Sewage sludge ash in concrete: a comprehensive review on properties, performance, and environmental sustainability

Finding an environmentally responsible and cost-effective way to dispose of sewage sludge is a global challenge. This review explores how turning incinerated sewage sludge ash (SSA) into a sustainable concrete ingredient could address both waste management challenges and the construction sector’s significant carbon footprint. The literature shows that SSA performance in concrete depends on how it is produced and what it contains. Although adding SSA can slow down setting and reduce early strength due to its lower initial reactivity compared to cement, these drawbacks can be overcome with specific pre-treatments and accelerating additives. Evidence suggests that replacing 5–10% of cement or fine aggregate with SSA can produce concrete that matches or even exceeds the long-term strength of traditional mixes, while maintaining other key properties, such as tensile and flexural strength. SSA can also make concrete more durable by reducing water absorption, blocking chloride ions, and refining the pore structure. Altogether, these results show that SSA can be safely and effectively used in concrete, helping to cut landfill use, reduce reliance on clinker, and promote a circular economy in construction. However, thorough leaching tests are essential to ensure that SSA-based concrete is environmentally safe before it is widely adopted.