Coupled reactive transport modelling of the international Long-Term Cement Studies project experiment and implications for radioactive waste disposal

Abstract

Cementitious materials are commonly included as encapsulants and backfill or barrier materials in geological disposal facilities for radioactive wastes. An understanding of the long-term behaviour of cement-based materials is therefore required to provide confidence in their safety functions. Reactive-transport models have been produced of an in-situ experiment carried out at the Grimsel Test Site in Switzerland, as part of the collaborative Long-Term Cement Studies (LCS) Project, called “LCS Experiment 2”. The experiment involved the emplacement of hardened Portland cement through a borehole into a fully-saturated, water-conducting shear zone (fracture) in granite for a period of six years, to explore cement leaching/degradation and the potential interaction of highly-alkaline fluids with the rock. Reactive transport models of the cement present in the experiment illustrate that it is possible to simulate the behaviour of Portland cement interacting with a low ionic strength groundwater, namely leaching of the cement (dissolution of primary solids, especially portlandite) and precipitation of carbonate minerals and secondary aluminosilicates, using a complex 3D model geometry. However, the model results highlight uncertainties surrounding cement solid dissolution rates and rates of secondary mineral formation, both of which could be explored in future research. The study illustrates the importance of modelling large-scale experimental systems which, along with natural/industrial analogue data, can be used to build confidence in the long-term behaviour of engineered barriers in radioactive waste disposal systems.