Formation of zeolites in metakaolin-based geopolymers and their potential application for Cs immobilization

Like low-Si NaOH-based geopolymers, the mixed CsOH-NaOH geopolymers contain a significant amount of crystalline material which is imbedded within an amorphous matrix. Formulations with 1%Cs yielded the crystalline phases zeolite A and zeolite X. At 50%Cs the Cs-bearing zeolite F was formed. All three phases were observed at an intermediate Cs content (7%Cs). Pore water analysis indicated a preference for Cs uptake from the activating solution, while leaching experiments indicated selectivity for Cs immobilization in the mixed CsOH-NaOH geopolymers. Correlation of the apparent diffusion constants for both Na and Cs, as obtained from the leaching experiments, with the structural data lead to the conclusion that Cs is more efficiently bound by zeolite F, whereas Na binding is preferred by zeolites A and X. Nevertheless, the leachability indices for both Cs and Na were well above 6, indicating that such matrices may be considered as waste forms for 137Cs.