Freeze–Thaw Characteristics of Pb(II)-Contaminated Soil Stabilized Using NaOH- and Na 2 SiO 3 -Activated Geopolymer

Abstract

Lead [Pb(II)] contamination poses a significant environmental concern due to its detrimental effects on soil physicochemical (i.e., physical and chemical) properties and structural stability. This issue is further exacerbated by freeze–thaw (F-T) cycles, which accelerate the migration and redistribution of heavy metals. Traditional solidification/stabilization (S/S) technology has been widely employed for soil remediation, with cement-based stabilization being the most commonly adopted approach. However, ordinary portland cement, the primary binder used in conventional S/S systems, is associated with significant environmental drawbacks, including high carbon emissions and energy consumption. As a sustainable alternative, geopolymer stabilization has attracted increasing attention due to its lower environmental impact and strong capacity for immobilizing heavy metals. In this study, slag and fly ash were utilized as aluminosilicate precursors, while solid sodium hydroxide (NaOH) and sodium silicate (Na 2 SiO 3 ) served as alkaline activators to synthesize NaOH-activated geopolymer (SHG) and Na 2 SiO 3 -activated geopolymer (SSG) via a one-part geopolymer system. The stabilized Pb(II)-contaminated soil was subjected to F-T cycles, and their macroscopic characteristics and mechanical properties, including mass loss, unconfined compressive strength (UCS), UCS loss, and pH value, were systematically evaluated. Pb(II) leaching concentration was determined using inductively coupled plasma mass spectrometry. Furthermore, the F-T resistance mechanism was investigated via microstructure analysis, and structure degradation was quantified using Image-Pro Plus (IPP, version 6.0) to evaluate soil looseness. An F-T damage prediction model was also established to characterize the evolution of mechanical deterioration. The results revealed that mass loss increased with increasing Pb(II) concentration and F-T cycles but remained below 5%. UCS loss exhibited a similar trend, with the most significant reduction occurring after the first F-T cycle. Although SSG exhibited higher initial UCS, it showed greater susceptibility to F-T damage compared with SHG. Performance degradation was primarily attributed to pore expansion and crack propagation, resulting in increased structural looseness. Despite a slight increase in Pb(II) leaching after F-T cycles, the stabilization efficiency remained above 99%. The proposed prediction model provides valuable insights for optimizing one-part geopolymer systems for stabilizing heavy metal–contaminated soil in seasonally frozen regions.