Alteration behavior of high- and low-pH cement paste–compacted bentonite samples in contact with seawater

Abstract

This study investigated the interactions between high- and low-pH cement pastes and compacted bentonite in seawater environments, which is relevant to geological disposal of radioactive waste. The investigation combined immersion tests, various analytical techniques, and conceptual reactive transport modeling to examine mineralogical transformation and structural changes at cement–bentonite interfaces. The results showed that seawater infiltration pathways significantly influenced interactions at the interface. When seawater passed through the cement and made contact with bentonite, interactions were limited to near-surface areas of the bentonite side. However, more extensive alterations of the bentonite were observed at the interface when seawater reached the cement via bentonite. High-pH cement specimens exhibited significant mineral alterations, including cement hydrate dissolution and secondary mineral formation, while low-pH cement specimens showed less extensive mineral changes. The research revealed that pH fronts, determined by cement type, controlled elemental distributions and mineral transformations. High-pH cement generated steeper chemical gradients, resulting in more substantial alterations in both materials. Low-pH cement showed minimal bentonite alteration due to its lower pH than high-pH cement, but underwent significant structural changes when exposed to seawater-derived magnesium. Near cementitious materials, exchangeable cation content of montmorillonite changed from Na-dominant to Ca-dominant, independent of cement type. Although montmorillonite dissolution was limited, secondary mineral formation significantly affected mass transfer properties. These findings suggest that solely selecting low-pH cement to prevent montmorillonite dissolution may not be optimal, and the effects of secondary mineralization should be accounted for in disposal facility design.