New insights into the immobilisation of Ce(IV) in Portland cement: Properties, hydration, microstructure and occurrence status of Ce

The immobilisation mechanism of Pu(IV) by Portland cement-based material is not well understood. This study aims to reveal the immobilisation mechanism and occurrence status of Ce(IV) (chemical analogue of Pu(IV)) in Portland cement paste. The addition of Ce(NO₃)₄ first slightly retarded then accelerated the early-age hydration of Portland cement, thereby resulting in an increase of compressive strength by 29 %. The main contributor to immobilisation of Ce is Portland cement rather than zeolite. The hydration products were not altered by Ce(NO₃)₄ addition, whereas the hydration degree and mean chain length of C-(Al/Ce)-S-H increased. Over 99.99 % of Ce was immobilised by synthesised C-S-H. A analysis of the synthesised C-S-H confirmed the formation of C-Ce-S-H, where Ce tetrahedra took the bridging site of silicate chain of C-Ce-S-H. Nano-crystalline CeO₂ grains were identified, which intermixed with C-Ce-S-H. Approximately 90 % of Ce were immobilised by C-Ce-S-H, and the rest exist as nano-crystalline CeO₂ grains.