Modeling experimental results of diffusion of alkaline solutions through a compacted bentonite barrier

The interaction between concrete/cement and swelling clay (bentonite) has been modeled in the context of engineered barrier systems for deep geological disposal of high-level radioactive waste. The geochemical transformations observed in laboratory diffusion experiments at 60 and 90 oC between bentonite and different high-pH solutions (K-Na-OH and Ca(OH)2-saturated) were reconciled with the reactive transport code CrunchFlow. For K-Na-OH solutions (pH = 13.5 at 25 oC) partial dissolution of montmorillonite and precipitation of Mg-silicates (talc-like), hydrotalcite and brucite at the interface are predicted at 60 oC, while at 90 oC the alteration is wider. Alkaline cations diffused beyond the mineralogical alteration zone by means of exchange with Mg2+ in the interlayer region of montmorillonite. Very slow reactivity and minor alteration of the clay are predicted in the Ca(OH)2-bentonite system. The model is a reasonable description of the experiments but also demonstrates the difficulties in modeling processes operating at a small scale under a diffusive regime.