Céline Cau Dit Coumes

This study investigates the structure and microstructure of hydroxyapatite substituted with iodate (IO ₃ ⁻ ) and/or carbonate (CO ₃ ²⁻ ) ions, within the context of long‐lived radionuclide waste conditioning. The role of the synthesis protocol, “reverse” or “direct” precipitation from aqueous solution, as well as the nature and amount of substituted anions on the structure and microstructure of hydroxyapatite samples, was evaluated using XRD, SEM, and HRTEM. The anisotropic crystallite shape assessed by HRTEM analyses was taken into account for the Rietveld refinements of the XRD patterns. Considering the reverse synthesis protocol, these refinements revealed the formation of a single hydroxyapatite phase. Despite the use of long counting times, no crystalline secondary phases were detected by XRD, regardless of the nature and amount of the substituting anions. Iodate ions substituting for A sites tend to expand the lattice, while carbonate ions located in B sites induce lattice contraction. Clearly, the influence of iodate substitution on the structural modifications is lower than that of carbonate substitution. Moreover, co‐substitution seems to modify the incorporation mode of iodate ions within the structure. The intended application of this work requires studying high substitution rates, thereby addressing gaps in the literature regarding the structural evolution of hydroxyapatite with increasing substitution rates. In this study, substitution levels reached up to 15.4 wt.% for iodate mono‐substitutions, 11.9 wt.% for carbonate mono‐substitutions, and approximately 20 wt.% for combined iodate and carbonate ions co‐substitutions.

This work investigates the potential of wollastonite-based brushite cement (WBC) for the stabilization and solidification of radioactive waste contaminated by ⁹⁰Sr. This phosphate binder was formed by the reaction of wollastonite (CaSiO₃) with a phosphoric acid solution containing borax and metallic cations (Al³⁺, Zn²⁺). Two cement pastes were investigated: a commercial binder (WBC-C) and an optimized formulation (WBC-O), produced using a zinc-free mixing solution with a higher aluminum content than that of WBC-C. Mineralogical characterizations using XRD, TGA, XRF, SEM-EDX, and Raman spectroscopy showed that both materials mainly contained amorphous hydrated silica and calcium aluminophosphate, along with crystalline brushite, residual wollastonite, and quartz. The stability of WBC-C under γ-irradiation was evaluated up to a dose of 1 MGy. The only observable effect was water radiolysis, leading to dihydrogen production at yields comparable to Portland cement matrices and geopolymers. Strontium leaching, assessed using the ANSI/ANS-16.1-2003 (R2008) procedure, followed a two-stage release mechanism combining surface wash-off and diffusion. The apparent diffusion coefficient D_a of Sr in WBC-C was markedly lower than typical values reported for Portland cement matrices. WBC-O exhibited enhanced Sr retention, possibly due to its higher aluminum content, which refines mesopores and reduces diffusion pathways accessible to Sr. WBC binders therefore appear to be promising candidates for strontium immobilization.

This work investigates the potential of wollastonite-based brushite cement (WBC) for the stabilization and solidification of radioactive waste contaminated by ⁹⁰Sr. This phosphate binder was formed by the reaction of wollastonite (CaSiO₃) with a phosphoric acid solution containing borax and metallic cations (Al³⁺, Zn²⁺). Two cement pastes were investigated: a commercial binder (WBC-C) and an optimized formulation (WBC-O), produced using a zinc-free mixing solution with a higher aluminum content than that of WBC-C. Both materials mainly contained amorphous hydrated silica and calcium aluminophosphate, along with crystalline brushite, residual wollastonite, and quartz. The stability of WBC-C under γ-irradiation was evaluated up to a dose of 1 MGy. The only observable effect was water radiolysis, leading to dihydrogen production at yields comparable to Portland cement matrices and geopolymers. Strontium leaching, assessed using the ANSI/ANS-16.1 procedure, followed a two-stage release mechanism combining surface wash-off and diffusion. The apparent diffusion coefficient D_a of Sr in WBC-C was three orders of magnitude lower than typical values reported for Portland cement matrices. WBC-O showed enhanced Sr retention due to its higher aluminum content, which refines mesopores and reduces diffusion pathways accessible to Sr. WBC binders therefore appear to be promising candidates for strontium immobilization.

We report the first miniaturized device for studying cement curing, enabling operando measurements and higher experimental throughput than previously possible.

Owing to the high carbon emissions associated with the production of Portland cement clinker and its use in concrete, the construction industry is interested in the development of more sustainable materials and processes. One area of research is on the curing of cements through carbonation rather than hydration in order to offset emissions by sequestering CO2 in the built environment. Here, we present a simple 3D-printed gas-flow microreactor to facilitate rapid studies of the carbonation curing of millimeter-thin cement samples. The device is paired with a gas and humidity control system to deliver precise environmental conditions. It can be coupled with various analytical techniques, including X-ray diffraction and Raman spectroscopy, thus allowing in situ monitoring of carbonation processes. We validate and characterize the carbonation system and measurement sensitivity and subsequently utilize the microreactor to investigate a large parameter space in the carbonation curing of a hydrated lime model system. Finally, we demonstrate the industrial utility of the device by following the carbonation curing of ordinary Portland cement (OPC) paste.

Beryllium metal is used as neutron moderator and reflector or multiplier in certain types of fission or fusion reactors. Dismantling of these reactors will produce radioactive beryllium waste, classified as low- or intermediate-level waste, that will need to be stabilised and solidified before being sent to disposal. The cementation process is under consideration because it may offer a good compromise between simplicity of implementation, cost, and quality of the final cemented wasteform. Nevertheless, knowledge of the corrosion behaviour of Be metal in a cement-based matrix is still limited, partly due to the high toxicity of Be that complicates testing. This study thus investigates Be corrosion in cement suspensions using potentiometry, voltammetry, and electrochemical impedance spectroscopy. Among the five different investigated systems (Portland cement blended without or with 40 wt.% silica fume, calcium sulfoaluminate clinker blended without or with 15% anhydrite, and calcium aluminate cement), Portland cement blended with 40% silica fume and calcium sulfoaluminate cement comprising 15% anhydrite are the most effective in mitigating beryllium corrosion. They allow reduction in the corrosion current by factors of 4 and 50, respectively, as compared to Portland cement.

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.