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

https://doi.org/10.1016/j.apgeochem.2020.104624Get rights and content

Highlights

  • 13 years of in situ bentonite-concrete interaction from FEBEX experiment were characterized.

  • Spatial distribution of porosity and mineralogy were given to depict the alteration of both the man-made barriers.

  • Mineralogical mapping and autoradiography porosity mapping were combined to describe the geochemical processes.

  • 2:1 Mg Phyllosilicate precipitate in concrete at the interface between bentonite and concrete.

Abstract

The Engineered Barrier System (EBS) implemented in full-scale experiments are designed to provide an understanding 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.

Introduction

Deep Geological Repositories (DGR) are currently the most acceptable solution for safe long-term storage of high-level radioactive waste (HLRW) (Chapman, 2019). The waste is thus isolated from the biosphere by a system of engineered and natural barriers. The Engineered Barrier System (EBS) usually consists of a metal sleeve surrounding the waste, forming a canister cylinder, followed by a compacted bentonite clay barrier. The host rock, in which access galleries are excavated, will need concrete vaults to support the walls (clay rocks) or concrete plugs (crystalline rocks) to seal and stop the swelling of the hydrated bentonite backfill inside the gallery. Increased confidence in DGR could be achieved based on the detailed multi-scale analysis and characterization carried out after the dismantling of long-term (10–30 years) in situ simulated experiments.

The concrete-clayrock/bentonite interface is considered as one of the most reactive interfaces in the EBS system. Due to the large geochemical contrasts between concrete (pH ~13) and clays (pH ~7), chemical reactions are expected to induce modifications in both the chemical and physical properties of clayey and cementitious materials. These highly contrasted pH and Eh conditions will lead to ion diffusion. A series of reactions are likely to occur, such as ion exchange on the clay minerals’ surface, dissolution of some minerals, and precipitation of secondary phases at the interfaces. In addition, these reactions will change the pore network and therefore the hydraulic properties of both the clay and the cementitious materials (Chagneau et al., 2015; Xie et al., 2015). The difference in the pore water pH in both materials is ruled by the equilibrium of the assemblage of solid phases. The pH of ordinary Portland cement (OPC) concrete equilibrates at pH 13–12 (Bullard et al., 2011), and clay or bentonite at 7–8 (Fernández et al., 2004; Turrero et al., 2006). While clay tends to be stable in the long-term (Bildstein and Claret, 2015), concrete materials are subject to continuous evolution by means of groundwater or clay pore water hydrating and dissolving the major cement phases. In response to the progressive degradation of the cementitious materials, the pH of the pore water in cement is controlled by the solubility of portlandite (Ca(OH)2) and later the dissolution of calcium silicate hydrates (C–S–H) with variable Ca/Si from high (1.6–1.2) to low ratio (1.0–0.7). Calcite also precipitates in replacement of cement phases, in as far as soluble inorganic carbon species are present in the bentonite or clay pore water environments (Glasser and Matschei, 2007). The pH gradient produces an alkaline front with the capacity to partially alter the mineralogy in both the clay and the concrete (Dauzeres et al., 2014, 2016; Gaboreau et al., 2011, 2012; Jenni et al., 2014). The intensity and propagation of these reactions depend on the type of cement and clay materials, the available pore volume and the efficiency and magnitude of water and solute transport.

One of the most recognized dissolution/precipitation reactions at these cementitious materials-clay interfaces is the precipitation of Mg-silicate hydrates (M-S-H), described as Mg-perturbation (Bildstein and Claret, 2015; Lerouge et al., 2016). It is a common place representing the result of the reactive transport processes caused by the chemical gradients in clay/cementitious materials (Dauzeres et al., 2014; Fernández et al., 2018b; Lerouge et al., 2017). The increase in Mg concentration is usually characterized, either in the concrete (Jenni et al., 2014; Lerouge et al., 2017), both in cementitious materials and bentonite (Fernández et al., 2017), or just in the clay, sometimes several mm from the interface in clay rocks (Mäder et al., 2017).

The release of dissolved Mg in clay pore water may originate from cation exchange from the bentonite exchange complex, through the dissolution of Mg minerals in the clay rocks (i.e. dolomite) or the dissolution of Mg phases in the cementitious materials. At low temperature in alkaline conditions at pH > 10.5, brucite precipitates (Pokrovsky and Schott, 2004). At relatively lower pH values of 10–9, Mg silicates such as saponite, and Ca and Mg di-trioctahedral smectites are predicted to precipitate (Marty et al., 2014) and have been characterized in clay-concrete interactions (Lerouge et al., 2017). The nature of these neogenic M-S-H phases in concrete materials reveals similar structural properties to that of 2:1 and 1:1 magnesium layer phyllosilicates (Nied et al., 2016; Roosz et al., 2015; Vespa et al., 2018). Those precipitated in clay or bentonite are very difficult to characterize because they are mixed with the present assemblage of phyllosilicates. Nevertheless, what is known from their synthesis in the laboratory is that they have a very limited capacity to form solid solutions with C–S–H phases and precipitate as separate phases, containing very small amounts of Ca (Bernard et al., 2018a, 2018b; Lothenbach et al., 2015).

