Effect of Ca(OH)2 on the immobilization of simulated radioactive borate waste in metakaolin-based geopolymer waste forms

Geopolymers are promising candidates for immobilizing radioactive borate waste (BW), but their performance needs improvement as borate negatively affects their physicochemical properties such as compressive strength and setting time. This study investigated the effect of Ca(OH)₂ on geopolymer waste forms by varying the Ca/Al ratio (0.25–1.0). At a Ca/Al ratio of 0.25, Ca(OH)₂ tripled the compressive strength (5 to 14 MPa) by promoting geopolymerization reaction and forming C-(A)-S-H gel. The increased pH and the reactive Ca ions led to the decomposition of immobilized BW. A low Ca/Al ratio caused minimal structural changes, while higher ratios promoted C-(A)-S-H gel formation. Characterization results confirmed the formation of extensive geopolymers and a compact microstructure. Notably, in the geopolymer waste form with a Ca/Al ratio of 0.25, the cumulative fraction leached of B decreased by more than twofold and the leachability index increased from 7.5 to 8.7, compared to the Ca-free geopolymer waste form. This research provides insights into the role of calcium in immobilizing radioactive BW in geopolymers, emphasizing the importance of optimizing the Ca/Al ratio to enhance immobilization performance.