Cementation of ILW ion exchange resins : Impact of sulfate ions released by radiolysis on hydrated matrix.

International audienceSome of the ion exchange resins used during treatment of spent nuclear fuels are intermediate level radioactive wastes which may be damaged by radiolysis process, releasing sulfate ions directly into the cement-based encapsulating material. This work consists in an experimental study of the resulting sulfate attack on the properties of the hydrated matrix dimensional stability, mineralogy and microstructure of the samples as well as variations in the chemical composition of the curing solution were studied during six months. Three sites of delayed ettringite formation were detected into the cement matrix near the surface exposed to solution, localized in the interfacial transition zone between cement matrix and resins, or progressively replacing the portlandite that initially fulfilled the cracks of anionic resins. During the experiment period, the ettringite precipitation and the expansion detected were moderate, and did not lead to cracking. The material involved was considered as having a good resistance to sulfate attack

Étude des potentialités des ciments sulfo-alumineux bélitiques pour le conditionnement du zinc (de l'hydratation à la durabilité)

Les ciments silico-calciques sont largement utilisés pour le conditionnement des déchets nucléaires de faible activité. Cependant, les interactions existant entre certains constituants du déchet et les phases cimentaires peuvent perturber l hydratation de ces dernières. Ainsi, le zinc ZnII, fréquemment rencontré sous forme de sel chloré dans les cendres d incinération de déchets alpha contenant du PVC ou du néoprène, est connu pour inhiber la prise du ciment Portland. Une solution a ce problème serait d utiliser un liant de chimie différente, qui présenterait une meilleure compatibilité avec le déchet à conditionner tout en conservant les qualités d une matrice cimentaire (coût modeste, mise en œuvre aisée, hydratation avec l eau apportée par les déchets ). Le potentiel des ciments sulfo-alumineux bélitiques pour le conditionnement du zinc ZnII a donc été évalué. Quatre aspects ont été abordés : le déroulement de l hydratation, les propriétés des liants hydratés, les mécanismes assurant la rétention du zinc et la durabilité des matériaux obtenus (lixiviation et modélisation à l aide d un code de transport réactif). Les influences du dosage en gypse, de l échauffement induit par l hydratation de pièces massives, et de la teneur en chlorure de zinc ont été déterminées.Il en résulte que les matériaux à base de ciment sulfo-alumineux gypsé à hauteur de 20 % sont de bons candidats pour le conditionnement du zinc : pas de retard d hydratation, assemblage minéralogique robuste vis-à-vis de la température (dans la gamme 20 85C), bonne résistance mécanique, stabilité dimensionnelle et très bonne rétention du zinc, même lors d une lixiviation prolongée par l eau pure.Calcium silicate cements are widely used for low- and intermediate-level radioactive waste conditioning. However, wastes produced by nuclear activities are very diverse and some of their components may chemically react with cement phases. For instance, ashes resulting from the incineration of technological wastes including neoprene and polyvinylchloride may contain substantial amounts of soluble zinc chloride. This compound is known to strongly delay or inhibit Portland cement setting. One approach to limit adverse cement-waste interactions is to select a binder showing a better compatibility with the waste while keeping cement matrix advantages (low cost, simple process, hydration with water provided by the waste ).This work thus investigates the potential of calcium sulfoaluminate cement for zinc ZnII immobilization. Four aspects were considered: hydration (kinetics and products formed), properties of hydrated binders, mechanisms of zinc retention and durability of the cement pastes (based on leaching experiments and modelling). The influence of three main parameters was assessed: the gypsum content of the cement, the concentration of ZnCl2 and the thermal evolution at early age.It follows that materials based on a calcium sulfoaluminate cement containing 20% gypsum are interesting candidates for zinc ZnII stabilization/solidification: there is no delay in hydration, mineralogy of the hydrated phases is slightly dependent on thermal history, mechanical strength is high, dimensional changes are limited and zinc ZnII is well immobilized, even if the cement paste is leached by pure water during a long period (90 d).LILLE1-Bib. Electronique (590099901) / SudocSudocFranceF

