Water Treatment Sludge as a Sustainable Supplementary Cementitious Material: A Review

Water treatment sludge (WTS) is a by-product of water treatment plants, which is often landfilled and risks contaminant leakage into the environment. However, it is rich in aluminosilicate content, which suggests potential suitability as a supplementary cementitious material (SCM), offering a sustainable approach for waste recycling and carbon emission reduction in cement production. While prior reviews have broadly addressed WTS reuse across construction materials, the mechanisms governing its pozzolanic performance as a cement replacement remain insufficiently synthesized, including its emerging use in alternative binders. This systematic review addresses this gap by synthesizing literature from 2010 to 2026 on WTS as a partial cement replacement in cement-based materials (CBMs), in both binary and ternary blends. Findings show that WTS can exhibit high pozzolanic reactivity after grinding and calcination at 600–800 °C, though performance varies depending on source and composition and processing. Partial replacement of cement with 10% calcined WTS was most frequently identified as the optimal substitution level, improving mechanical properties by promoting C-S-H and C-A-S-H formation, although some studies report favourable long-term strength at higher substitution. However, most studies reported that further increasing WTS content in the mix (beyond 10%) leads to a decrease in performance due to the dilution effect, which limits the formation of C-S-H. This review further discusses the durability aspects and environmental impact of using WTS, which remain underexplored in the literature, and highlights areas for future investigations.