Caractérisation rhéologique de bitumes 70/100 utilisé comme matrice de confinement de déchets radioactifs

National audienceL'objectif de cet article est une étude rhéologique approfondie du bitume pur 70/100. Le bitume étant constitué de deux composants majeurs, les asphaltènes dispersés dans une matrice maltène, l'évolution de la microstructure du bitume en fonction de la température rend l'applicabilité du Principe d'Equivalence Temps-température (PETT) incertaine. Deux formes de représentation du PETT seront présentées et l'applicabilité du principe discutée. Le profil d'évolution de la viscosité en fonction du taux de cisaillement et un test de créneaux de contraintes confirment la présence d'un seuil d'écoulement, inférieur à 1 Pa, qui semble thermodépendant en dessous de 50°C. En appliquant la règle de Cox-Merz, une courbe maîtresse de viscosité peut être obtenue sur une très large gamme de taux de cisaillement pour les températures supérieures à 50°C. Elle est modélisée avec précision par une loi de Carreau-Yasuda à seuil

Synthesis of geopolymer foams for decontamination of liquid nuclear waste

Liquid radioactive waste is produced in the nuclear industry and has to be treated to firstly minimize their impact on environment and secondly to propose an ultimate confinement matrix. One way to decontaminate these waste is to synthesize inorganic monolithic filters that are less sensitive to radiolysis phenomena than organic ones. Geopolymer cements are good candidates to fulfill these specifications since intrinsically they are mesoporous with high specific surface area [1] and compatible with specific grafting agents which allow to trap selectively radionucleides of interest (especially the cesium) [2]. For this purpose, a monolithic geopolymer with good mechanical resistance and hierarchical porous network (tailored open macroporosity) was synthesized. From this geopolymer foam, the precipitation of copper hexacyanoferrate into the porous network has been performed in order to trap selectively the cesium. The functionalized foams were characterized and the trapping capacity of Cs was assessed. After having determined the sorption kinetics, sorption isotherms were performed and the maximum sorption capacity, Q = 120 mg/g, was measured. Tests in a radioactive environment were also carried out in order to validate the performance of the material in real conditions (traces of Cs in fresh water). The results show that the functionalized material is capable of selectively trapping Cs with a distribution coefficient Kd of 2.37 105 ml/g. The results demonstrate remarkable potential of this innovative material for Cs removal from liquid nuclear waste. [1] Steins, P., A. Poulesquen, O. Diat, and F. Frizon, Structural Evolution during Geopolymerization from an Early Age to Consolidated Material. Langmuir, 2012. 28(22): p. 8502-8510. [2] Poulesquen. A, A. Gerenton, D. Lambertin, T. Piallat, F. Frizon, Y. Barré, A. Grandjean, Procédé de préparation d’une mousse de géopolymère fonctionnalisée, ladite mousse fonctionnalisée et ses utilisations », Brevet, FR 15/5386

Synthesis of geopolymer emulsions

The understanding of emulsion geopolymer synthesis is a major issue for several industrial applications such as the formation of hierarchically porous material for filtration, lightweight materials for civil engineering or even the conditioning of radioactive mineral oil. Emulsion stability (irreversible coarsening, creaming…) are mainly controlled by the interfacial properties (surface tension and nature of the surfactant) and the viscosity ratio between the dispersed (hd) and the continuous phase (hc). The aim of this paper is thus to study model emulsions (composed of hexadecane (C16) as dispersed phase and metakaolin based geopolymer as continuous phase) with the highest volume fraction of C16 as possible. Surfactant was added to the mixture to stabilize the C16 droplets and geopolymer emulsion was synthesis under shear stirring. The influence of the viscosity of the geopolymer paste controlled by the water content was studied and results show that emulsions are unstable for a viscosity ratio hd/hc lower than 0.01. Up to 70% in volume of C16 was incorporated within the geopolymer and hierarchical porous network was thus obtained. Indeed after removal the C16 phase, the porous network was characterized and a specific surface area of 90 m²/g, a mean mesopore diameter of 19 nm, a macropore size distribution ranged between 10-200 µm (fig 1.) and a compression strength of around 0.5 MPa were obtained. Please click Additional Files below to see the full abstract