Etude de l'hydratation des ciments sulfo-alumineux par des solutions de borate de sodium (de la spéciation du bore au retard à l'hydratation)

Dans le circuit primaire des réacteurs nucléaires à eau pressurisée, le bore participe au contrôle des réactions de fission. Le traitement de cette solution génère des déchets aqueux contenant une forte concentration en bore (de 1 à 3 mol/L). Le conditionnement de ces déchets à l'aide d'un ciment silico-calcique est compliqué par le fort pouvoir retardateur des ions borate sur l'hydratation du liant. Un traitement des déchets à la chaux est nécessaire pour précipiter les ions borate sous forme d'hexahydroborite. Cette stratégie, si elle limite le retard d'hydratation, ne le supprime pas. Par ailleurs, l'hexahydroborite est instable en milieu cimentaire et se convertit dans le temps en boroaluminate de calcium. Une autre approche pourrait consister à utiliser un ciment sulfoalumineux bélitique à forte teneur en ye'elimite. Ce liant présente en effet l'avantage de former en quantité importante des phases de type AFm et/ou AFt lors de son hydratation, phases qui peuvent incorporer des ions borate dans leur structure.Au cours de ce travail, l'hydratation de ciments sulfoalumineux par des solutions de borate de sodium a été étudiée au jeune âge et à plus long terme (sur une durée de 2 ans) dans l'objectif de préciser l'influence d'un ensemble de paramètres (pH du déchet, concentration en bore, taux de gypse du ciment) sur la vitesse d'hydratation du liant, la nature des hydrates formés, et les propriétés du matériau obtenu (résistance mécanique, stabilité dimensionnelle). Pour ce faire, une démarche analytique, procédant par complexification progressive des systèmes étudiés, a été mise en œuvre. Ainsi ont été successivement abordées la spéciation du bore en solution alcaline, l'étude des phases précipitant au sein des systèmes {CaO, B2O3, Na2O, H2O}, {CaO, B2O3, Al2O3, H2O} et {CaO, Al2O3, B2O3, SO3, H2O}, puis celle des pâtes de ciment gâchées avec une solution boratée simulant le déchet. L'approche expérimentale a été complétée par des modélisations thermodynamiques s'appuyant sur une base de données spécialement développée pour les besoins de l'étude.Il apparaît que le gypse joue un rôle primordial dans le contrôle de la réactivité du ciment. L'ajout de gypse fixe, par un mécanisme indirect, le pH de la solution interstitielle à une valeur proche de 11, ce qui favorise la précipitation transitoire d'un composé boraté faiblement cristallisé, l'ulexite. La dissolution des phases anhydres du ciment est alors fortement ralentie jusqu'à l'épuisement du gypse, conduisant ainsi à des retards de prise considérables. En l'absence de gypse, le retard à l'hydratation est de plus faible amplitude. Dans ces conditions, le pH de la solution interstitielle atteint des valeurs plus élevées, ce qui permet de déstabiliser rapidement l'ulexite. A plus long terme, les ions borate sont incorporés au sein d'une phase de type AFt, en solution solide avec les ions sulfate. Les résultats obtenus permettent de conclure que ce sont les ciments sulfo-alumineux contenant une faible teneur en gypse qui sont les plus adaptés au conditionnement de solutions à forte concentration en bore.In the primary circuit of pressurized water reactors, boron helps controlling the fission reactions. The treatment of this solution produces aqueous low-level or intermediate-level and short lived radioactive with a high boron concentration (up to 1 to 3 mol/L). Stabilization/solidification of such wastes with calcium silicate cement is complicated by the strong retarding effect of borate ions on cement hydration. A calcium hydroxide addition is required to precipitate borate ions into hexahydroborite. With this approach, the hydration delay is limited, but not suppressed. Besides, hexahydroborite is unstable in the cement paste and is progressively converted into a hydrated calcium boroaluminate phase. Another strategy may consist in using belite calcium sulfoaluminate cement with high ye'elimite content. During hydration, this binder forms indeed large amounts of AFm and/or AFt phases which can incorporate borate ions into their structure.In this work, hydration of calcium sulfoaluminate cement by borated solutions was investigated at early age, and over a 2-year period, in order to determine the influence of a set of parameters (boron concentration and pH of the waste, gypsum content of the cement) on the hydration rate of the binder, on the phase assemblage formed, and on the properties of the resulting material (mechanical strength, volume change). An analytical approach was adopted, based on a progressive increase in the complexity of the investigated systems. The focus was successively placed on the speciation of boron in alkaline solution, on the study of the phases formed within the {CaO, B2O3, Na2O, H2O}, {CaO, B2O3, Al2O3, H2O} and {CaO, Al2O3, B2O3, SO3, H2O} systems, and on the characterization of cement pastes prepared with a borate solution which mimicked the waste. The experimental approach was completed by thermodynamic modelling using a database specially developed for the needs of the study. Gypsum appears to play a key role in controlling the reactivity of cement. The gypsum addition sets, by an indirect mechanism, the interstitial solution pH at a value close to 11, which promotes the precipitation of a poorly crystallized borated compound, ulexite. Dissolution of the anhydrous phases is strongly slowed down until the exhaustion of gypsum, and major delays are observed. Without any gypsum, the hydration delay is shorter. Under these conditions, the pore solution pH reaches higher values after mixing. Ulexite is consequently quickly destabilized. Borate anions are then incorporated into a mixed borate/sulphate AFt type phase. It appears that calcium sulfoaluminate cements with low gypsum contents should be recommended to solidify borated solutions.MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Alkali binding by magnesium silicate hydrates

