Some legacy radioactive waste containing aluminum (Al) metal need to be stabilized and solidified before their final disposal. Currently, Portland cement (PC) is extensively used for conditioning low- or intermediate-level radioactive waste. However, the high alkalinity of PC leads to strong corrosion of Al metal, which is associated with significant dihydrogen release. Therefore, it is important to investigate alternative binders that show better chemical compatibility with Al metal. Magnesium potassium phosphate cements (MKPCs), comprising equimolar amounts of MgO and KH2PO4, are interesting candidates since their pore solution pH may fall within the passivation domain of Al metal. The understanding of their long-term durability, especially under alkaline conditions, is however incomplete. Hence, MKPC paste samples (with fly ash as a filler) were submitted to semidynamic leaching tests using an alkaline solution under well-controlled conditions. The leachates were analyzed over time using ICP-AES, and the leached solids were characterized by XRD, SEM/EDS, and 31P MAS-NMR spectroscopy. Leaching induced a decrease in the content of crystalline K-struvite (MgKPO4.6H2O), the main hydrate of the paste samples, as well as the precipitation of calcium-deficient hydroxyapatite (CDHA), brucite (Mg(OH)2) and possibly magnesium silicate hydrates, OH-LDH or PO4-LDH phases. The experimental data were then used as an input for numerical simulations using the reactive transport code HYTEC.

The long-term durability of magnesium potassium phosphate cement (MKPC) pastes was investigated by examining their leaching behavior. MKPC comprised magnesium oxide (MgO) and potassium dihydrogen phosphate (KH2PO4) in equimolar amounts and yielded K-struvite (MgKPO4·6H2O) and a nearly neutral pore solution pH upon hydration. Semi-dynamic leaching tests were performed on MKPC paste samples using demineralized water with a pH set at 7, and the leached solids were analyzed using XRD, SEM/EDS, 11B and 31P MAS-NMR spectroscopy. Leaching was mainly governed by diffusion of dissolved species through the pore network of the paste. Three main zones were observed in the leached solids: (i) a poorly cohesive residual layer where K-struvite was fully depleted, (ii) an intermediate zone where K-struvite coexisted with cattiite (Mg3(PO4)2·22H2O), and (iii) a third zone without any cattiite. Reactive transport modeling made it possible to predict the extent of degradation and the phase evolution in the MKPC paste samples.

Mitigating the release of dihydrogen resulting from metal corrosion or water radiolysis is an important issue for the disposal of certain types of cemented radwaste packages. The approach investigated in this work consists in adding an oxide getter (gamma-MnO2/Ag2CO3) to the cement matrix. Since the efficiency of the getter decreases under wet environment, two self-desiccating binders (calcium sulfo-aluminate and magnesium potassium phosphate cements), are used to obtain significant desaturation of the pore network by the sole hydration reactions. The getter slightly influences the rate of cement hydration at early age, but has no effect afterwards. Sorption of ions released by dissolution of cement phases onto gamma-MnO2 is evidenced, as well as partial or total destabilization of silver carbonate. Nevertheless, the getter still enables to reduce strongly the outgassing of dihydrogen from mortars encapsulating Al-metal, which opens new perspectives to improve the conditioning of waste producing H2 in a cement matrix.

Portland 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.

Calcium sulfoaluminate (CSA) cements with high ye'elimite content are of interest for the production of self-desiccating binders. In this work, the influence of cement composition and calcium sulfate source on the content of water bound by hydration was investigated, with the future goal of designing non-expansive materials with a dry internal environment. A thermodynamic approach was used first. Ten cement compositions were prepared by blending a CSA clinker comprising 54·3% ye'elimite and 29·1% belite with anhydrite and calcium oxide, respectively, within the ranges 80–95%, 5–20% and 0–15%. Cement suspensions (water/cement ratio (w/c) = 6) were maintained under stirring at 20°C for 1 month. The cement comprising 80% clinker and 20% anhydrite led to the highest contents of ettringite and bound water, which was simulated well by thermodynamic calculations. A kinetic study was then performed on cement pastes (w/c = 0·5 or 0·6), with the cement comprising 80% clinker and 20% calcium sulfate (introduced as anhydrite or gypsum). The addition of gypsum, which dissolves faster than anhydrite, led to the rapid formation of a dense microstructure, which tended to limit the progress of hydration at later age. With anhydrite, higher hydration degrees were reached and specimens cured in a wet or dry environment exhibited smaller volume changes.

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.

The purpose of this work was to study the role of cesium in sodium‐based geopolymer and its thermal stability for nuclear waste management. A series of mixed sodium and cesium geopolymer samples (Na1−x Cs x )2O·Al2O3·SiO2·12H2O (referred to as (Na1− x Cs x )‐GP, where x = 0, 0.08, 0.15, 0.42, 1) have been prepared. All geopolymer samples were heated at 1100°C for 24 h. Pollucite (CsAlSi2O6) and feldspathoid (CsAlSiO4) were crystallized from Cs‐GP. Nepheline (NaAlSiO4) and a small amount of crystallized silica were obtained from Na‐GP. The other geopolymers (Na1− x Cs x )‐GP (x = 0.08, 0.15, 0.42) led to pollucite and nepheline main phases. Amorphous silica phase was observed in all the geopolymer samples with various amounts. Phase quantification and scanning electron microscope revealed that higher Cs concentrations in Na‐GP tend to decrease the amorphous phase while improving pollucite and nepheline phase quantification. The amorphous geopolymers have also been studied by pair distribution function analysis. Tetrahedral chains formed by T–O bonding (with T = Si, Al) were shown to be more tighten around Cs+ than around Na+. It led to shorter Cs–T bond than Na–T bond matching the higher solvation property of Na+. Furthermore, thermal study analysis pointed out the fact that geopolymer samples (Na1− x Cs x )‐GP, can be considered as solid solutions.

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. (C) 2014 Elsevier B.V. All rights reserved.