Optimizing NaOH or Na2SiO3 Activated Geopolymer for Pb Immobilization: Freeze–Thaw Enhancement and Resistance Mechanisms

Abstract The rapid industrialization and urbanization of cold regions have intensified environmental challenges, particularly lead (Pb) contamination from industrial activities. Solidification/stabilization ( S / S ) technology is a widely adopted engineering method for immobilizing heavy metal contaminants, because it reduces their mobility, toxicity, and environmental impact through combined physical encapsulation and chemical stabilization. Recently, geopolymer technology has emerged as a promising alternative binder for S / S applications. Geopolymer is synthesized by activating aluminosilicate-rich industrial byproducts, such as blast furnace slag (BFS) and fly ash (FA), with alkaline activators to form dense and chemically stable materials. Their three-dimensional network structure enhances Pb immobilization by integrating chemical bonding with physical encapsulation. Geopolymer preparation methods are generally categorized into one-part and two-part systems. The one-part method, which uses solid alkaline activators, improves safety and applicability in field conditions, especially in cold regions. However, limited research has examined the Pb immobilization performance of one-part geopolymer systems under freeze–thaw (F-T) conditions. Addressing this gap, this study evaluated one-part NaOH activated geopolymer (NHG) and Na 2 SiO 3 activated geopolymer (NSG), synthesized from BFS and FA, as sustainable binders for Pb immobilization in cold climates. This study first assessed the mechanical properties of NHG and NSG under non-F-T cycles, considering different BFS/FA ratios (i.e., Ca/Si ratios) and various proportions of alkaline activator. NHG and NSG with an optimized Ca/Si ratio were then subjected to controlled F-T cycles at varying freezing temperatures, with Pb contamination incorporated at different concentrations. Performance was evaluated through unconfined compressive strength (UCS), appearance analysis, mass loss, pH value, and Pb leaching test. In addition, microstructure analysis was analyzed using scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) to elucidate durability and immobilization mechanisms under F-T cycles. Results revealed that under non-F-T cycles, NSG achieved higher UCS values than NHG, with both systems reaching peak performance at a Ca/Si ratio of 1.2. However, under F-T cycles, NHG displayed superior F-T resistance compared to NSG. Despite these differences, both systems maintained excellent Pb immobilization performance, with Pb leaching efficiency below 1%. This is primarily attributed to their dense matrix, reduced number of pores and cracks, and strong chemical interactions with Pb ions. Even after repeated F-T cycles, the geopolymer matrix preserved structural integrity and ensured Pb immobilization. The outcomes demonstrate that one-part geopolymer activated by NaOH or Na 2 SiO 3 are environmentally sustainable and technically viable materials for the immobilization of Pb contaminants in cold regions. It offers valuable insights into sustainable waste utilization, long-term environmental protection, and the development of durable remediation technology suitable for harsh climatic conditions.