Influences of Aluminum Ash Slag on Mechanical and Microstructural Characteristics of Stabilized/Solidified Heavy Metal Contaminated Soils

Solidification/stabilization (S/S) is a widely adopted technique in the remediation of contaminated soils. However, how to efficiently and economically modified contaminated soil remains a challenging issue. In this study, Secondary aluminum ash slag (AAS) was employed as a novel stabilizer for the S/S treatment of lead (Pb) and zinc (Zn) contaminated soils. Internal and external addition methods were compared in sample preparation process by unconfined compressive strength (UCS) and hydraulic conductivity tests. Toxicity characteristic leaching procedure (TCLP) tests were further conducted on stabilized soils sampled by external addition methods to evaluate the retention capacity of contaminated ions. Hereafter, microstructural characterizations, including scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) and mercury intrusion porosimetry (MIP) analyses was performed to elucidate the stabilization mechanisms. Results shown that moderate incorporation of AAS can significantly enhance both mechanical and stability behaviors. Additionally, there exists an optimal dosage of AAS (4% in this study), which can yield a UCS exceeded 2000 kPa, the hydraulic conductivity was also hindered accordingly. Moreover, the leaching concentration of Pb and Zn was reduced by over 95%. Microstructural analysis indicated that external addition of AAS promoted the formation of calcium aluminosilicate hydrate (C-A-S-H) and ettringite (AFt), thus contributing to pore filling and improved the compactness. These findings highlight the potential of AAS as a sustainable stabilizer in contaminated soil remediation, which possess both environmental and economic benefits in terms of S/S contaminated soil.