Conventional validation of low-carbon stabilization binders often relies on short curing periods and batch leaching tests, which do not adequately capture long-term durability or transport-controlled contaminant release. In this study, a bioash–GGBFS binder was evaluated for stabilization/solidification of metal-contaminated soil from Näsudden, Sweden, using an integrated program of extended curing, wet–dry and freeze–thaw cycling, and standardized percolation and diffusion leaching tests. The treated mixture (50% soil, 35% bioash, and 15% GGBFS) developed unconfined compressive strength in the MPa range and maintained high strength after durability exposure, with 1635 ± 308 kPa after wet–dry cycling and 2047 ± 100 kPa after freeze–thaw cycling. Percolation testing at L/S = 10 showed strong reductions in leaching compared with untreated soil, including 96% for As, 98% for Cd, 90% for Pb, 92% for Zn, 88% for Ni, and 65% for Cu. Diffusion testing confirmed low release for most elements, while Cu showed the highest cumulative release and mobility, indicating an element-specific limitation. Overall, the results demonstrate that the bioash–GGBFS binder can provide both durable mechanical performance and sustained immobilization of most priority contaminants under transport-relevant conditions. The findings support its potential as a low-carbon alternative for stabilization and reuse of contaminated soils, although additional measures may be needed where Cu governs compliance