Concrete perturbation in a 13 years in-situ concrete/bentonite interaction from FEBEX experiments. New insight of a 2:1 Mg Phyllosilicate precipitation at the interface

Abstract

The Engineered Barrier System (EBS) implemented in full-scale experiments are designed to provide an under standing of the long-term performance of Deep Geological Repositories (DGR) as nuclear waste repositories. The existing interfaces between the engineered barrier materials, such as concrete and bentonite, constitute reactive surfaces on which the thickness and intensity of degradation of the material, which may affect the confinement properties, are under investigation. This study focuses on a concrete projected on a saturated compacted bentonite from the in situ FEBEX experiment, emplaced in the Grimsel Test Site (GTS, Switzerland) and dismantled during 2015 after 13 years of functioning. Preserved sections crossing the interface have shown macroscopic heterogeneities in 1–2 cm of the shotcrete from the contact with bentonite that presumably affected the porosity distribution. In this area, the distribution of mineral and chemical components has been analyzed in detail, both in concrete that is distant from the interface and in contact with the bentonite. The information provided by detailed mineralogical mappings was consistent with quantitative chemical analysis. Chemical mappings are used to explain the distribution, nature and evolution of the phases in the concrete at the interface with clay. The role of porosity, presumably affected by the initial application of the shotcrete, has influenced the characteristic geochemical reactions in the bentonite-concrete interaction. The chemical composition of di- and tri-octahedral Al–Mg smectites, in the mixing trend of high-charge beidellite saponite, were identified in the concrete in the degraded area at the interface.