Development of metakaolin-based geopolymer for solidification of sulfate-rich HyBRID sludge waste

Highlights: • K-based geopolymer can solidify more HyBRID sludge waste up to 53.8 wt%. • Sulfate-rich HyBRID sludge waste causes formation of ettringite during cementation. • The dissolution of cristobalite was observed during geopolymerization. • MK based geopolymer can be used for solidification of sulfate-rich nuclear waste. - Abstract: Radioactive waste which contains high sulfate ion is an abiding challenge for cementitious solidification due to the formation of sulfate mineral such as ettringite. Such secondary mineral formation can cause the expansion and disintegration of cement waste form, resulting in the increase of leachability. To alleviate this problem, we proposed metakaolin-based geopolymer waste form to solidify the radioactive waste containing high sulfate content generated from the Hydrazine Based Reductive metal Ion Decontamination (HyBRID) process. Although ettringite formation was observed in the cement waste form, this mineral did not form in the geopolymer waste form. The addition of HyBRID sludge waste significantly increased the compressive strength of geopolymers at Si/Al ratio of 1.6 from 13.6 MPa to 49.6 MPa. The partial dissolution of cristobalite was confirmed during geopolymerization induced by potassium silicate solution, which increases the amount of silicon that enters geopolymerization. The maximum waste loading of 53.8% was achieved when geopolymer composition was K₂O∙2.8SiO₂∙Al₂O₃∙15.2H₂O, which had compressive strength of 14.3 MPa, exceeding the repository acceptance criterion (3.44 MPa). Potassium-based geopolymer showed high compressive strength, low leachability, and high waste loading. These results increase our understanding of the solidification mechanism for HyBRID sludge waste using geopolymer waste form, and highlight the importance of considering the type of alkali cations and different Si/Al ratios when designing the chemical compositions of geopolymer waste form.