• Replacing cement with water treatment plant (WTP) sludges proves feasible. • Composites containing 5% sludges match or improve in strength. • Dehydration processes of WTP influence the reactivity and sustainability of sludges. • Filter press drying results in lower carbon emissions compared to centrifugation. • Calcination increases carbon emissions, with limited mechanical benefits. The growing demand for sustainable construction materials and the drive towards carbon neutrality have intensified the search for alternative supplementary cementitious materials (SCMs). Although sludges from water treatment plants (SWTPs) have shown potential as SCMs, their large-scale application remains limited due to compositional variability and insufficient understating of their performance. This study investigates the feasibility of using calcined and non-calcined SWTPs from two southern Portuguese facilities employing different drying processes, while also quantifying carbon footprint benefits (kg CO 2-eq /m 3 of material). An innovative and integrated experimental approach, which directly links physicochemical properties, calcination-induced transformations, and cement hydration behaviour with mechanical performance, was developed to elucidate the role of processing on material reactivity and sustainability. Comprehensive characterisation included particle size analysis, chemical and mineralogical composition, thermogravimetric behaviour, release of dangerous substances, and pozzolanic reactivity. Cement pastes and mortars were then produced with 5% and 10% cement replacement by processed sludges to assess hydration kinetics, mechanical strength, and environmental performance. Results demonstrated that dehydration and calcination routes strongly influence chemical composition and pozzolanic activity, leading to distinct reactivity patterns. While 5% replacement maintained or even improved mechanical strength in specific cases, 10% substitution generally reduced performance. Importantly, incorporating SWTPs led to consistent greenhouse gas reductions of up to 4.9%, confirming tangible environmental benefits. Overall, this work highlights the innovative valorisation of water treatment residues as viable SCMs through controlled processing and mix design, reinforcing the potential of circular economy practices in the cement industry and offering a scalable pathway toward lower-carbon construction materials.
Machini et al. (Wed,) studied this question.