Hydration performance and optimization of multi-solid waste composite calcined sewage sludge

The rapid increase in sewage sludge (SS) production and the global pursuit of carbon neutrality have driven the demand for the high-value construction material utilization of SS. In this study, calcium-rich solid wastes-carbide slag (CS) and phosphogypsum (PG)-were co-calcined with SS at elevated temperatures (800-1000 °C) to address the insufficient cementitious activity of thermally activated sludge. The resulting products were used to replace 30% of ordinary Portland cement (OPC) in pastes. The effects of calcination temperature, CS addition, and PG addition on the properties of calcined SS and its pastes were systematically investigated using XRD, FTIR, SEM, T₂NMR, and TG-DSC. Results showed that calcination at 800 °C converted kaolinite and muscovite in SS into amorphous reactive phases, significantly enhancing pozzolanic activity and mechanical strength. Co-calcination with CS and PG facilitated the formation of anorthite and calcium aluminate gels, further improving paste hydration (AFt, CH formation) and reducing its porosity. When CS and PG replaced 30% and 15% of SS, respectively, and the mixture was calcined at 800 °C, a clear synergistic hydration effect was observed. Response surface methodology identified the optimal conditions as 882.45 °C, 14.78% CS, and 2.62% PG, under which the compressive strength reached 85% of that of OPC, with a production cost of 190.19 RMB per t and carbon emissions of 529.17 kg CO₂ e per t, both notably reduced compared to OPC. These results demonstrate that MWCS is a promising sustainable cementitious material for low-strength engineering applications or as a partial cement replacement.