A bioash–cement composite binder was evaluated as a low-cement stabilization material for metal-contaminated soils, with emphasis on mechanical performance and long-term leaching behavior under field conditions. Two fine soil fractions from the Näsudden area (Skellefteå, Sweden), classified as hazardous (HS) and non-hazardous (NHS), were treated in laboratory trials to optimize binder composition. An optimum formulation containing 35 wt.% bioash and 5 wt.% cement (dry basis, relative to soil) improved unconfined compressive strength (UCS) to 696 kPa (HS) and 479 kPa (NHS) after 28 days and reduced leaching of Zn, Cd, Pb, and Co. Arsenic immobilization improved in HS but decreased in NHS, while Cu and Ni leaching increased, consistent with elevated pH and dissolved organic carbon (DOC) promoting soluble complexation. The optimized binder was then applied to a third soil (“Pilot soil”) and validated at pilot scale by treating 100 tonnes of soil and constructing a 2 m high noise barrier. Parallel laboratory tests on the Pilot soil yielded UCS values of 1000 kPa and confirmed effective retention of Zn and Cd, with generally good Pb stabilization, whereas As remained the most mobile element across soil types. Two-year field monitoring showed decreasing leachate concentrations of As, Cu, Ni, Pb, and Zn over time, and field samples exhibited improved Cu and Ni retention compared with laboratory results, suggesting progressive aging effects such as carbonation and mineral transformations. Overall, the results demonstrate that bioash–cement binders can produce mechanically stable treated materials suitable for low-load applications while reducing cement demand; however, performance is strongly controlled by soil-specific chemistry (notably DOC) and field execution (mixing and compaction), and further binder optimization is required to address arsenic mobility.

The objective of this study is to evaluate and optimize an eco-friendly bioash–ground granulated blast furnace slag (GGBFS) binder for solidification/stabilization (S/S) of contaminated sandy–silt soil by identifying formulations that provide both mechanical strength and effective multi-element immobilization across curing time and carbonation aging. The soil contained elevated trace elements (As 403 mg/kg, Pb 806 mg/kg, Zn 398 mg/kg, Cu 526 mg/kg), exceeding Swedish guideline values for sensitive land use and requiring stabilization. A design-of-experiments (DoE) approach was used to define binder formulations. Mixtures were prepared at optimum moisture content (from Proctor compaction) and evaluated using unconfined compressive strength (UCS) testing and standardized batch leaching (SS-EN 12,457–2, L/S = 10). Leachates were analyzed for pH, electrical conductivity (EC), total organic carbon (TOC), inorganic carbon (IC), and dissolved trace elements. The dataset was analyzed using principal component analysis (PCA) and response-surface mapping to identify formulation regions that balance strength and leaching performance. Formulations (bioash 10–35%; GGBFS 5–15%) were cured for 28, 56, and 115 days. Carbonation aging was conducted for three weeks in sealed containers at laboratory temperature (19–24 °C) under CO₂ exposure. The formulation 35% bioash:15% GGBFS achieved the highest UCS (1438 ± 111 kPa at 56 days; n = 2) and strongly reduced leaching of cationic metals. Zn and Cd were below analytical limits (Zn < 2 µg/L; Cd < 0.05 µg/L), and Pb decreased by 99% relative to untreated soil. Arsenic leaching decreased by up to 43% after 28 and 115 days but increased transiently (20%) at 56 days. This increase coincided with CaCO₃ formation and lower-pH eluates with elevated Ca and IC, consistent with carbonation-driven changes in As retention. Compared with a cement-based binder, the bioash–GGBFS system moderated alkalinity while maintaining strength and improving Pb and As immobilization. Overall, the bioash–GGBFS system shows strong potential for sustainable remediation of metal-contaminated soils, although Cu and Ni require further optimization.

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