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 {sup o}C between bentonite and different high-pH solutions (K-Na-OH and Ca(OH){sub 2}-saturated) were reconciled with the reactive transport code CrunchFlow. For K-Na-OH solutions (pH = 13.5 at 25 {sup o}C) partial dissolution of montmorillonite and precipitation of Mg-silicates (talc-like), hydrotalcite and brucite at the interface are predicted at 60 {sup o}C, while at 90 {sup o}C the alteration is wider. Alkaline cations diffused beyond the mineralogical alteration zone by means of exchange with Mg{sup 2+} in the interlayer region of montmorillonite. Very slow reactivity and minor alteration of the clay are predicted in the Ca(OH){sub 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.