In situ reaction and alteration of mudstone-cement interfaces at the Horonobe URL: Comparison between ordinary Portland cement and low alkaline cement

Interactions between cement and host rock in geological repositories for radioactive waste will result in the formation of a chemically disturbed zone which may affect repository safety. The chemical evolution at the interface between cement (Ordinary Portland Cement: OPC and Low Alkaline Cement: LAC) and mudstone after 11 years of in situ reactions at the Horonobe Underground Research Laboratory is described. Various analytical techniques were used to identify the key reactions at the cement-rock interface, which included cement dissolution, precipitation of secondary minerals such as calcite and C-(A-)S-H phases, cation exchange in clay minerals, and reduction of rock porosity. The results show that the mudstone contacted by both OPC and LAC was altered to depths of a few millimetres, whereas cement alteration was observed over a wider area in LAC. Calcite formation occurred at both interfaces due to the ingress of carbonate ions in the groundwater. A relatively denser calcite layer formed at the OPC interface, suggesting a more favourable environment for calcite precipitation, as suggested by thermodynamic calculations. Furthermore, C-(A-)S-H phases were prevalent in the mudstone, suggesting complex interactions depending on porewater composition, pH, and mineral stability. The study also highlights the effects of cement-mudstone interactions on radionuclide migration, such as reduction of diffusivity due to reduced porosity and enhancement of sorption or incorporation into secondary minerals in the altered mudstone. Overall, the research provides valuable insights into long-term cement-mudstone interactions and their effects on radionuclide behaviour.