Synergistic use of water treatment sludge and silica fume in geopolymer binders for stabilization/solidification of heavy metals in construction waste

This study investigates the potential of a novel geopolymer binder, synthesized from water treatment sludge (WTS), silica fume (SF), and a recycled liquid alkaline activator (RLAA), for the stabilization and solidification (S/S) of construction and demolition (C&D) waste contaminated with Pb and Cd. For this purpose, response surface methodology (RSM) was used to design and optimize the experiments by assessing the combined effects of WTS content, SF content, and heavy metal (HM) concentration through two 17-run Box–Behnken design matrices. The mechanical performance and leachability of the samples were evaluated after 28 days of curing using unconfined compressive strength (UCS) and toxicity characteristic leaching procedure (TCLP) tests, while the results were further verified through a set of microstructural analyses. The findings revealed that the optimal mechanical performance was achieved in Pb-contaminated samples with 15% SF and 5% WTS and Cd-contaminated samples with 15% of both additives, with all stabilized samples exhibiting significant strength enhancement (UCS > 1000 kPa) from the WTS and SF addition. Moreover, the immobilization efficiencies of Pb and Cd exceeded 97%, while the leachate concentrations were far below the EPA regulatory limits. The high correlation coefficients of the developed RSM models (R 2 = 0.953 for UCS Pb , 0.974 for S/S efficiency Pb , 0.973 for UCS Cd , and 0.995 for S/S efficiency Cd ) demonstrated their strong predictive capability for both strength and leachability. These results confirm that the synergistic use of WTS and SF in geopolymer binders provides a sustainable and highly effective approach for the S/S of HM-contaminated construction waste. The synergistic use of WTS, SF, and RLAA provides a cleaner-material geopolymer binder that converts multiple industrial waste streams into a value-added stabilization material, reducing dependence on Portland cement and commercial alkaline activators while providing a sustainable and highly effective approach for the S/S of HM -contaminated construction waste.