# Bioash-Based Stabilization/Solidification for Heavy Metal(oid) Soil Remediation: A Case Study in Northern Sweden

## TL;DR

This study explores using a bioash-cement mix to stabilize heavy metal-contaminated soils in Sweden, showing improved strength and reduced metal leaching, though performance depends on soil chemistry and field conditions.

## Contribution

The study introduces a low-cement bioash–cement binder for soil remediation, validated at pilot scale with field performance data.

## Key findings

- The optimized binder achieved compressive strengths of 696 kPa and 479 kPa for two soil types after 28 days.
- Leaching of Zn, Cd, and Pb was reduced, but As mobility remained a challenge, especially in non-hazardous soils.
- Field monitoring over two years showed decreasing leachate concentrations of several metals, with improved Cu and Ni retention over time.

## Abstract

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.