Environmentally friendly geopolymers; Synthesis and characterization of pH-Controlled metakaolinite; Phosphate-based hardened bodies

香西 直文   ; 徳永 紘平   ; 土肥 輝美   ; 高橋 嘉夫*

Kozai, Naofumi; Tokunaga, Kohei; Dohi, Terumi; Takahashi, Yoshio*

In this study, we achieved pH control (up to 7) of phosphate-based geopolymer hardened bodies by reacting metakaolinite with a mixed aqueous solution of phosphoric acid and a soluble phosphate salt. This pH control was achieved by adjusting the total amount of phosphate ions and varying the molar ratio of phosphoric acid to phosphate salt. The hardened body with a neutral pH of 6.0 displayed mechanical and spectroscopic characteristics considerably similar to those of conventional phosphoric acid geopolymer hardened bodies with a pH below 3. To evaluate its potential application as a radioactive waste solidifier, a Sr-doped hardened body with a neutral pH (using an SrCl aqueous solution as a simulant of Sr) was subjected to static leaching testing. The Sr-doped hardened body demonstrated high resistance to Sr leaching. Sr K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy of the Sr-doped hardened body revealed that its high Sr confinement capability was primarily due to inner-sphere complexation of Sr with the phosphate groups bridging metakaolinite particles.