Toward Sustainable Radioactive Waste Management: Geopolymerizationof Sewage Sludge Ash as a Viable Solution

This study examines the geopolymerization potential of sewage sludge ash (SSA) for immobilizing radioactive waste through a series of experimental phases. The initial phase of the study involved processing sewage sludge from three different treatment plants, followed by calcination and subsequent characterization. The initial synthesis of geopolymers was conducted using 100% SSA, followed by compressive strength testing. In the second phase, a full factorial design was employed to optimize a metakaolin-based geopolymer formulation, with adjustments made to five variables: metakaolin (MK), activating solution (AS), sand, water, and lime. The optimal conditions were identified as 120 g MK, 125 g AS, 360 g sand, 55 g water, and 14.5 g lime. Under these conditions, the compressive strength increased from 15.0 ± 1.0 to 21.3 ± 0.6 MPa when the specimens were cured at 60 °C for 6 h. The optimized formulation was then augmented with SSA, and its characteristics were examined through a series of analytical techniques, including ICP–OES, XRF, XRD, SEM, and EDS. In the third phase of the study, immobilization of simulated radioactive activated carbon and ion-exchange resin wastes contaminated with ¹³⁷Cs within the geopolymer matrix was investigated. Leaching and compressive strength tests were conducted to evaluate the performance of the material, and the results indicated that the release rates of ¹³⁷Cs were between 2.55 × 10^–5 and 3.23 × 10^–5 cm d^–1. These findings suggest that SSA-derived geopolymers can effectively immobilize radioactive waste, offering a sustainable alternative to traditional Portland cement.