Sustainable valorization of detergent industry wastewater treatment sludge in cement mortar for heavy metal immobilization and durability assessment

The increasing generation of industrial wastewater treatment sludge presents significant environmental challenges owing to its heavy metal content and limited disposal options. This study evaluates the feasibility of using detergent-industry wastewater treatment sludge as a partial cement replacement (5–30%) in mortar while assessing its environmental safety and durability. The originality of the current research lies in linking standard dynamic leaching tests (NEN 7345) with electrochemical impedance spectroscopy (EIS) and complex conductivity to correlate heavy metal immobilization, ionic transport, and microstructural evolution during the process of leaching. The amount of heavy metals (As, Cr, Zn, Cu, Fe, and Al) released from the samples was determined by means of ICP-OES, while EIS was used as a non-destructive technique to characterize pore connectivity, dielectric relaxation, and ionic transport inside the cementitious materials. It was found that heavy metal content increased as a function of time and sludge content; however, their values were still lower than permitted, which indicated the high effectiveness of immobilization inside the cementitious materials. Two relaxation phenomena, related to localized dielectric polarization and diffusion of ions, were revealed by electrical measurements. As the amount of sludge content was increased, impedance decreased and relaxation times changed due to the pore structure evolution and ionic transport properties alteration.