Immobilization and leaching behavior of cobalt in alkali-activated fly ash/dehydrated cement paste fine blends

Abstract

The primary radioactive isotope in concrete waste from decommissioned nuclear power plants is ⁶⁰Co, which must be securely immobilized and disposed of to protect human health and the environment. The leaching behavior of Co²⁺ in alkali-activated fly ash/dehydrated cement paste (DCP) blends was studied by focusing on the influence of different curing temperatures on phase evolution. When cured at 25 °C, alkali-activated fly ash/DCP blends predominantly formed the C-N-A-S-H gel phase, whereas at 60 °C, the C-A-S-H gel phase was more prevalent. By contrast, fly ash-based geopolymers without DCP predominantly form an N-A-S-H gel phase. The Co²⁺ ion leaching tests indicated that Co retention was most effective during phase evolution into the C-N-A-S-H gel. Conversely, fly ash-based geopolymers lacking DCP tend to form significant amounts of NaCl, which negatively impacts the Co retention. In these geopolymers, Co is mainly immobilized in the form of Co(OH)₂.