Thermal and chemical parameters controlling phase stability and caesium immobilisation in phosphate-based geopolymers, using a chabazite-rich volcanic tuff

Acid-activated phosphate-based geopolymers emerged as alternatives to Portland cement thanks to their promising mechanical and thermal properties, as well as their potential for the solidification/stabilisation of hazardous wastes. In this regard, a peculiar application field can be found in radioactive waste management operations, where the immobilisation of radionuclides is one of the primary objectives. However, limited knowledge is available regarding the effectiveness of this kind of binders in this context. This study investigated the use of a natural, chabazite-rich, volcanic tuff as a novel precursor for phosphate-based geopolymers in radioactive waste management applications. The effects of the phosphoric acid concentration and the curing temperature on the geopolymerisation process were assessed. Elemental dissolution tests were used to estimate the release of aluminium, silicon, and iron from the volcanic tuff, showing extensive aluminium release due to the instability of chabazite in acid environments. The mineralogical and microstructural evolution of the samples was examined via quantitative phase analysis by X-ray diffraction and scanning electron microscopy, confirming chabazite dissolution and the subsequent formation of crystalline taranakite phase primarily governed by phosphoric acid concentration. Caesium retention, a key requirement for radioactive waste conditioning, was assessed through leaching tests. All samples demonstrated excellent caesium immobilisation capabilities, outperforming traditional cementitious matrices and comparable to alkali-activated geopolymers. The samples also met the Italian waste acceptance criterion for radioactive waste disposal. Therefore, chabazite-rich volcanic tuff can serve as a viable, natural precursor for phosphate-based geopolymers, providing a lower-impact alternative to other commonly used precursors.