Sorption experiments with HTO, 36-Cl, 125-I and 14-C labeled formate on aged cement matrices retrieved from long-term in-situ rock laboratory experiments

Abstract

The long-term performance of a cement-based barrier system, envisaged worldwide in many concepts for deep geological storage of radioactive waste, depends on its material properties and how they evolve with time. Chemical interactions with the service environment may lead to mineralogical alterations and related physico-chemical changes at material interfaces, influencing radionuclide migration. Predictive (reactive) transport modelling therefore requires information on transport properties (e.g. sorption properties, porosity) of aged cement matrices. Radionuclide binding by fresh and aged cement matrices (> 10 years) has been investigated using two different aged cementitious materials retrieved from long-term in-situ rock laboratory (at the Mont Terri and Grimsel test sites, Switzerland) experiments to provide sorption data for use in predictive modelling and interpretation of the field diffusion experiments. For this purpose, the uptake of 36Cl, 125I, 3H (HTO) and 14C has been investigated in a series of batch sorption experiments. Fresh cement paste shows the strongest sorption for 36Cl and 125I due to the largest proportion of radionuclide sorbing cement phases (ettringite, C-S-H, AFm). Measured sorption values for 36Cl and 125I on the aged cement matrices are about an order of magnitude lower. Sorption values for HTO and 14C-formate on cementitious materials are generally very low, suggesting only weak interaction with the surfaces of the cement minerals.