Hydration and rheological properties of municipal sludge co-processing cement

The recycling and co-processing of municipal sludge in the production of cement clinker is a promising technology for the valorization of municipal solid wastes. However, the Al and Si rich in municipal sludge and the presence of heavy metals usually results in the different performance of municipal sludge co-processed cement (CPC) from normal ordinary Portland cement (OPC). To understand the variation in fresh behaviour, this study evaluated the flowability and rheological properties of CPC and OPC pastes with dosages of superplasticizers (SP). The hydration kinetics and hydration products of pastes in the fresh state were characterized. The influences of SP on the hardening properties of CPC and OPC samples were further addressed. The result indicated that the incorporation of SP provided a superior flowability of CPC pastes compared to OPC after mixing, however, the fast decline in flowability of CPC pastes in the first 40–80 minutes was confirmed. Consequently, CPC pastes presented faster development of static yield stress and dynamic yield stress compared to OPC with the SP dosage from 0.05 % to 0.125 %. CPC consumed significantly higher SP to achieve similar workability to OPC pastes. The characterization of reaction products indicated that hydration of CPC produced more ettringite and less portlandite in the first few minutes compared to OPC with or without SP. The application in OPC and CPC all reduced the volume of pores larger than 100 nm. The optimal SP could effectively promote the strength development of CPC and OPC at early ages.