Low-Silica Metakaoline-Based Geopolymers as Candidate Matrices for Immobilization of Cesium Ions

Alkali-activated aluminisilicate materials, also known as geopolymers, have been considered as attractive candidates for nuclear waste immobilization due to their ability to incorporate cations, combined with high chemical resistance and suitable mechanical and thermal properties. The goal of the present research was to study the incorporation and immobilization of Cs in low-Si geopolymers (SiO₂:Al₂O₃ molar ratio ≤ 2) which are known to have a relatively high crystalline phase content.

Low-Si geopolymers was prepared from metakaoline using activating solutions containing CsOH and NaOH at different proportions. Some formulations also included sodium nitrate, NaNO₃, which is a major component in many nuclear waste streams. The structural evolution of the resulting products was followed using X-ray diffraction and FTIR spectroscopy, the incorporation of Cs in the geopolymer was followed by pore water analysis, and immobilization efficiency was determined from leaching tests following the ANSI/ANS-16.1 standard procedure.

The crystalline phases formed within nitrate-free geopolymer matrices were zeolite A, zeolite X and the Cs-bearing zeolite-F (also named zeolite D in previous publications). The relative amount of the Cs-bearing zeolite as well as the fraction of Cs within this phase were both found to increase with CsOH content in the activating solution. The introduction of NaNO₃ into the geopolymer mix led to the formation of two nitrate-bearing crystalline phases, nitrate sodalite and nitrate cancrinite. The Cs-bearing zeolite F was not identified within these geopolymer matrices.

Poue 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. The apparent diffusion constants for both Na and Cs, as obtained from the leaching experiments, were correlated with the structural data for both nitrate-free and nitrate-bearing formulations. These correlations indicate that the Cs-bearing zeolite-F as well as nitrate sodalite and nitrate cancrinite immobilize Cs ions more efficiently than does zeolite A. Nevertheless, the leachability indices for both Cs and Na in all of the matrices studied were well above 6, indicating that such matrices may be considered as waste forms for Cs ions.