The binding of Na+, K+, and Li+ by magnesium silicate hydrate (M–S–H) was investigated in batch sorption experiments. Sorption isotherms and cation exchange measurements indicated the binding of alkalis in cation exchange sites compensating the negative surface charge of M–S–H. Higher pH values led to further deprotonation of the silanol groups and a higher alkali uptake by M–S–H. No significant incorporation of alkalis in the main silica or magnesium oxide sheets was observed. However, the silica sheets were less polymerized in the presence of higher alkali hydroxide concentrations

Understanding the setting and hardening process of wollastonite-based brushite cement. Part 1: Influence of the Ca/P ratio and H3PO4 concentration of the mixing solution

International audienceWollastonite-based brushite cement is prepared by mixing wollastonite with a phosphoric acid solution containing metallic cations and boric acid. This work investigates simplified systems comprising wollastonite and H3PO4 solutions only, in order to clarify the influence of the H3PO4 concentration, Ca/P and l/s ratios (this latter parameter being correlated to the two others) on the setting and hardening process.At constant H3PO4 concentration, increasing the Ca/P ratio, and decreasing the l/s ratio, accelerates the early stages of hydration but limits its progress at 7 d. At high Ca/P ratio, more basic calcium orthophosphates form in addition to dicalcium phosphate.At constant l/s ratio, increasing the H3PO4 concentration in the range 7–10 mol·L−1 retards cement hydration, and inhibits the setting at concentrations ≥12 mol·L−1. A good compromise is obtained for H3PO4 concentrations between 9 and 10 mol·L−1: hydration is not too fast and yields the maximum amount of dicalcium phosphate

Understanding the setting and hardening process of wollastonite-based brushite cement. Part 2: Influence of the boron and aluminum concentrations in the mixing solution

International audienceThe mixing solution of wollastonite-based brushite cement is a phosphoric acid solution containing metallic cations and borax. This work complements a previous study devoted to the influence of the H3PO4 concentration, Ca/P and liquid-to-solid (l/s) ratios on the setting and hardening process of the binder by providing new insight into the role of aluminum and boron.Boron retards the setting and decreases the heat released during the process. It also contributes to reduce the macroporosity of the hardened material but yields to poor compressive strength. With aluminum in the mixing solution, the mechanical properties are greatly improved thanks to the precipitation of an amorphous aluminophosphate which increases the density of the cement matrix. But aluminum alone leads to fast setting. A joint addition of boron and aluminum to the mixing solution makes it possible to get a material with optimized properties both in fresh and hardened states

