Sustainable Stabilization of Cadmium (Cd)-Contaminated Soil with Biochar-Cement: Evaluation of Strength, Leachability, Microstructural Characteristics, and Stabilization Mechanism

To facilitate the solid waste reuse of cadmium (Cd)-contaminated soil and biochar and reduce the high emission of cement materials, we employed biochar-cement composite stabilizer to remedy Cd-contaminated soil. We assessed the effects of the biochar-cement composite stabilizer dosage and curing time on stabilized soil strength by using unconfined compressive strength (UCS) tests, leaching concentration and stabilization rate in Cd-contaminated soil stabilized by biochar-cement by using toxicity characteristic leaching procedure, and mechanism of change in strength and leaching characteristics by using scanning electron microscopy and X-ray diffraction. The research results demonstrate that biochar addition to cement-stabilized soil promoted cement hydration reaction and increased particle agglomeration. A low dosage of biochar improved stabilization effects. The optimal biochar dosage was 5%, which enhanced the early growth speed of the UCS. More cement improved the stabilized soil's UCS. At a 5% cement dosage, adding biochar significantly reduced Cd leaching concentration, and improved the stabilization rate. At a 10% cement dosage, the biochar-cement composite stabilized soil's stabilization rate was >99.8%. Calcium silicate hydrate, calcium aluminate hydrate, and ettringite were the main products of biochar-cement stabilized soil that filled soil and biochar pores; these substances encapsulated, adsorbed, or exchanged ions of Cd2+, achieving stabilization effects.