Modelling of the Interaction Between an Engineered Clay Barrier and Concrete Structures in a Deep Storage Vault

Abstract

AbstractThe degradation of concrete structures in a storage vault will release an alkaline plume influencing the geochemical evolution of clay engineered barriers or plugs. The studied configuration has a representative scale and is composed of a rectangular clay barrier interfaced on one side with a cement block and on the other side with an external granitic field. A clay model including major and accessory minerals is used; cement material is supposed to be made of portlandite and CSH minerals. The clay barrier and the cement block are assumed to be initially saturated and in equilibrium with their respective interstitial water. Diffusive transport of aqueous species coupled to chemistry is simulated in a one dimension space up to 10000 y. The feed-back of the porosity evolution on the transport properties is not taken into account. The temperature is supposed to be constant (25°C). Calculations are performed with the TRIO-EF coupled transport-chemistry code.The main results deal with the evolution of the pH profile across the clay barrier, the dissolution of clay minerals and the precipitation of new solid phases in the clay such as albite, gibbsite and CSH. These simulations show that the pH excursion includes only a minor part of the clay barrier at 10 000 y and that the CSH precipitation retards the alkaline plume progression. The appearance of local accumulations of newly precipitated minerals might reduce strongly the porosity. Future developpements of this work include: (i) the completion of the geochemical model by introducing other minerals in concrete (other CSH phases and sulfo-aluminates) and ion exchange sites within the clay, (ii) the implementation of a precipitation/dissolution model in order to describe on a realistic way the porosity profile evolution and possible clogging phenomena.