Synergistic solidification of radioactive sludge containing An3+ via geopolymer encapsulation–microwave sintering vitrification: Structural regulation and long-term stability mechanisms of glass–ceramic waste forms

Radioactive sludge, a typical multiphase dispersed radioactive pollutant, presents significant challenges for nuclear waste management owing to its complex composition, high water content, and persistent radiotoxicity. This study introduces, for the first time, a dual-stage treatment strategy combining geopolymer encapsulation with microwave-assisted vitrification to simultaneously achieve rapid solidification and long-term stabilisation of sludge. A slag–metakaolin–silica fume based geopolymer matrix was developed, into which 45 wt% simulated sludge (using Nd 3+ as a surrogate for trivalent actinides) was incorporated. After 3 days of standard curing, the solidified bodies exhibited compressive strengths exceeding 31.13 MPa, surpassing IAEA regulatory requirements for the transport and storage of radioactive waste. Microwave sintering at 1300 °C for 90 min resulted in complete vitrification. For Nd 2 O 3 contents ≤32 wt%, Nd 3+ was incorporated into the glass network as a modifier, forming a homogeneous glass phase. At loadings between 32 and 50 wt%, partial substitution of Ca 2+ /Na  +  by Nd 3+ induced the formation of oxyapatite-type crystals, Ca 2 Nd 8 (SiO 4 ) 6 O 2 and NaNd 9 (SiO 4 ) 6 O 2 , resulting in a glass–ceramic composite. Product Consistency Tests (PCT) conducted over 42 days revealed an Nd- normalised leaching rate as low as 1.61 × 10 −6  g m −2 ·d −1 , which is four orders of magnitude below current nuclear industry standards. This integrated approach significantly enhances sludge loading capacity (up to 45 wt%) while reducing the sintering duration to one-third of that required by conventional Joule heating methods, offering a cost-effective and robust solution for the immobilisation and disposal of complex radioactive sludge.