Study of geopolymerization mechanisms by 27Al-NMR and calorimetry correlation

Geopolymers are alumino-silicate binders prepared by reacting a powdered alumino-silicate source (metakaolin) with an alkali silicate “activating” solution. The geopolymerization reaction is a complex process but it is consensual that geopolymers are formed by dissolution of the metakaolin and condensation reactions between silicates and aluminates initially in solution or as dissolution products. However, those two processes occur concomitantly during the geopolymerization. It makes it difficult to study geopolymerization mechanisms in detail for kinetics or thermodynamics purposes. This could explain why detailed mechanistic descriptions are scarce in the literature and why this topic is still a matter of debate. In this study, an experimental method highlighting the different mechanisms involved in the geopolymerization is proposed, allowing the determination of a thermodynamic parameter of the system. The different processes constituting the geopolymerization were dissociated by varying the metakaolin content in geopolymers, for a given activating solution. Reactivity of such mixes was investigated by isothermal conduction microcalorimetry (ICC). Time resolved 27Al static Nuclear Magnetic Resonance (NMR) was used to monitor the concentration of aluminate centers in solution during the reaction. The correlation as function of time of the total heat release measured by ICC with the aluminate centers concentration in solution exhibited the existence of a master curve allowing the determination of a reaction enthalpy. The influence of alkali cations, silicate species and aluminate ions on this reaction enthalpy was then investigated. For the first time, the dependence of the geopolymer thermodynamics over the initial composition of the system was highlighted

Un modèle de cinétique d'évolution de populations de bulles dans un fluide à seuil

International audienceThe context of this study is to predict the swelling of bitumen drums in which radioactive salts are mixed (60 wt% of bitumen and 40 wt% of salts). Radioactivity generates uniform volume production of hydrogen by radiolysis of bitumen chains. The creation of gas occurs on very large time scales (more than a hundred years), hence the need to provide model in order to predict the swelling. It has been shown that bitumen is a yield stress fluid. Therefore this work proposes to study the influence of a continuous gas generation and a yield stress on the kinetic evolution of a bubble population. Usually, when a gas is dissolved beyond its solubility limit without yield stress or gas creation, one observes a nucleation-growth-ripening scenario (growth of large bubbles to the detriment of small ones). It selects a distribution of large bubbles over long times, independently of the initial distribution of nuclei. Our work shows that in some cases the gas creation competes with the ripening kinetics and the yield stress can make the bubble population bimodal during its evolution.Le contexte de cette étude est de décrire les processus physico-chimiques élémentaires de base conduisant au gonflement d’enrobés de bitume (suspensions de sels dans une matrice bitume) dans lequel sont conditionnés des sels de coprécipitation de radionucléides. La radioactivité génère une production volumique uniforme de dihydrogène par radiolyse des chaînes de bitume. Ce taux de création de gaz perdure sur des échelles de temps très grandes (plus de cent ans), bien que décroissant au cours du temps. L’étude scientifique vise à conforter les modèles actuels de prédiction du gonflement sous radiolyse des enrobés bitume. Il a été montré que les enrobés de bitume sont des fluides à seuil. Ces travaux proposent donc d'aborder l'influence d’une génération continue de gaz (appelée terme source dans la suite de cet article) et d’un seuil d’écoulement sur la cinétique d'évolution d'une population de bulles. Classiquement, pour un gaz dissous au delà de sa limite de solubilité et en l’absence de seuil et de terme source, on observe un scénario de germination-croissance-mûrissement (croissance des grosses bulles au détriment des petites) qui sélectionne aux temps longs une distribution de grosses bulles, indépendante de la distribution initiale de germes. Nos travaux montrent qu’il existe des régimes où le terme source entre en compétition avec la cinétique de mûrissement et où le seuil d’écoulement peut étaler ou rendre bidisperse la distribution de taille de bulles au cours de son évolution

Rheo-acoustic gels: Tuning mechanical and flow properties of colloidal gels with ultrasonic vibrations

Colloidal gels, where nanoscale particles aggregate into an elastic yet fragile network, are at the heart of materials that combine specific optical, electrical and mechanical properties. Tailoring the viscoelastic features of colloidal gels in real-time thanks to an external stimulus currently appears as a major challenge in the design of "smart" soft materials. Here we introduce "rheo-acoustic" gels, a class of materials that are sensitive to ultrasonic vibrations. By using a combination of rheological and structural characterization, we evidence and quantify a strong softening in three widely different colloidal gels submitted to ultrasonic vibrations (with submicron amplitude and frequency 20-500 kHz). This softening is attributed to micron-sized cracks within the gel network that may or may not fully heal once vibrations are turned off depending on the acoustic intensity. Ultrasonic vibrations are further shown to dramatically decrease the gel yield stress and accelerate shear-induced fluidization. Ultrasound-assisted fluidization dynamics appear to be governed by an effective temperature that depends on the acoustic intensity. Our work opens the way to a full control of elastic and flow properties by ultrasonic vibrations as well as to future theoretical and numerical modeling of such rheo-acoustic gels.Comment: 21 pages, 14 figure

