Evolution of the Reaction and Alteration of Granite with Ordinary Portland Cement Leachates: Sequential Flow Experiments and Reactive Transport Modelling

The construction of a repository for the geological disposal of radioactive waste will include the use of cement-based materials. Following closure, groundwater will saturate the repository, and the extensive use of cement will result in the development of a highly alkaline porewater, pH > 12.5; this fluid will migrate into and react with the host rock. The chemistry of the fluid will evolve over time, initially with high Na and K concentrations, evolving to a Ca-rich fluid, and finally returning to the natural background groundwater composition. This evolving chemistry will affect the long-term performance of the repository, altering the physical and chemical properties, including radionuclide behaviour. Understanding these changes forms the basis for predicting the long-term evolution of the repository. This study focused on the determination of the nature and extent of the chemical reaction, as well as the formation and persistence of secondary mineral phases within a granite, comparing data from sequential flow experiments with the results of reactive transport modelling. The reaction of the granite with the cement leachates resulted in small changes in pH and the precipitation of calcium aluminium silicate hydrate (C-(A-)S-H) phases of varying compositions, of greatest abundance with the Ca-rich fluid. As the system evolved, secondary C-(A-)S-H phases redissolved, partly replaced by zeolites. This general sequence was successfully simulated using reactive transport modelling.