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

Dei, Shuntaro  ; Shibata, Masahito*; Negishi, Kumi*; Sugiura, Yuki  ; Amano, Yuki   ; Bateman, K.*; Wilson, J.*; Yokoyama, Tatsunori   ; Kagami, Saya   ; Takeda, Masaki ; Sakurai, Akitaka; Tachi, Yukio  

Interactions between cement and host rock in geological repositories for radioactive waste will result in a chemically disturbed zone, which may potentially affect the long-term safety. This paper investigates 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. The study combines various analytical techniques to identify the key reactions at the cement-rock interface, including cement dissolution, precipitation of secondary minerals such as calcite and C-(A-)S-H phases, cation exchange in montmorillonite and reduced porosity in mudstone. 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 due to incorporation into secondary minerals in the altered mudstone.