SF and 30% FA. [Copyright &y& Elsevier]

[Display omitted] • Carbonates persist after calcination at 600 °C. • Dolomite is partially decomposed at 750 °C. • A kaolinite gangue hinders the decomposition of dolomite at 750 °C. • Geopolymers based on high carbonates content exhibit lower shrinkage. • Crystallisation of different calcium and/or magnesium silicates was evidenced. In order to develop fire resistant geopolymer materials, it is necessary to understand the parameters controlling their thermal behavior such as the content of alkali or rare earth cations. This study highlights the effect of carbonate nature and amount on the thermal behavior of geopolymers in order to propose a model of thermal behaviour and predict shrinkage. For this, various mixtures of kaolin and calcite and/or dolomite with different percentages were calcined at 600 and 750 °C. The feasibility of consolidated materials based on these mixtures were evaluated and the thermal behavior of the obtained materials was investigated. The characterization of the mixtures revealed the persistence of carbonates after calcination at 600 °C and their partial decomposition after calcination at 750 °C. This fact was explained in the case of high dolomite content by the existence of kaolinite gangue hindering its decomposition. Consolidated materials were obtained from the different mixtures. After thermal treatment at 1000 °C, it was evidenced that the amorphous phase crystallizes to form in a major part leucite. At lower available alkaline earth species (Ca2+ and Mg2+) content, only wollastonite is formed and the shrinkage varies between 15 and 12%. However, at higher content, different calcium and magnesium silicates can be formed such calico-olivine and larnite in the case of calcite based materials and akermanite and merwinite for dolomite based materials. This fact leads to lower shrinkage value (from 6 to 8%). Thus, it is possible to control the thermal behavior and predict the shrinkage of geopolymer materials by controlling the available alkaline earth cations. [ABSTRACT FROM AUTHOR]

Kaolinite and its dehydroxylated forms are widely used in industry, including the growing field of geopolymers. Until now, a deep understanding of the dissolution mechanism of kaolinite particles in alkaline media is still lacking. This work aims to investigate the dissolution mechanisms through aluminum and silicon leaching of two references kaolinites (KGa-1b and KGa-2) regarding various 2 M alkali treatment (KOH, NaOH and 50/50 NaOH + KOH) as a function of time and at 80 °C. For this, a wide range of methods have been used, from the determination of [Si] and [Al] in solution to the analysis of solids by combining X-ray diffraction (XRD), Fourier Transformed Infrared (FTIR), X-ray Absorption Near Edge Structure (XANES) and Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) spectroscopies and Transmission Electronic Microscopy (TEM) observations. The results evidence a higher dissolution rate with poorly crystallized kaolinites, confirming the role of the crystallinity of the samples toward reactivity in an alkaline media. In addition, the nature of the cation significantly affect the degree of dissolution following the sequence KOH < NaOH + KOH < NaOH. This process is combined with a change in Al speciation from 6-coordinated to 4-coordinated sites, as evidenced by XAS and MAS-NMR experiments. Altogether, these results allow to propose different dissolution mechanisms at the particle scale depending on the cations. The dissolution in presence of KOH proceeds uniformly from the edges, starting from the edges of the octahedral sheet, whereas in the case of NaOH, the edges dissolution is oriented with respect to areas of less coherence starting from the tetrahedral sheet. [Display omitted] • The degree of dissolution of kaolinites is linked to the type of alkaline treatment. • The dissolution rate is higher in the poorly crystallized kaolinites. • The dissolution is accompanied by a change in aluminum speciation. • The particle edges dissolution is homogeneous in KOH and oriented in NaOH. [ABSTRACT FROM AUTHOR]

Knowing the water transport phenomena is fundamental for understanding the durability of cement-based materials. The moisture content within cementitious materials is of primary importance because it has a direct influence on mechanical (shrinkage, creep) and chemical (penetration of aggressive agents) degradation. Many models based on the determination of the transfer properties of the material under study already exist but require fitting of the parameters to find the experimental kinetics. Transfer parameters are generally estimated from the empirical approaches developed for soils by Van Genuchten and the water vapour diffusion reduction factor is often calculated via Millington's semi-empirical model. In this paper, we make a proposal to improve the prediction of moisture transfer, through mechanisms such as drying, capillary imbibition and water vapour permeability in cement-based materials, by using a porous network model to access the transfer parameters. [ABSTRACT FROM AUTHOR]

Concrete shrinkage and creep under variable hydric conditions are important factors for the safety and durability of concrete especially in nuclear reactor or nuclear waste storage background. A large (and long – more than 900 days) experimental campaign, conducted on two different concretes, has therefore been designed to study the strains of non loaded and loaded concretes submitted to drying and liquid water imbibition cycles. For the purposes of comparison, concrete strains and mass variations during drying only (50% RH) and/or following cycles of drying/rewetting were also recorded. This allowed the identification of desiccation shrinkage, basic creep and drying creep at 10 MPa of axial stress. Important results were found and have shown that the final mass and strain are not deeply modified by the introduction of a rewetting phase either for a loaded or a non loaded material. [ABSTRACT FROM AUTHOR]

The conditions that lead to Transient Thermal Deformation (TTD), also called Thermal Transient Creep (TTC), of concrete subjected to various temperatures and compressive loads in sealed conditions are analysed and a model based on a two-scale porosity is proposed. In this model, water expands in the smaller pores and diffuses progressively to the capillary pores at the upper scale. It is then shown that the water overpressure in smaller pores and the duration of the diffusion process between the two scales of porosity control the TTD. It is also shown that creep or shrinkage before the temperature transition mitigates TTD. The aim is to provide a model able to predict the conditions for TTD to appear and the amplitude of the TTD under various thermo-mechanical conditions. The TTD model is implemented in an existing basic creep model, fitted and compared with different experimental results. [ABSTRACT FROM AUTHOR]

The influence of atmospheric carbonation on gas diffusion was investigated using four hardened cement pastes (CEM I, CEM III/A, CEM V/A, and a low-alkalinity binder) with common water-to-binder ratio (0.4). The diffusivity of the non‑carbonated and carbonated pastes with respect to helium and nitrogen was measured at different relative humidities. Carbonation decreased the diffusivity of the CEM I paste, whereas that of the other binders significantly increased after carbonation. These results show the competition between porosity clogging and cracking induced by carbonation. The consequences of carbonation are therefore believed to depend on the considered binder. Clogging dominates in ordinary Portland cement (OPC), leading to a decrease in its transport properties after carbonation; cracking dominates in blended cements, leading to a significant increase in its transport properties after carbonation. [ABSTRACT FROM AUTHOR]