Low-carbon alkali-activated materials for Cu(II)-contaminated soil stabilization: Reaction kinetics, immobilization mechanisms, and leaching behavior

Alkali-activated cementitious materials (AAMs) are recognized as low-carbon binders with considerable potential in the solidification/stabilization (S/S) field. This study aimed to explore the influence of Cu^{2 +} on the reaction kinetics and products of AAMs and examined the immobilization mechanisms of Cu²⁺ within the reaction products. Results indicated that the early reaction kinetics of AAMs was only slightly impacted by the incorporation of Cu²⁺, showing a strong compatibility between the AAMs and Cu²⁺. The addition of an increased alkali equivalent facilitated the formation of reaction products, optimized the pore structure of AAMs, and significantly improved the compressive strength and Cu²⁺ encapsulation efficiency. However, incorporating Cu²⁺ increased the content of large pores, which deteriorated the mechanical properties of AAMs. Notably, Cu²⁺ chemically bonded in the amorphous phase by forming covalent bonds with silicon tetrahedra, which resulted in an increase in the MCL of C/N-A-S-H. Moreover, Cu²⁺ could achieve chemical immobilization in the amorphous gel by ion substitution and bonding to anionic sites. Actual S/S analysis revealed that with 70 % Cu-contaminated soil, the leaching concentration of Cu in S/S blocks was as low as 0.61 mg/L, with a compressive strength of 8.9 MPa after 28 days. Compared to ordinary Portland cement (OPC), utilizing AAMs for S/S treatment of Cu-contaminated soil reduced carbon emissions by 58.5 %. Overall, this research affirmed AAMs as a viable low-carbon alternative for S/S treatments, providing a sustainable and economical option for the management of heavy metal-contaminated soils.