Resource utilization of urban sludge via pyrolysis: impacts on cement performance and environmental safety

Abstract

This work utilizes pyrolysis technology to convert urban sludge into a pyrolyzed sludge at temperatures ranging from 300 °C to 600 °C and investigates its potential application in cement-based materials. The results showed that as the pyrolysis temperature increases, the yield of the pyrolyzed sludge decreases (from 73.7% to 58.7%), while the ash content and specific surface area increase (specific surface area increases from 11.9 m² g⁻¹ to 186.5 m² g⁻¹). The pH shifts from neutral to alkaline (approximately 6.9 to 9.5), indicating that the product is more stable and possesses a good porous structure. Performance tests reveal that untreated sludge significantly delays cement setting and reduces compressive strength (with a 10% dosage, the 3 days strength is only 11.5 MPa), whereas pyrolyzed sludge significantly improves mechanical properties. At an appropriate dosage, the 3 days strength increases by approximately 37%, and the 28 days strength improves by up to 35%. Pyrolyzed urban sludge (600 °C, 3–5%) significantly improved splitting tensile strength, while untreated sludge and higher replacement ratios reduced it. Environmental risk assessment shows that although the pyrolysis process concentrates heavy metals, the leaching concentrations of all metals in the concrete are well below the limits specified in GB 5085.3-2007, indicating that the environmental risks are controllable. In conclusion, pyrolysis not only enables the reduction and stabilization of sludge but also enhances its performance in concrete, demonstrating that pyrolyzed sludge is a feasible resource utilization approach as cement admixtures.