Reaction kinetics of FEBEX bentonite in hyperalkaline conditions resembling the cement–bentonite interface
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.
Section snippets
Experimental
Batch experiments were run in tightly-closed Teflon reactors. The bentonite (80 g) and three alkaline solutions (0.240 l: 0.1, 0.25 and 0.5 M NaOH; pH: 12.90, 13.26 and 13.52, after speciation) were mixed. The system was initially buffered at portlandite saturation by including an excess of portlandite (6 g: about 4 times the cation exchange capacity CEC of the bentonite (100 ± 2 cmol (+)/Kg)) in the reactor. The experimental t/T grid was 1, 6, 12 and 18 months and 25, 75, 125 and 200 °C.
The
Characterization of the aqueous phase
Bentonite is able to buffer the alkaline solutions at stationary (540 days) values between pH 12.5 and 8.5 depending on temperature and initial NaOH concentration. The rise in temperature favored neutralization of the initial hydroxide alkalinity, so that pH is reduced from 25 to 200 °C; 12.5–11.9 at 25 °C, 12.5–11.3 at 75 °C, 12.0–9.5 at 125 °C and 10.0–8.5 at 200 °C. Na+ and total alkalinity constitutes the major cation and anion components and their concentration displayed the same trend as
Bentonite–alkaline solutions interaction: a model of reaction
The decreased pH and alkalinity as a result of the alkaline reaction of bentonite can be explained by the hydrolysis of montmorillonite in basic medium (Cama et al., 2000; modified for aqueous silica speciation and hydroxides solubilities at pH > 11: e.g.: Stumm and Morgan, 1981):(Si7.72Al0.29)IV(Al2.69Fe3+0.41Mg0.85)VIO20(OH)4(Ca0.25Mg0.20Na0.28K0.11)1.29 + 4.56 H2O + 16.28(OH)− ↔ 0.41 Fe(OH)4− + 2.97 Al(OH)4− + 7.72 H2SiO42− + 0.25 Ca2+ + 0.20 Mg(OH)2 + 0.28 Na++0.11 K+
This is an important reaction to
Conclusions
The hyper-alkaline reaction of FEBEX bentonite (NaOH 0.5–0.1 M) in the presence of portlandite (Ca(OH)2) is characterized by the dissolution of montmorillonite and the precipitation of zeolites (analcime (200–125 °C) and phillipsite (75 °C)), saponite and calcium hydrated silicates (gel-CSH (125–25 °C), 11 Å-tobermorite (200–125 °C) and gyrolite (200 °C)). The reaction presents a temperature-kinetic control driven by montmorillonite dissolution. This type of reaction has been experimentally
Acknowledgements
The European Commission (contract n° FI4W-CT-2000-00028) and ENRESA have supported this work. We want to express our gratitude to professor Alain Meunier and an anonymous reviewer for their valuable comments.
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