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Influence de la température sur la rétention du sélénite par une pâte de ciment altérée et par ses phases pures constitutives

Abstract : In the French design of a deep repository for radioactive waste, cementitious materials will undergo a constraint in temperature, due to cement hydration exothermic reactions and to the heat generation of radioactive waste. Such temperature changes (to a maximum value of 60-70°C) could affect the cement paste mineralogy and could therefore impact radionuclides retention properties. Selenium, whose the radioisotope $^{79}$Se (T$_{1/2}$ = 6,5.10$^5$ years) is a fission product, has been chosen for the retention study because of its sensitivity in the changes of the mineralogical composition of the cement pastes. The main objective was to study the effect of temperature on selenite sorption with thermally altered cement pastes (CEM I) and relative individual cement phases (portlandite, C-S-H, ettringite and hydrogarnet. With increasing temperature, we observed: 1) modifications of the mineralogical composition of altered cement pastes, which seem irreversible and modifications of the ionic activity products of pure cement phases. These changes have been predicted with a thermodynamic calculation; 2) a decrease of selenite retention affinity for all studied solid phases. This effect can not be explained only by mineralogical modification, but also by the modification of equilibrium solution composition (particularly with calcium and sulphate concentrations in solution). Modelling attempts proposed in this study didn’t achieve to build a predictive model but must be considered as a first step in the right direction to obtain a mechanistic model. Describing selenite sorption onto cementitious phases as a function of temperature needs more information in order to complete the thermodynamic database for cement materials (Log_K and ΔH°), to characterize surface sites of the pure cementitious phases and determine their complexation constants as a function of temperature. Finally, it is necessary to determine the complexation constants of competitive and complexe ions (respectively sulfate and calcium).
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Nathalie Mace. Influence de la température sur la rétention du sélénite par une pâte de ciment altérée et par ses phases pures constitutives. Chimie. Université Paris XI - Orsay, 2006. Français. ⟨tel-01697604⟩



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