Misato Shimbashi

The alteration of bentonite under alkaline conditions and the subsequent changes in properties such as permeability and self-sealing ability should be evaluated for the performance assessment of radioactive waste repositories. As the period of evaluation for alterations is extremely long, natural analog (NA) studies that can observe long-term phenomena similar to the system of radioactive waste repositories have significant advantages. However, locations that can be set up as NA study sites with significant similarity are limited and should be pursued by localizing analog systems. This literature review summarized studies reporting secondary phases formed at low temperatures (<100°C) under a broader range of natural sites which are chemically similar to the alkaline conditions expected at bentonite in radioactive waste repositories, including near pyrometamorphic rocks, near ophiolites, and in alkaline saline lakes. This review provides insights into the species, formation conditions, and stability of secondary phases that could be formed during cement–bentonite interactions and the timescale for mineralogical transitions from a metastable to a stable phase. The findings could be useful for selecting secondary phases to be considered in reactive transport modeling for predicting cement–bentonite interactions in radioactive waste repositories.

Understanding the behavior of secondary minerals under alkaline conditions is important for predicting the potential alteration of the constituent minerals in radioactive-waste disposal facilities. A previous study reported the formation of uncommon Fe- and Mg-bearing clays under natural alkaline conditions in the Philippines; these were referred to as iron-magnesium-silicate-hydrates (F-M-S-H) and nontronite-like minerals. The current study aimed to investigate the structural and chemical characteristics and to understand the formation pathways of these clays by performing a detailed characterization. F-M-S-H comprised tetrahedral–octahedral–tetrahedral (TOT) layers, imperfect interlayer hydroxide sheets, and interlayer Ca ions. The systematic changes in the characteristics of F-M-S-H at different sampling depths, such as the gradual decrease of the interlayer hydroxide sheets to form smectitic domains, were caused by the differing interaction periods between each sediment at different sampling depths and alkaline seepage. Furthermore, F-M-S-H was ferrous in form prior to oxidation. In contrast, a nontronite-like mineral comprised nontronite and part of an interlayer hydroxide sheet. This mineral was inferred to be formed under chemically different conditions from F-M-S-H, and probably formed in the presence of aqueous Fe³⁺ and Mg ions.

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.

In radioactive waste disposal facilities, low-permeability engineered barrier materials are important for inhibiting radionuclide migration. However, dissolution–precipitation reactions under alkaline conditions change the permeability of engineered barriers. To understand long-term dissolution–precipitation reactions under alkaline conditions in chemically complex systems, trenches and drill holes were excavated at Narra in Palawan, where alkaline fluids (pH > 11) have been naturally produced, seeping into clastic sediments derived from serpentinized ultramafic rocks and gabbro of Palawan ophiolite. Interaction between the alkaline seepage and clastic sediments, which have been deposited since 15,000 radiocarbon years before present (¹⁴C yr BP), led to dissolution of minerals and the precipitation of Si-bearing phases which were divided into two main categories: Fe-Mg-Si infillings and Ca-Si infillings. The former category was composed of iron-magnesium-silicate-hydrate (F-M-S-H) and a nontronite-like mineral and was widely recognized in the clastic sediments. The nontronite-like mineral likely formed by interaction between silicates and alkaline seepage mixed with infiltrated seawater, whereas F-M-S-H formed by the reaction of silicates with alkaline seepage in the absence of seawater infiltration. Ca-Si infillings included 14 Å tobermorite and were precipitated from alkaline seepage combined with the Ca and Si supplied by the dissolution of calcite and silicates in the clastic sediments.