Water treatment sludge in the production of red-ceramic bricks: effects on the physico-mechanical properties

Water treatment plants generate sludge, which is mainly composed of inorganic minerals and a reduced fraction of organic matter. Even though the water treatment sludge (WTS) can be classified as non-inert and non-hazardous waste, there exists a clear interest in its potential valorization and reuse in other industrial sectors. This study presents the assessment of WTS, obtained after coagulation, flocculation, and decantation (using polyaluminum chloride coagulant), as raw material for the production of red ceramic bricks. Clay was substituted with sludge in the green mixture in ratios from 0 to 20 wt.%. The extrudability of the plastic mixture was adjusted after evaluating the rheological properties by applying an amplitude sweep test to prismatic bars produced with different sludge and moisture contents. The presence of sludge increases the stiffness of the green mixture, and more water is required for the extrusion of the ceramic products without any superficial or visual defects. The linear shrinkage was within the limits required for ceramic bricks, regardless of the sludge content. The results of the compressive strength and water absorption of the reduced-scale bricks also indicate that the inclusion of up to 20 wt.% of sludge in clay mixture is feasible for the production of red ceramics products.