Sustainable Ultra-High-Performance Concrete with Sewage Sludge Ash: A Multi-Dimensional Assessment of Mechanical Performance, Safety, and Life-Cycle Impacts

Ultra-high-performance concrete exhibits excellent mechanical performance but relies on a high binder content, resulting in substantial carbon emissions. This study investigates sustainable ultra-high-performance concrete incorporating sewage sludge ash, aiming to balance mechanical performance, environmental safety, and life-cycle impacts within an integrated material system. High volumes of sewage sludge ash were incorporated into ultra-high-performance concrete under autoclave curing, with mixture proportions designed based on particle packing theory. Fresh properties, mechanical performance, shrinkage behavior, microstructural characteristics, heavy-metal leaching, as well as life-cycle environmental and economic impacts were systematically evaluated. The incorporation of porous sewage sludge ash modified the pore structure of ultra-high-performance concrete, thereby enabling a substantial reduction in cement content. At a sewage sludge ash replacement level of 60%, life-cycle assessment results indicate a 42.7% reduction in carbon emissions while maintaining a compressive strength of approximately 147 MPa under autoclave curing, remaining within a practically viable range for ultra-high-performance concrete. This confirms that sewage sludge ash can be safely incorporated into ultra-high-performance concrete, delivering a favorable sustainability–performance trade-off alongside significant environmental and economic benefits.