A Strategy for Complete Utilization by Coincinerating Phosphorus-Rich Sewage Sludge in a Cement Kiln: Clinker Mineral Phase Transformation and Carbon Emission Reduction

The coincineration of sewage sludge in cement kilns enables the comprehensive utilization of sludge but may affect clinker quality due to its high phosphorus (P) content. This study investigated P transformation during coincineration and its impact on clinker. The results indicate that when sludge is added to cement raw meal (CRM), P in the sludge initially transforms into apatite at 600 °C, followed by its transformation into 2CaO·SiO2–Ca3(PO4)2 (C2S–C3P), Ca4P2O9, Ca5P2SiO12, and Ca7P2Si2O16 in the clinker. The P distribution in clinker is 75.7%–87.3% in the calcium silicate phase, 9.9%–21.7% in the free oxides, and 2.4%–4.7% in the interstitial phase. A 1.0% increase in P2O5 decreases 3CaO·SiO2 (C3S) by 7.07% and increases 2CaO·SiO2 (C2S) by 9.07% in the clinker, lowering cement compressive strength by 7.77, 11.92, and 12.17 MPa at 3, 7, and 28 days, respectively. To meet the PO42.5 strength requirement (77.60 MPa at 28 days), the P2O5 dose in CRM should not exceed 1.58%. The coincineration of 1 t of sludge in a cement kiln results in a net increase of 0.057 tCO2eq The results of this study provide theoretical guidance for the sustainable development of cement production.