The Tournemire industrial analogue: reactive-transport modelling of a cement–clay interface

C Watson,D Savage,S Norris,C Walker,S Benbow,J Wilson

doi:10.1180/claymin.2013.048.2.02

Abstract

AbstractIn 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.

Highlights

The role of Ordinary Portland Cement (OPC)-based materials in geological disposal facilities (GDFs) for radioactive wastes is often a crucial one
This plume will be in chemical disequilibrium with both the other materials used in the construction of the GDF and the natural host rock formation, with potentially deleterious effects on the capability of those materials to function in their intended manner as safety barriers (e.g. Wilson et al, 2011)
One may look to natural cement-clay analogue systems that have been in situ for many tens, hundreds or thousands of years (e.g. Gaucher & Blanc, 2006; Savage, 2010), which are more relevant on both temporal and spatial scales applied to the GDF

Summary

Introduction

The role of Ordinary Portland Cement (OPC)-based materials in geological disposal facilities (GDFs) for radioactive wastes is often a crucial one. Industrial or ‘manmade’ analogues sit somewhere between large-scale experimental studies and natural analogues They may have been in place for decades or even centuries (relatively long compared to experimental studies, but short compared to natural analogues), and the initial and boundary conditions are often better understood than those for natural analogue systems. They offer an opportunity to inform mathematical models which are in turn used to understand the evolution of the GDF over the very long time scales considered by safety cases

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Conclusion