Sustainable immobilization of lead in high-moisture surplus clay using paper sludge ash: Comparative assessment with ordinary Portland cement on strength and leaching behavior

Paper sludge ash (PSA) is a promising low-carbon binding agent for the solidification/stabilization (S/S) of heavy-metal (HM) contaminated soils. This study evaluates PSA’s effectiveness in immobilizing lead (Pb) in high-moisture Ao clay surplus soil as compared to ordinary Portland cement (OPC) under conditions relevant to embankment construction in Japan. First, binder dosages were determined from cone index tests on uncontaminated Ao clay with initial water contents w = w L and w = 1.3 w L , targeting q c = 800 kN/m 2 , the Japanese criterion for embankment applications. The resulting optimum ranges were A PSA = 10–15% and A OPC = 5–10%. Next, these dosages were applied to Ao clay artificially contaminated with Pb(NO₃) 2 at 500–10,000 mg/kg. Cone index, dry density, pH, and toxicity characteristic leaching procedure (TCLP) tests were performed after curing periods up to 28 days, along with XRD, SEM-EDS, FTIR, and TG-DTA analyses to elucidate immobilization mechanisms. Both PSA and OPC treatments reduced leached Pb by over 90% at appropriate dosages, achieving q c ≥ 800 kN/m 2 after 28 days. OPC developed higher early-age strength and required lower binder contents, particularly at w = 1.3 w L , whereas PSA provided comparable long-term Pb immobilization despite its slower pozzolanic reaction. Mineralogical and microstructural analyses confirmed the formation of C-S-H gels, ettringite, calcite, and cerussite, indicating that Pb is immobilized through precipitation, co-precipitation, isomorphic substitution, and microstructural encapsulation. Thus, PSA is a technically viable and more sustainable alternative to OPC for S/S of Pb-contaminated surplus soils, supporting the beneficial reuse of industrial residues.