Geochemical modelling of multimineral evolution for a 15-month experiment

Abstract

The impact of hyper-alkaline leachate released from the cementitious barrier of a geological disposal facility on the host rock is a significant issue for the safety assessment of long-term nuclear disposal. However, modelling of multimineral transformations, multiple chemical reactions and multiple secondary-phase pathways remains a challenge due to uncertainties in parameters and a limited available database describing the kinetics of dissolution/precipitation reactions. In this study, a new modelling approach, mixed kinetic–equilibrium, was employed to study the complex reactions occurring in an experimental system consisting of Borrowdale Volcanic Group rocks permeated with a hyper-alkaline leachate for over 15 months. The modelling suggests that dissolution of primary dolomite, quartz, calcite and potassium (K) feldspar in the host rock initially drives the chemical evolution of this system. The subsequent precipitation of several secondary phases, including calcite, brucite, talc and calcium–silicate–hydrate phases, is predicted to control the long-term chemical equilibria and mineralogical composition of the host rock impacted by the alkaline leachate. The results from the modelling provide a deeper understanding of the long-term interactions between the host rock and high pH leachate, with dolomite predicted to be a major controlling phase on the geochemical evolution of the system.