The FEBEX project (Full-scale Engineered Barriers Experiment in crystalline host rock) was one of the experiments implemented in the Underground Research Laboratory (URL) located at the Grimsel Test Site in Switzerland. It was based on the Spanish reference concept for disposal of HLRW in crystalline rock (AGP Granito: Huertas et al. (2006)). The present article focuses on the characterization of a 13-year (2002–2015) concrete-bentonite interface from this experiment. The concrete plug placed to retain the bentonite swelling in the in situ GTS gallery represents an ideal heterogeneous interface for analyzing the microstructural aspects of a real EBS. This document intends to show an example of the chemical evolution at the interface between concrete and bentonite, in a real case of gallery sealing with the heterogeneities that could be induced by the man-made set up. The evolution of the concrete phases in contact with clay materials and the porosity distribution will be analyzed in regard to the characteristics of shotcrete. The present study is part of a global and exhaustive characterization work performed on the FEBEX project (FEBEX-dp (http://www.grimsel.com/gts-phase-vi/febex-dp/febex-dp-introduction). This study comes under the European project CEBAMA (https://www.cebama.eu/) which attempts to describe the microstructural and geochemical perturbation of some engineered cement-based materials.

The paper attempts to show in detail microstructural and mineralogical evolution at the interface between concrete and bentonite, revealed by porosity and mineralogical mapping methodologies described by (Gaboreau et al., 2012; Gaboreau et al., 2017; Gaboreau et al. (2011)). The alteration of the hydrated cement phases and the characteristic of the formation of neogenic phase at the concrete-bentonite interface are described within the microstructural framework acquired by electron microscopy, quantitative electron probe microanalysis (EPMA) and quantitative porosity mapping.

Section snippets

In situ bentonite/concrete interface

The studied material is part of the FEBEX in situ experiment. The FEBEX project simulated the waste canisters that are placed horizontally in drifts and surrounded by a clay barrier constructed from highly compacted bentonite blocks (ENRESA, 1995). Two cylinder heaters were maintained at a constant temperature of 100 °C on their surface, facing an annulus of compacted bentonite that filled the gap up to the granitic wall. A concrete plug was applied to seal the drift end. The operational phase

Quantitative porosity mapping by autoradiography

An impregnated 14C-PMMA polished section of the bentonite/concrete interface is presented in Fig. 1. The sample size is 4 × 8 cm and it displays a dark green bentonite and a gray concrete. Some grains/aggregates are apparent over the bentonite surface while some optical heterogeneities are observable in the concrete at the interface, enhanced with the dotted red lines (Fig. 1). The autoradiography reveals that these color contrasts in the concrete correspond to porosity heterogeneities.

Discussion

The precise description of the modification of mineral assemblages to be developed at the pore scale in the EBS reactive interfaces is a key issue to build reliable predictive modelling exercises, carried out to gain confidence in the long term safe performance of these systems. The most studied geochemical systems concerning EBS repository stability are (i) the evolution of phases involved in the long-term hydration reaction of the cement/water paste (Fernández et al., 2018a; Ridi et al., 2011

Conclusion

The reactivity of concrete-clay interfaces has been long studied to assess the degradation of such geochemical contrasted engineered and natural barriers. Laboratory experiments, natural analogues, in situ interfaces and geochemical reactive transport modeling have been implemented to robustly predict their behavior over decades. Many of these studies were conducted on homogeneous model reactive systems, while heterogeneities, nature and sources are expected to exist at the interface between

Declaration of competing interest

None.

Acknowledgments

The research leading to these results has received funding from the European Union's Horizon 2020 Research and Training Programme of the European Atomic Energy Community (EURATOM) (H2020-NFRP-2014/2015) under grant agreement n° 662147 (CEBAMA). The FEBEX-DP Consortium (Nagra, SKB, Posiva, Ciemat, Kaeri) financed the dismantling and sampling operation in 2015.

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      Rather than brucite, M-S-H may thus act as a sorbent for B in soil, with B being released into pore water if secondary M-S-H dissolves. In radioactive waste disposal, in situ concrete–clay interaction experiments in underground test sites have shown an Mg-rich zone in clayey rock around the concrete–clay interface (Dauzeres et al., 2016; Fernández et al., 2017; Gaboreau et al., 2020; Jenni et al., 2014; Mäder et al., 2017). Some experimental studies have investigated Mg perturbation in bentonite (Fernández et al., 2018; González-Santamaría et al., 2021), in which M-S-H phases were considered secondary phases that may affect engineered barrier properties in the long term.

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      The FEBEX in-situ experiment was performed at Grimsel (Switzerland) with two operation periods: the 1st period was from 1997 to 2002, the 2nd period started from 2002 and ended on June 2015 [136]. Fernandez et al. [75], Alonso et al. [78] and Gaboreau et al. [137] tested the bentonite/concrete interactions in the 2nd period of the FEBEX experiment. Fernandez et al. [75] studied the impact of shotcrete concrete plug on bentonite and observed a limited mineralogy alteration.

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      Some scholars have also studied concrete interfaces from the microscopic perspective. For instance, Gaboreau et al. [25] studied the existing interface between concrete and bentonite, and the interface was found to be characterized by macroscopic inhomogeneity. Lei et al. [26] studied the interface transition zone of concrete via microhardness and scanning electron microscopy characterization.

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      In the CEM-I, LpH-experiments, and even CEM-II-experiments once excluding Ca from the analysis, crusts morphologies correspond to rich Mg-phases and would be formed by groups of messy small crystal size as M-S-H gels. Another more plausible possibilities are the formation of 2:1 trioctahedral clay minerals like saponite-vermiculite (Gaboreau et al., 2020). Furthermore, the presence of montmorillonite with intercalated brucite layers like a pseudochlorite structure has been previously purposed by Cuevas et al. (2018); Cuevas et al. (2019) and González-Santamaría et al. (2020b).

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