Synergistic effects of sewage sludge ash and limestone in cement-based composites: hydration kinetics and mechanical performance†

The use of solid waste as a supplementary cementitious material (SCM) has the potential to reduce CO₂ emissions in the cement industry. Sewage sludge ash (SSA), generated during the incineration of municipal wastewater treatment residues, has emerged as a viable SCM candidate for fabricating ternary cementitious systems incorporating cement and limestone. This study systematically investigated the hydration kinetics, phase evolution, and mechanical behavior of SSA-modified ternary composites. The research particularly focused on elucidating the synergistic interactions between SSA components and conventional cement hydrates under different compositions of ternary composites. Experimental results showed that fine SSA particles provided crystal nuclei for the crystallization and precipitation of hydration products and further increased the initial release of hydration heat. Besides, the reactive aluminate and silicate in SSA facilitated its reaction with limestone and the formation of aluminum-containing hydrates and carboaluminate phases. The formation of additional ettringite was also observed when the gypsum content was increased. These additional hydrates filled in the large capillary pores of hardened cement paste, resulting in a denser microstructure and higher compressive strength. The ternary composite incorporating 20% SSA, 10% limestone and 2% gypsum exhibited a compressive strength of 49.54 MPa with a strength activity index (SAI) of 94.56% at 28 days. The excellent mechanical performance of this ternary composite demonstrates its significant potential for the development of low-carbon cementitious materials.