Résumé
In the French design of a deep repository for radioactive waste, cementitious materials willundergo a constraint in temperature, due to cement hydration exothermic reactions and tothe heat generation of radioactive waste. Such temperature changes (to a maximum value of60-70°C) could affect the cement paste mineralogy and could therefore impact radionuclidesretention properties. Selenium, whose the radioisotope $^{79}$Se (T$_{1/2}$ = 6,5.10$^5$ years) is a fissionproduct, has been chosen for the retention study because of its sensitivity in the changes ofthe mineralogical composition of the cement pastes. The main objective was to study theeffect of temperature on selenite sorption with thermally altered cement pastes (CEM I) andrelative individual cement phases (portlandite, C-S-H, ettringite and hydrogarnet.With increasing temperature, we observed: 1) modifications of the mineralogical compositionof altered cement pastes, which seem irreversible and modifications of the ionic activityproducts of pure cement phases. These changes have been predicted with a thermodynamiccalculation; 2) a decrease of selenite retention affinity for all studied solid phases. This effectcan not be explained only by mineralogical modification, but also by the modification ofequilibrium solution composition (particularly with calcium and sulphate concentrations insolution).Modelling attempts proposed in this study didn’t achieve to build a predictive model but mustbe considered as a first step in the right direction to obtain a mechanistic model. Describingselenite sorption onto cementitious phases as a function of temperature needs moreinformation in order to complete the thermodynamic database for cement materials (Log_Kand ΔH°), to characterize surface sites of the pure cementitious phases and determine theircomplexation constants as a function of temperature. Finally, it is necessary to determine thecomplexation constants of competitive and complexe ions (respectively sulfate and calcium).