Incorporation of boron into metakaolin-based geopolymers for radionuclide immobilisation and neutron capture potential

Metakaolin-based geopolymers have attracted significant interest in decontaminating radioactive debris from the Fukushima nuclear accident. This study explored the incorporation of boron (B) into geopolymers using boric acid as the source, with the goal of developing B-enriched geopolymers for enhanced radionuclide immobilisation and neutron capture potential. The addition of boric acid lowered the pH of the alkali activator, reducing metakaolin solubility and impeding alkali-activated geopolymerisation. B formed an unstable BO 4 (xB, 4-xSi) structure with extra short-range Si tetrahedra in low-temperature curing conditions, making it prone to be leached out. High-temperature curing facilitated alkali-activated geopolymerisation, mitigating some negative effects of boric acid. It also promoted partial incorporation of BO 4 into the framework, reducing leaching. Additionally, in acid-activated geopolymers, boric acid absorbed substantial reaction heat during the initial dealumination phase by reacting with PO 4 , thereby enhancing the overall geopolymerisation degree and increasing the relative content of near-Si terminal P and Al 6 units. B could be incorporated into the framework by bonding with numerous Al-unsaturated Si tetrahedra to form a stable BO 4 (0B, 4Si) structure. Although B introduction slightly reduced the positive charge of the acid-activated geopolymer's structure, decreasing its capacity to immobilise anionic SeO 3 2− through electrostatic adsorption, the decrease was negligible. Conversely, B introduction increased structural compactness, which improved Cs + immobilisation through physical entrapment. Overall, the B-containing acid-activated geopolymer effectively incorporated B into the main matrix while maintaining radionuclide immobilisation capacity. This study provides valuable insights into the selection and incorporation mechanisms of the B-containing geopolymer matrix, contributing to effective strategies for radioactive waste disposal. • Low-temp cured alkali-activated geopolymer has abundant extra-framework Si tetrahedra, forming unstable structures with BO 4 . • High-temp curing boosts Al-unsaturated Si tetrahedra and promotes minor BO 4 incorporation into the structure. • BO 4 forms stable structures in acid-activated geopolymers. • The addition of boric acid promotes acid-activated geopolymerisation. • B-added acid-activated geopolymers retain strong radionuclide immobilisation capabilities.