Mechanical Performance and Microstructural Characteristics of Concrete Incorporating Drinking Water Treatment Sludge as Fine Aggregate Replacement

The management of sludge generated during drinking water treatment poses an environmental challenge due to the large volumes produced and the associated disposal costs. This study evaluates the potential use of sludge from the El Bosque Drinking Water Treatment Plant (Cartagena, Colombia) as a partial replacement of fine aggregate in concrete production. The sludge was characterized through physical, chemical, mineralogical, and microstructural analyses, including particle size distribution, Atterberg limits, specific gravity, X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), and petrographic examination. Five concrete mixtures were produced by replacing the fine aggregate with sludge at 0%, 5%, 10%, 15%, and 20% by mass. Mechanical performance was assessed through compressive strength and splitting tensile strength tests at curing ages of 7, 14, and 28 days. The results showed that increasing sludge content led to a progressive reduction in mechanical strength. Nevertheless, the mixture containing 5% sludge achieved a 28-day compressive strength of 26.15 MPa, corresponding to 96.9% of the design strength and comparable to the control mixture. Microstructural analyses revealed that higher sludge incorporation levels increased porosity and microcracking while reducing the relative proportion of cementitious matrix, explaining the observed decline in mechanical performance. These findings demonstrate the technical feasibility of incorporating DWTS from the El Bosque Drinking Water Treatment Plant as a limited replacement for fine aggregate without substantial losses in mechanical performance. The study provides site-specific physicochemical, mineralogical, and microstructural information that supports the sustainable valorization of this locally generated residue within circular economy strategies for the construction sector.