In the post-closure period of a geological disposal facility for radioactive waste, leaching of cement components is likely to give rise to an alkaline plume which will be in chemical disequilibrium with the host rock (which is clay in some concepts) and other engineered barrier system materials used in the facility, such as bentonite. An industrial analogue for cement-clay interaction can be found at Tournemire, southern France, where boreholes filled with concrete and cement remained in contact with the natural mudstone for 15–20 years. The boreholes have been overcored, extracted and mineralogical characterization has been performed. In this study, a reactive-transport model of the Tournemire system has been set up using the general-purpose modelling tool QPAC. Previous modelling work has been built upon by using the most up-to-date data and modelling techniques, and by adding both ion exchange and surface complexation processes in the mudstone. The main features observed at Tournemire were replicated by the model, including porosity variations and precipitation of carbonates, K-feldspar, ettringite and calcite. It was found that ion exchange needed to be included in order for C-S-H minerals to precipitate in the mudstone, providing a better match with the mineralogical characterization. The additional inclusion of surface complexation, however, led to limited calcite growth at the concrete-mudstone interface unlike samples taken from the Tournemire site that have a visible line of crusty carbonates along the interface.

Highly alkaline environments induced by cement-based materials are likely to cause the physical and/or chemical properties of the bentonite buffer materials in radioactive waste repositories to deteriorate. Assessing long-term alteration of concrete/clay systems requires physicochemical models and a number of input parameters. In order to provide reliability in the assessment of the long-term performance of bentonite buffers under disposal conditions, it is necessary to develop and verify reactive transport codes for concrete/clay systems. In this study, a PHREEQC-based, reactive transport analysis code (MC-CEMENT ver. 2) was developed and was verified by comparing results of the calculations with in situ observations of the mineralogical evolution at the concrete/argillite interface. The calculation reproduced the observations such as the mineralogical changes in the argillite limited to within 1 cm in thickness from the interface, formation of CaCO3 and CSH, dissolution of quartz, decrease of porosity in the argillite and an increase in the concrete. These agreements indicate a possibility that models based on lab-scale (∼1 year) experiments can be applied to longer time scales although confidence in the models is necessary for much longer timescales. The fact that the calculations did not reproduce the dissolution of clays and the formation of gypsum indicates that there is still room for improvement in our model.