Co-immobilization of cesium and strontium containing waste by metakaolin-based geopolymer: Microstructure, mineralogy and mechanical properties

The present study investigated the possibility of co-immobilization of caesium and strontium containing waste by metakaolin-based (MK) geopolymer. 4% of CsNO3 and 4% of Sr(NO3)2 by weight were used as simulants for radioactive Cs and Sr. The effect of Cs and Sr on the geopolymerization kinetics was determined via isothermal calorimetry. Furthermore, the evolution of mechanical properties, microstructure, and mineralogy was analysed. The results showed that Cs exerted no noticeable effect on reaction kinetics while Sr lowered the geopolymerization degree of MK geopolymer due to part of OH− from the activating solution being consumed by Sr2+, thereby reducing the activating capacity of pore solution. In addition, the pore structure analysis showed that the incorporation of Sr coarsened the microstructure of MK geopolymer while no significant change was observed with Cs. The phase analysis showed that no new phase was formed because of Cs or Sr incorporation. The mechanical properties decreased with Cs and Sr addition and a higher strength loss was observed for Sr-bearing geopolymers than that of Cs-bearing matrices. The strength reduction in Cs-bearing matrices is associated with the accompanying nitrate ions while a more pronounced strength reduction in Sr-bearing specimens is due to the synergistic effect of nitrates and Sr2+ on the geopolymerization. Despite that, the compressive strengths for all tested MK recipes met the minimum Belgian waste form acceptance criteria indicating that MK geopolymer preserved sufficient strength even at high Cs and Sr waste loadings. Thermodynamic modelling based on speciation and saturation index revealed that Cs+ was mainly bound through ion exchange, whereas Sr2+ largely precipitated as SrCO3. More importantly, it was observed that the co-immobilization of Cs and Sr by MK geopolymer is largely influenced by Sr behaviour. Based on the findings, it can be concluded that MK geopolymer is an excellent host material for Cs and Sr-based waste.