Durability and mechanical performance of eco-friendly portland cement mortars containing sewage sludge ash as a cement substitute

This study explores the potential use of sewage sludge ash (SSA) as a sustainable supplementary material for the partial replacement of Portland cement in mortar production. The objective is to evaluate the influence of SSA on the mechanical and durability properties of cement-based mortars while promoting the valorisation of wastewater treatment residues. In this work, dried sewage sludge obtained from a wastewater treatment plant was calcined at 550°C and 700°C to produce two types of ash, namely SSA550 and SSA700. Prior to their use in mortar formulations, the ashes were characterised through physicochemical and mineralogical analyses. SSA was then incorporated as a partial replacement of cement at substitution levels of 0%, 5%, 10%, 15%, and 20% by mass. The prepared mortars were evaluated in terms of mechanical strength, apparent porosity, sulfate resistance, and chloride ion permeability in order to assess both their structural and durability performance. The results showed that the calcination temperature significantly affects the behavior of mortars containing SSA. Among the tested formulations, the mortar containing 5% SSA550 exhibited the most balanced performance, combining satisfactory mechanical strength with improved resistance to sulfate attack while maintaining chloride permeability comparable to that of the reference mixture. In contrast, the mortar containing 10% SSA700, although showing relatively higher initial strength, presented increased porosity and reduced chemical resistance. Overall, the results indicate that SSA calcined at 550°C can be effectively used as a partial cement replacement. The incorporation of 5% SSA550 appears particularly promising, offering a viable approach to reducing clinker consumption while contributing to the sustainable reuse of sewage sludge in construction materials.