Electrochemical Behavior of Al/Mg Alloys Immobilized in a Magnesium Potassium Phosphate Cement-Based Mortar

International audiencePortland cement is extensively used for the conditioning of radioactive waste. However, its high alkalinity is a serious obstacle to the stabilization of waste containing aluminum metal since aluminum is oxidized by the pore solution with the production of dihydrogen. This work investigates the potential of an alternative binder, magnesium potassium phosphate (MKP) cement, for the stabilization of Al–Mg alloys comprising 2 to 4.5 wt% of Mg and other metallic impurities. The objective is to assess the influence of the alloy composition on its reactivity in the cementitious matrix at earlier ages, as well as at later ages, when the cement has reached a significant reaction degree. Two complementary techniques are used. Gas chromatography shows that the dihydrogen release, resulting from the corrosion process, is not influenced by the magnesium content in the alloy. Electrochemical impedance spectroscopy provides qualitative information about the corrosion but also makes it possible to assess the corrosion current using an equivalent electrical circuit linked to the kinetic parameters of the postulated corrosion mechanism. Over a one-year period, the corrosion current of the alloys, regardless of their Mg content, is reduced by almost three orders of magnitude in MKP mortar as compared to Portland-cement-based mortar.</jats:p

Pickering emulsions stabilized by inside/out Janus nanotubes: Oil triggers an evolving solid interfacial layer

International audienceHypothesisIn the field of Pickering emulsion, original inside/ouside Janus clays nanoparticles are investigated for their emulsification properties. Imogolite is a tubular nanomineral of the clay family having both inner and outer hydrophilic surfaces. A Janus version of this nanomineral with an inner surface fully covered by methyl groups can be obtained directly by synthesis (Imo-CH3, hybrid imogolite). The hydrophilic/hydrophobic duality of the Janus Imo-CH3 allows the nanotubes to be dispersed in an aqueous suspension and enables emulsification of non-polar compounds due to the hydrophobic inner cavity of the nanotube.ExperimentsThrough the combination of Small Angle X-ray Scattering (SAXS), interfacial observations and rheology, e stabilization mechanism of imo-CH3 in oil-water emulsions has been investigated.FindingsHere, we show that interfacial stabilization of an oil-in-water emulsion is rapidly obtained at a critical Imo-CH$_3$ concentration as low as 0.6 wt%. Below this concentration threshold, no arrested coalescence is observed, and excess oil is expelled from the emulsion through a cascading coalescence mechanism. The stability of the emulsion above the concentration threshold is reinforced by an evolving interfacial solid layer resulting from the aggregation of Imo-CH3 nanotubes that is triggered by the penetration of confined oil front into the continuous phase

Aluminum incorporation into magnesium silicate hydrate (M-S-H)

The incorporation of aluminum in magnesium silicate hydrate (M-S-H) phases was investigated. Magnesium (alumino) silicate hydrate (M-(A-)S-H) with Mg/Si ratios equal to 1.1 or 1.7 and Al/Si ranging from 0 to 0.2 were synthetized in batch experiments and equilibrated at 20, 50 and 70 °C. pH values between 9 and 10.5 were observed and aluminum up to Al/Si ~0.15–0.18 was incorporated in M-(A-)S-H. Thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray pair distribution function (PDF) analysis, transmission electron microscopy (TEM), 29Si and 27Al MAS NMR data showed that the M-(A-)S-H phases formed were similar to M-S-H with limited coherent size and a comparable polymerization degree of the tetrahedral silicates. Aluminum was incorporated in both tetrahedral and octahedral sites of M-S-H, while no aluminum was present as exchangeable cation on the surface sites

Formulating a Low-Alkalinity, High-Resistance and Low-Heat Concrete for Radioactive Waste Repositories

International audienceInvestigations 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 ($\Delta T$<20°C under semi-adiabatic conditions) and high strength ($\approx 70-80$ 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