Hydrothermal synthesis of pollucite from metakaolin-based geopolymer for hazardous wastes storage

Abstract

In this contribution, a low-temperature (∼200 °C) hydrothermal route is developed to fabricate pollucite from metakaolin-based geopolymer for the immobilization of Cs. The macroscopic immobilization performance upon hydrothermal treatment has been correlated with the microscopic structural evolution and process parameters for the first time. Upon hydrothermal treatments, analcime and pollucite first crystallize from Na-rich and Cs-rich amorphous geopolymer at 170 °C, respectively, and further grow with the increasing hydrothermal temperature. By probing the interplay between hydrothermal temperature, hydrothermal time and initial Na/Cs ratio of geopolymer in the immobilization performance of Cs using orthogonal experimental design, the optimal procedure has been screened as treating (0.2Na, 0.8Cs)-GP at 230 °C for 6 h. Moreover, the results demonstrate the role of Cs-contained hydrothermal medium in enhancing the immobilization performance of Cs and mechanical properties of the encapsulation matrix due to the accelerated nucleation of pollucite. The converted dominant leaching mechanism, i.e., from diffusion for the untreated geopolymer to surface wash off for the hydrothermal product, further emphasizes the superiority of trapping Cs in pollucite structures due to their stronger blocking effect.