Coupling effect of binary sludge thermal treatment products on the workability and hydration process in the Portland cement system

Abstract

The cement industry's carbon reduction initiatives are constrained by the uneven regional distribution of existing supplementary cementitious materials (SCMs), while the heat treatment products (SHP) of municipal sewage sludge face significant storage pressures and potential environmental risks. Base on this, this study innovatively evaluates the combined utility of binary SHPs, namely incinerated sewage sludge ash (ISSA) and sludge biochar (SBC), in ordinary Portland cement (OPC). The workability, strength evolution and hydration mechanisms of SHP-OPC pastes at various replacement levels were systematically explored. The addition of SHP can reduce the bleeding rate of the composite system and SBC can mitigate the negative impact of high ISSA content on the workability of the system. The combined replacement of 2% SBC and 10% ISSA increased the 28d compressive strength of OPC by 42.7%. The addition of SHP causes the main hydration peak of OPC paste significantly delayed. This is attributed to the PO43- ions released from the phosphorus components in SHP. The internal regulation ability of SBC helps mitigate the inhibitory effect of ISSA on the hydration of the system. Additionally, SHP facilitated the formation of highly polymerized C-A-S-H, which effectively filled the mesopores in the paste. ISSA and SBC demonstrated distinct hydration enhancement effects, and their combined replacement of OPC exhibited a positive synergistic utility. These findings provide a critical basis for the development of novel SCMs and the resource-efficient utilization of municipal sludge.