Authigenic Clay Minerals from Interface Reactions of Concrete-Clay Engineered Barriers: A New Perspective on Mg-Clays Formation in Alkaline Environments

Abstract

Artificial and singular geochemical environments are created around the engineered barrier systems (EBS) designed to isolate high level nuclear wastes in deep geological repositories. A concrete-bentonite interface takes place within the EBS and it builds a significant chemical gradient (pH), approximately from pH 8 (bentonite) to pH 12 (low alkali concrete), in a few millimetre thickness. This disequilibrium triggers dissolution and precipitation reactions and form a thin altered region. In this area, poorly ordered authigenic clay minerals, mainly hydrated magnesium silicates, are formed adjacent to hydrated calcium silicates and calcite precipitates adhered to the interface with concrete. This paper presents the development of this authigenic mineral layer comparing 6–18 months to 13 years interfaces. Scanning Electron Microscopy with Energy Dispersive X-ray spectroscopy (SEM-EDX) morphological and chemical characterization with the aid of ternary plots, X-ray diffraction (XRD) and infrared (IR) data show the young to old interface evolution from single brucite layers to stevensite-saponite silicates composition. Geochemical calculations indicate that this layer acts as a pH~11 buffer useful to minimize bentonite alteration and to favour the retention of amphoteric metal ions.

High-pH/low pH ordinary Portland cement mortars impacts on compacted bentonite surfaces: Application to clay barriers performance

“…S3). This fact was shown in previous studies (Fernández et al, 2013;Cuevas et al, 2018). However, clay mineral tri-octahedral sheet characteristic XRD reflections (06l reflection at 1.53 Å) were also identified in HpH cement mortar experiments, in agreement with timedependent compositions trending to saponitic chemical coordinates.…”

“…LpH cement mortar induced the precipitation of brucite, which were not identified in the other cement mortar interfaces (CEM I and CEM II). Initially (6 months), there was not sufficient silica in solution to produce hydrated Mg-silicates, further identified at 18 months by SEM-EDX with honeycomb morphologies similar to those described in Cuevas et al, 2018, for the in situ 13-year-old concretebentonite FEBEX experiment. HpH experiments XRD θ/2θ scans in the interface (Fig.…”

Bentonite/CEM-II cement mortar INTERFACE EXPERIMENTS: A proxy to in situ deep geological repository engineered barrier system surface reactivity

“…The variety of Mg-silicates identified in literature as a result of cement/clay interaction are compatible with the presence of tri-octahedral phyllosilicates 1:1 or 2:1 sheet silicate (Shimbashi et al, 2018). The complex chemical compositions of the zone with anomalous high Mg concentrations have been found to be compatible with a mixture of brucite and Mnt, as the Al content maintained constant, and the di-octahedral character of the Mnt remained, as observed by the XRD-diffraction patterns (Cuevas et al, 2018b;Fernández et al, 2017). In fact, Lerouge et al (2017) identified grain compositions related to Mg perturbation that were highly rich in Mg and compatible with these types of mixtures.…”

Tetracycline Removal from Water by Adsorption on Geomaterial, Activated Carbon and Clay Adsorbents

“…The GM composite geomaterial was synthesised from Algerian Na-clay (Maghnia deposit), activated carbon produced from coconut shell, portland cement, and polyvinyl alcohol (PVA) polymer [100]. These constituents were selected based on the sorption properties of clay, activated carbon, and cement, on the mechanical properties related to clay and cement [101][102][103]. The addition of the polymer improves the sealing and the lifespan of the synthesised composite material.…”