Reaction kinetics of FEBEX bentonite in hyperalkaline conditions resembling the cement–bentonite interface

Abstract

Bentonite of the Serrata de Níjar (Almería, Spain) and concrete made with an ordinary portland cement (OPC) are candidate materials to be used as engineering barriers in the high level radioactive waste disposal in argillaceous rock. This experimental alteration study has been conducted in order to determine the kinetics of alteration of a montmorillonitic bentonite under hyperalkaline conditions (NaOH 0.5 to 0.1 M) in the presence of portlandite. The amount of montmorillonite destroyed and the secondary minerals formed have been measured by means of X-Ray Diffraction (XRD) in the solid phase after the performance of batch reactions carried out in airtight cells (3/1 liquid/solid) during 30 to 540 days. Scanning Electron Microscopy (SEM-EDX) and chemical analysis of a < 0.5 μm Ca-homoionized fraction have been used to characterize the reaction by-products, mostly zeolites (analcime and phillipsite type); calcium silicate hydrates (amorphous and 11 Å-tobermorite type) and saponite.

The nature of the solution chemistry determined in the aqueous phase has allowed us to calculate the chemical speciation and the saturation indices for the observed minerals. The evaluation of the equilibrium state in the system supports the dissolution of montmorillonite from undersaturation conditions as the driving process for the alkaline reaction of bentonite. The rate of montmorillonite dissolution has been calculated from the mineralogical quantification of smectite in the 75–200 °C tests. The global kinetics for the conversion of montmorillonite can be fitted to R (mol s⁻¹) = A(m²) k[OH⁻]^0.5; ln k = (− 20.09 ± 1.37) − (2731 ± 543) ^⁎ (1 / T); E_a = 22.7 ± 4.4 kJ/mol. These data are in close agreement with the rate of dissolution of montmorillonite obtained by methods based on solution chemistry.

Introduction

The study of the cement–bentonite interface reactivity is a key issue in the performance assessment of deep geological repositories of high level radioactive wastes. Cement is used as an engineered barrier itself, for practical requirements in the construction of underground vaults, and in the final sealing of access routes (Glasser, 2001). The cement plugs and seals held in contact with the bentonite barrier act as sources of alkaline fluids in wet conditions. Then, an alkaline plume is expected to evolve at the interface with bentonite.

During concrete degradation, the pH of the leached pore water may rise above 13 due to the release of sodium and potassium hydroxides (Taylor, 1987). When these hydroxides have been eliminated, the pH is controlled by the dissolution of portlandite (pH = 12.5) and, later, by the dissolution of calcium silicate hydrates or gel CSH (pH = 12.6–10) (Adenot and Buil, 1992, Glasser and Atkins, 1994, Lovera et al., 1997, Faucon et al., 1998). These pore fluids have the potential to react with montmorillonite-based bentonites, which may affect their physical and chemical properties. Montmorillonite dissolution and the formation of some other non-swelling crystalline phases induce changes in porosity and the loss of swelling and sorption capacities (Savage, 1997).

Previous studies on the reactivity of montmorillonite in alkaline solutions have focused on the collapse of expandable smectite layers, in particular on the formation of illite or illite/smectite mixed-layers. Using Wyoming montmorillonite, Eberl et al. (1993) observed the increase of illite layers up to 25% after 9 months at 35 °C in potassium hydroxide solution (KOH 3 M). Likewise, in sodium hydroxide solution (NaOH 0.5 M) at 35 °C, a random mixed-layer containing non-expanding layers is also obtained. In general, the formation of non-expanding layers, both in potassium and in sodium solutions, depends on solution concentration rather than on the temperature and time of reaction. According to Bauer and Berger (1998) and Bauer and Velde (1999) the smectite reacts in potassium alkaline medium (KOH 4 M) up to 80 °C to form an illite/smectite mixed-layer. However, formation of mixed-layer phases is an intermediate step in a series of dissolution–precipitation processes. New phases precipitate in the following sequence: discrete mica, KI-zeolite, phillipsite, K-feldspar and quartz.

The pH and concentrations of these types of experiments largely exceeded the actual environments found in cementitious materials. The system will be influenced by the presence of Ca(OH)₂ and the alkali concentrations will be less than 1 M (Huertas et al., 2000). More recently, Wyoming bentonite interaction tests at 0.5 M concentrations (NaOH, KOH, Ca(OH)₂) and 90 °C, have shown minor interlayering (collapse) effects but the formation of beidellitic smectites (Rassineux et al., 2001). These authors have argued a self-organization in the stacking order of the chemically heterogeneous layers coexisting in the original montmorillonite. Ramírez et al. (2002a), using similar conditions with FEBEX bentonite have shown, in addition, the formation of small amounts (< 3 wt.%) of zeolites (phillipsite (KOH) and analcime (NaOH)) and a slight increase of the structural magnesium content in the smectitic clay fraction. The determination of layer charge distribution in these experiments suggests a preferential dissolution of some (octahedrally-charged) smectitic layers in order to explain the increase in tetrahedral layer charge (Ramírez et al., 2002b). In addition, the presence of amorphous CSH-gels was inferred from the dissolution of significant amounts of calcium during the extraction of exchangeable cations at pH = 8. The quantities of calcium plus alkaline cations were significantly higher than the measured cation exchange capacity (total charge) of the montmorillonite (Ramírez et al., 2002a). The source for calcium in these experiments was mainly the exchangeable calcium initially present in the unaltered montmorillonite. CSH-like gel has also been found by Claret et al. (2002) in order to explain 12–11 Å XRD peaks in their clay alkaline reaction experiments.

The formation of CSH-gels was virtually absent from the experiments carried out in the NaOH–KOH (Ca(OH)₂ saturated) solutions used in the alkaline alteration of bentonite. This was due to the fact that the presence of portlandite (Ca(OH)₂) has not been introduced in the alkali hydroxide hydrothermal batch experiments. However, the first stages of interaction in the cement–bentonite interface will be charaterized by significant amounts of portlandite in contact with alkali–hydroxide solutions and bentonite. Thus, a comprehensive study of this system has to take into account these phases as shown by Savage (1997) and Savage et al. (2002), in modelling calculations.

The main objective of this research is to assess the mineralogical and bulk physico-chemical changes affecting the clay engineered barrier during the degradation of an ordinary portland cement (OPC). This is done by mixing solid portlandite, alkaline solutions and bentonite.

We know that the alkaline reaction of FEBEX bentonite is significantly accelerated with increasing time and temperature (Ramírez et al., 2002a). Then, an additional objective is to achieve a higher degree of reaction progress in order to perform a kinetic interpretation. For this purpose, this research also considered the rise in temperature up to 200 °C.