Valorization of water treatment sludge in the production of structural ceramics: Shaping optimization and property analysis

The proper disposal of water treatment sludge (WTS) is a growing environmental challenge. This research proposes the sustainable recycling of WTS in the manufacturing of ceramic materials, evaluating its use as an alternative raw material to replace clay. Since sintering WTS typically requires temperatures above 950 °C to achieve optimal properties, firing at lower temperatures generally compromises the performance of the pieces. To mitigate the impacts of this reduced thermal densification, the present study optimized the shaping parameters to enhance initial particle packing. The pressing parameters (moisture and compaction load) of the reference sample were defined via response surface methodology and qualitative analysis. Mixtures ranging from 0% to 100% WTS were shaped into cylindrical molds (50 × 100 mm) and fired at 800 °C for 3 hours. The anhydrous samples were evaluated by granulometry, Atterberg limits, thermogravimetry (TGA/DTG), X-ray diffraction (with the Rietveld method), and SEM-EDS. After calcination, linear shrinkage, water absorption, mechanical strength, and loss on ignition were analyzed. A progressive increase in loss on ignition (from 6.19% to 19.66%) and water absorption (from 13.11% to 42.77%) proportional to the waste content was observed. Compressive strength decreased from 46.69 MPa (reference) to 7.22 MPa (100% WTS), with the mixture containing 15% WTS standing out, achieving a performance of 41.33 MPa. Mixtures with up to 20% WTS exhibited properties similar to results found in the literature, whereas the 100% WTS sample showed usage limitations due to its non-plastic behavior and high absorption. It is concluded that the optimized shaping enables the incorporation of up to 20% WTS at 800 °C, surpassing performances reported in the literature for temperatures above 950 °C, while combining technological quality and environmental sustainability.