Kinetics of the Reaction of Methyl Iodide with Hydroxylamine in an Aqueous Solution within the Framework of Nuclear Spent Fuel Reprocessing

International audienc

Influence of orthophosphate ions on the dissolution of tricalcium silicate.

International audienceTricalcium silicate dissolution in the presence of orthophosphate ions was monitored by measuring the concentrations of calcium and silicate ions in dilute suspensions using a special dissolution cell coupled to an optical emission spectrometer. Results show that increasing adsorption of orthophosphate ions slows down the dissolution of Ca3SiO5 and that a calcium-phosphate precipitate may form at certain orthophosphate concentrations. These observations are correlated with results of calorimetric experiments carried out during the hydration of silica-rich cement pastes in the presence of the same salts

Influence of high nitrate salts concentration on dimensional variations of mortars under wet curing

Cementation is a widely applied technique for the conditioning of low- and intermediate-level radioactive wastes. In particular, the use of cement is an attractive procedure to immobilize evaporator concentrates from aqueous effluents treatment. This study is devoted to bring information on the influence of concentrates mixing solution on the dimensional variations under wet curing of simulated solidified waste forms with high ionic strength. In this study, the salinity of the evaporator concentrates was simulated by adding sodium or potassium nitrates. The length changes were measured, with displacement gauges consisting in linear variable differential transducers. Mortars prepared with solutions of KNO3 or NaNO3 exhibited expansion which increased with the nitrate concentration in the mixing solution, whatever the associated cations. The measured swelling was shown to be governed by a concentration effect which involved diffusion and osmosis: diffusion of the ions of the pore solution into the less concentrated curing solution, and water uptake by the material due to the osmotic pressure gradient between the pore and curing solutions

Physico-chemical investigation of clayey/cement-based materials interaction in the context of geological waste disposal: Experimental approach and results

International audienc

Dimensional stability under wet curing of mortars containing high amounts of nitrates and phosphates.

International audienceInvestigations were carried out in order to solidify in cement some aqueous streams resulting from nuclear decommissioning processes and characterized by a high salinity (300 g/L), as well as important concentrations of nitrate (150–210 g/L) and phosphate ions (0–50 g/L). Special attention was paid to the influence of these compounds on the dimensional variations under wet curing of simulated solidified waste forms. The length changes of mortars containing nitrate salts only (KNO3, NaNO3) were shown to be governed by a concentration effect which involved osmosis: the higher their concentration in the mixing solution, the higher the swelling. The expansion of mortars containing high amounts of phosphates (≥ 30 g/L in the mixing solution) was preceded by a shrinkage which increased with the phosphate concentration, and which could be suppressed by seeding the cement used with hydroxyapatite crystals. This transitory shrinkage was attributed to the conversion into hydroxyapatite of a precursor readily precipitated in the cement paste after mixing

Selection of a mineral binder with potentialities for the stabilization/solidification of aluminum metal

In a strongly alkaline medium, such as that encountered in conventional cementitious materials based on Portland cement, aluminum metal is corroded, with continued production of hydrogen. In order to develop a mineral matrix having enhanced compatibility with aluminum, a literature review was first undertaken to identify binders capable of reducing the pore solution pH compared with Portland cement. An experimental study was then carried out to measure the hydrogen production resulting from corrosion of aluminum metal rods encapsulated in the different selected cement pastes. The best results were achieved with magnesium phosphate cement, which released very little hydrogen over the duration of the study. This production could be reduced still further by adding a corrosion inhibitor (lithium nitrate) to the mixing solution. Open circuit potential measurement and Electrochemical Impedance Spectroscopy of aluminum electrode encapsulated in two pastes based on Portland cement and magnesium phosphate cement showed different redox behaviors. In the Portland cement paste, the electrochemical data confirmed the corrosion of aluminum whereas this latter tended to a passive state in the magnesium phosphate binder

Recherche d’un liant hydraulique pour le conditionnement de déchets contenant de l’aluminium métallique

Le conditionnement de déchets contenant des quantités significatives d’aluminium métallique dans une matrice à base de ciment Portland est à proscrire : la forte alcalinité de ce matériau conduit à une corrosion du métal et à la production massive de dihydrogène. Un état de l’art sur les liants minéraux conduisant à un pH de solution interstitielle inférieur à celui des ciments silico-calciques conventionnels est donc effectué dans l’objectif de développer une matrice minérale présentant une compatibilité chimique améliorée avec l’aluminium. Une étude expérimentale permet ensuite de comparer les quantités de dihydrogène produites par la corrosion de barreaux d’aluminium enrobés dans les différentes pâtes de ciment sélectionnées. Le ciment phosphomagnésien conduit au plus faible dégagement de dihydrogène sur la durée de l’étude (4 mois) : l’extrapolation sur une année conduirait à une production de 0,026 L.m-2.an-1. Cette production peut encore être réduite d’un facteur supérieur à 100 par ajout au matériau d’un inhibiteur de corrosion de l’aluminium, le nitrate de lithium

LiAl2(OH)6OH.2H2O solubility product and dihydrogen radiolytic production rate under γ-irradiation

International audienc

Incorporation of aluminum into C-S-H structures: From synthesis to nanostructural characterization

International audienceC-A-S-H (C=CaO, A=Al$_2$O$_3$, S=SiO$_2$, H=H 2O in cement nomenclature) phases have been synthesized from CaO, SiO$_2$, and AlNaO$_2$. The initial CaO/SiO$_2$ (C/S$_{initial}$) ratios varied from 0.8 to 1.5 and the initial Al$_2$O$_3$/SiO$_2$ (A/S$_{initial}$) ratio was set to 0.1. Samples were characterized by X-ray diffraction and chemical analyses of their equilibrium solutions. This paper describes experiments using a low-voltage scanning transmission electron microscopy (STEM-in-SEM) imaging system that allows transmission observations in an environmental scanning electron microscope. Observations of the nanostructure were also performed by transmission electron microscopy (TEM). Two types of morphologies were clearly observed: fine fibrillar aggregates and small plates, the latter being particularly sensitive to beam damage. Despite their different appearance, both of these phases were amorphous, and the small plates were richer in aluminum. The fraction of the small plate phase increased with the C/S$_{initial}$ ratio. TEM interpretations showed that C-A-S-H phases were not stable under the electron beam and high-magnification observations could significantly modify their structure. Images and chemical analyses acquired with STEM-in-SEM appeared as valuable sources of information because they offered a large observation field comparable to a transmission electron microscope and better magnification resolution than a classical scanning electron microscope