Impact de l'irradiation γ sur les propriétés rhéologiques et chimiques du bitume

International audienceL'objectif de cet article consiste à évaluer et simuler les effets de l'irradiation sur les bitumes, en particulier en terme de propriétés physico-chimiques structurales et rhéologiques. Pour cela, des irradiations γ externes ont été effectuées à différentes doses d'irradiations, allant de 1 à 7 MGy. Une augmentation de la viscosité en cisaillement et du seuil d'écoulement avec la dose a été observée. De même, les modules élastiques et visqueux (G' et G'') augmentent avec la dose, avec une augmentation plus marquée pour G', ce qui traduit un renforcement du caractère élastique, induit par les réticulations résultant de l'irradiation. Par ailleurs, un plateau à basse fréquence apparaît sur G', traduisant ainsi un comportement pseudo-solide (G' ≈ G'') et conduisant à l'augmentation de la viscosité complexe. La réticulation du bitume, induisant son renforcement, a été confirmée par diverses techniques. L'augmentation de la température de transition vitreuse (Tg), est observée par DSC modulée. Une augmentation des fractions aromatiques, concomitante avec une décroissance des fractions aliphatiques, a été observée par chromatographie d'exclusion stérique (SEC), spectroscopie infrarouge (FT-IR) et diffraction de rayons X (DRX)

New insights into the role of hydroxide ions and silicate species during geopolymerization

The specific role of hydroxide ions in highly alkaline silicate solutions has been hardly investigated due to the difficulty to quantify them. In this study, Hammet acidity functions of sodium silicate solutions have been assessed for the first time. The low acidity function values found in these solutions, when compared to pure sodium hydroxide solutions, has been explained by the buffering effect of silicate species using liquid state 29Si NMR. Such a parameter has then been used to quantify the hydroxide ions ability to react during mixing alkali silicate solutions with metakaolin. Despite lower initial acidity function values for equivalent sodium hydroxide additions when compared to silicate-free solutions, it has been demonstrated that dissolution of the studied metakaolin is much more efficient in presence of silicate species. Such a phenomenon has been attributed to the gradual release of hydroxide ions resulting from silicate species condensation during the metakaloin dissolution

Interpenetration of fractal clusters drives elasticity in colloidal gels formed upon flow cessation

Colloidal gels are out of equilibrium soft solids composed of attractive Brownian particles that form a space-spanning network at low volume fractions. The elastic properties of these systems result from the network microstructure, which is very sensitive to shear history. Here, we take advantage of such sensitivity to tune the viscoelastic properties of a colloidal gel made of carbon black nanoparticles. Starting from a fluidized state under an applied shear rate $\dot \gamma_0$, we use an abrupt flow cessation to trigger a liquid-to-solid transition. We observe that the resulting gel is all the more elastic when the shear rate $\dot \gamma_0$ is low and that the viscoelastic spectra can be mapped on a master curve. Moreover, coupling rheometry to small angle X-ray scattering allows us to show that the gel microstructure is different from gels solely formed by thermal agitation where only two length scales are observed: the dimension of the colloidal and the dimension the fractal aggregates. Competition between shear and thermal energy leads to gels with three characteristic length scales. Such gels structure in a percolated network of fractal clusters that interpenetrate each other. Experiments on gels prepared with various shear histories reveal that cluster interpenetration increases with decreasing values of the shear rate $\dot \gamma_0$ applied before flow cessation. These observations strongly suggest that cluster interpenetration drives the gel elasticity, which we confirm using a structural model. Our results, which are in stark contrast with previous literature, where gel elasticity was either linked to cluster connectivity or to bending modes, highlight a novel local parameter controlling the macroscopic viscoelastic properties of colloidal gels

Fragmentation du combustible nucléaire UO2 en environnement oxydant : Etude cinétique et phénoménologique

Une des voies envisagées pour apporter de la flexibilité à la gestion du combustible UO2 usé/irradié est son entreposage à sec. Ce mode de gestion pose en particulier la question de la tenue du combustible en cas de scénario accidentel qui entraînerait la mise à l'air des pastilles de UO2. La cinétique d'oxydation et la phénoménologie de la fragmentation de UO2 entre 250 et 330°C ont été étudiées à partir de vieillissements sous O2 et sous air de monocristaux et de fragments de pastilles de combustible non irradiés. En environnement oxydant, l'initiation de la fissuration semble due à la relaxation des contraintes de traction induites par la contraction du réseau cristallin lors de la croissance d'une couche de U4O9/U3O7 en surface de UO2. Ainsi, une période d'incubation avant fissuration, dont la durée chute avec l'augmentation de la température, est mise en évidence. Durant leur oxydation, monocristaux et fragments présentent une fissuration progressive conduisant à leur subdivision jusqu'à une taille de grain submicronique