The increase in waste generation and demand for environmentally sustainable construction materials necessitates exploring alternative resources such as sewage sludge. Understanding the sources and treatment methods of sewage sludge is essential before its use in the production of construction materials to ensure the performance and environmental benefits of the material. This study systematically reviewed the chemical composition of sewage sludge ash (SSA) from diverse origins, including wastewater treatment plants (municipal and urban), industrial, septic, and water treatment plants, and across various treatment methods, including thermal, drying, and chemical processes. Laboratory analyses of samples from the Ardhi University and Mburahati wastewater treatment plants in Tanzania complemented the literature review. The results indicate that the thermal treatment yielded SSA with elevated SiO₂, Al₂O₃, and Fe₂O₃ contents exceeding 70%, meeting the ASTM C618 standards for supplementary cementitious materials. Sludge from septic tanks showed elevated SiO₂ and Al₂O₃ levels, whereas industrial sludge contained higher CaO levels. This study highlights the potential of SSA as a valuable resource for sustainable construction, promoting circular economy principles and reducing environmental impact by transforming waste into functional building materials. These findings suggest opportunities to optimise the performance of SSA-based materials by strategically blending SSA from different sources and applying innovative treatment methods to enhance SSA properties, thereby enabling SSA to serve as supplementary cementitious materials.