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

Abstract

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 BO₄(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 BO₄ into the framework, reducing leaching. Additionally, in acid-activated geopolymers, boric acid absorbed substantial reaction heat during the initial dealumination phase by reacting with PO₄, thereby enhancing the overall geopolymerisation degree and increasing the relative content of near-Si terminal P and Al₆ units. B could be incorporated into the framework by bonding with numerous Al-unsaturated Si tetrahedra to form a stable BO₄(0B, 4Si) structure. Although B introduction slightly reduced the positive charge of the acid-activated geopolymer's structure, decreasing its capacity to immobilise anionic SeO₃²⁻ 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.