Optimizing the Mechanical Properties of Concrete Through Partial Replacement of Fine Aggregates with Wastewater Sludge

This study investigated the potential of using wastewater sludge (WWS) as a partial replacement for fine aggregates in concrete to optimize its mechanical properties, while mitigating environmental impacts. Wastewater sludge from three wastewater treatment plants in Polokwane, South Africa was used to replace sand at: 0, 5, 10, 15, and 20% by weight. The leaching of heavy metals, including calcium, silicon, aluminium, iron, and phosphorus, was evaluated by using a toxicity characteristic leaching procedure (TCLP) on samples submerged in iodine water for 28, 90, and 140 days. Scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), and energy-dispersive X-ray analysis (EDX) were employed to characterize the organic compositions of the sludge and sludge-based concrete. The results demonstrate that the incorporation of wastewater sludge significantly, reduced leachable heavy metals, with concentrations remaining within acceptable limits. Energy dispersive X-ray analysis revealed a substantial decrease in the metal content of the sludge-based concrete when compared with that of the original wastewater sludge. The surface morphology of the sludge-based concrete exhibited heterogeneous, crystalline, and rocky features, in contrast to the spongy and porous morphology of the sludge. Compressive strength tests showed that by replacing up to 5% of sand with wastewater sludge, maintained the required compressive strength of 25 MPa after 90 days of curing. These findings suggest that the partial replacement of fine aggregates with wastewater sludge in concrete can mitigate environmental pollution, while potentially optimizing the mechanical properties of the resulting material, thereby contributing to sustainable construction practices.