Comparative durability of NaOH-activated and Na2SiO3-activated geopolymer for Pb solidification/stabilization under chemical attack.

Rapid industrial development and site reuse have increased the risk of lead (Pb) pollution under aggressive chemical conditions. Solidification/stabilization is widely applied to immobilize Pb by reducing its mobility through physical encapsulation and chemical stabilization. One-part geopolymer synthesized from industrial by-products and solid alkaline activators has attracted attention due to their improved safety and practical applicability compared with conventional two-part systems, while their long-term durability and Pb immobilization performance under chemical attack remain insufficiently understood. In this study, one-part NaOH-activated geopolymer (NHG) and Na2SiO3-activated geopolymer (NSG), synthesized from ground granulated blast furnace slag and fly ash, were evaluated as binders for Pb immobilization. Pb-doped NHG and Pb-doped NSG were immersed in H2SO4, HCl, Na2SO4, and NaCl solutions, with water immersion used as a reference. Durability was assessed through macroscopic appearance, mass loss, unconfined compressive strength (UCS), durability coefficient, and Pb leaching behavior, while microstructure evolution and Pb distribution were examined by SEM-EDS. The results showed that chemical erosion severity follows the order H2SO4 > HCl > Na2SO4 > NaCl, with a corresponding ion aggressiveness sequence H+ > SO42- > Cl- > Na+. Before immersion, NSG exhibited a much higher initial UCS than NHG. At Pb0.8, NHG reached 24.37 MPa, whereas NSG achieved 68.15 MPa. Under chemical attack, particularly in acidic environments, NSG experienced faster UCS degradation. In contrast, NHG showed superior long-term durability, characterized by better UCS retention and lower degradation. Despite these differences, Pb leaching concentrations remained below 5 mg/L under all tested conditions. SEM-EDS observations indicated that durability differences were associated with distinct binding gels, with NHG governed by a network-like C-S-H gel and NSG dominated by a gel-like N-A-S-H. Overall, Pb-doped NHG exhibited more favorable comprehensive performance when long-term durability was considered.