Alteration of Bentonite Reacted with Cementitious Materials for 5 and 10 years in the Mont Terri Rock Laboratory (CI Experiment)

Shingo Yokoyama,Andreas Jenni,Misato Shimbashi,Urs Mäder,Yasutaka Watanabe,Daisuke Minato

doi:10.3390/min11030251

Shingo Yokoyama, Andreas Jenni + Show 4 more

Open Access

https://doi.org/10.3390/min11030251

Copy DOI

Abstract

The cement–clay interaction (CI) experiment was carried out at the Mont Terri rock laboratory to complement the current knowledge on the influence that cementitious materials have on Opalinus Clay (OPA) and bentonite (MX). Drill cores including the interface of OPA, concrete (LAC = low-alkali binder, and OPC = ordinary Portland cement), and MX, which interacted for 4.9 and 10 years, were successfully retrieved after drilling, and detailed analyses were performed to evaluate potential mineralogical changes. The saturated compacted bentonites in core samples were divided into ten slices, profiling bentonite in the direction towards the interface, to evaluate the extent and spatial variation of the mineralogical alteration of bentonite. Regarding the mineral compositions of bentonite, cristobalite was dissolved within a range of 10 mm from the interface in both LAC-MX and OPC-MX, while calcite precipitated near the interface for OPC-MX. In LAC-MX and OPC-MX, secondary products containing Mg (e.g., M-S-H) also precipitated within 20 mm of the interface. These alterations of bentonite developed during the first 4.9 years, with very limited progress observed for the subsequent 5 years. Detectable changes in the mineralogical nature of montmorillonite (i.e., the formation of illite or beidellite, increase in layer charge) did not occur during the 10 years of interaction.

Highlights

Bentonite is used in radioactive waste disposal facilities as an engineered barrier owing to its suitable physicochemical properties, such as swelling properties [1], low hydraulic permeability under saturation conditions [1] and cation exchange capacity for radionuclide sorption [2]
No X-ray powder diffraction (XRD) analysis of the initial MX-80 bentonite (MX) used for the clay interaction (CI) experiment was conducted, Karnland (2010) summarized the mineral composition of MX-80 for the commercial batches of each year, with the mean composition determined as montmorillonite (81.4%), illite (0.8%), calcite (0.2%), cristobalite (0.9%), gypsum (0.9%), muscovite (3.4%), plagioclase (3.5%), pyrite (0.3%), quartz (3.0%), and tridymite (3.8%) [2]
These results indicate that the dissolution of montmorillonite did4.9-year not significantly progress precipitation of secondary phases occurred mainly during the interaction and had inlittle the interaction between and ordinary Portland cement (OPC)-MX for 10 years

Summary

Introduction

Bentonite is used in radioactive waste disposal facilities as an engineered barrier owing to its suitable physicochemical properties, such as swelling properties [1], low hydraulic permeability under saturation conditions [1] and cation exchange capacity for radionuclide sorption [2]. The evaluation of the alteration behavior of bentonite during its interaction with cementitious materials is important for performance assessments and safety analysis for radioactive waste disposal. The mineralogical alteration of bentonite under alkaline conditions has been reviewed in previous studies [3,4,5]. The dissolution of the major minerals of bentonite has been extensively investigated; previous studies have summarized the dissolution rates of these minerals [6,7], as well as the potential secondary phases in various chemical conditions [3,5]. Savage et al (2007) suggested that calcite, dolomite, chalcedony, C(A)SH at variable Ca/Si ratios, K-feldspar, illite, phillipsite, analcime, clinoptilolite, and heulandite are the most likely secondary phases to form in a low-temperature cement–bentonite systems [5]

Methods

Results

Conclusion