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.

Les opérations d'assainissement d'anciennes installations nucléaires peuvent générer des solutions acides contaminées par du strontium et du césium. Un des moyens de traitement de ces effluents consiste à les faire circuler sur des adsorbants en colonne. S'il existe des matériaux inorganiques permettant le piégeage du césium en milieu acide, aucun adsorbant efficace n'est actuellement disponible dans le cas du strontium. L'enjeu de cette thèse est donc d'élaborer un adsorbant minéral du strontium fonctionnant en milieu acide, mais également compatible avec l'environnement alcalin d'un stockage de déchets nucléaires qui constituera l'exutoire final du piégeur. Certains ciments phosphatés pourraient présenter des atouts pour l'application visée. Leurs performances restent toutefois à déterminer. On cherchera donc dans un premier temps à préciser les contributions des différentes phases qui les composent à la rétention du strontium et on identifiera les mécanismes mis en jeu. A partir des résultats obtenus sur phases pures, on formulera et caractérisera un matériau polyphasique dont l'assemblage minéralogique sera optimisé pour le piégeage. Dans la dernière partie de la thèse, on déterminera l'évolution des propriétés de ce matériau dans l'environnement alcalin d'un stockage.