Comparative Effects of Sewage Sludge and Sewage Sludge–Biomass Briquette Ash on Phase Evolution, Microstructure and Performance of Fired Clay Bricks

This study investigates the potential use of sewage sludge (SS) and sewage sludge ash obtained from the combustion of sewage sludge–wood-chip briquettes (BSSA) as additives in fired clay bricks. The novelty of this study lies in comparing these two waste-derived additives within the same clay matrix. Clay mixtures containing 5–20 wt.% SS or BSSA were prepared and fired at 1000 °C. The raw materials and fired products were characterized using chemical and thermal analyses, X-ray diffraction (XRD), and scanning electron microscopy with energy-dispersive spectroscopy (SEM–EDS). Compared with the control specimens, SS incorporation reduced bulk density by 10–35% and compressive strength by 32.5–62.8%, while BSSA reduced these properties by 8–18% and 29.6–58.8%, respectively. Water absorption increased with both additives. Microstructural observations indicated increased pore formation, particularly in SS-containing specimens, consistent with the burnout of organic constituents during firing. The results suggest that incorporation levels of 5–10 wt.% offer a more favorable balance of properties than 20 wt.%. The use of BSSA may support cascade waste utilization by combining energy recovery with subsequent ash reuse.