Abstract:This study explored the strength evolution and hydration mechanisms of composite paste when municipal sludge biochar (SBC) and incinerated sewage sludge ash (ISSA) were synergistically used to replace ordinary Portland cement (OPC). The synergistic replacement of 2% SBC and 10% ISSA increased the 28d compressive strength of the composite paste by 17.5%. In terms of fresh paste hydration, SBC enhanced the early hydration of ISSA at high replacement levels, while ISSA significantly improved the system's strength in the middle and late stages through pozzolanic reactions. Regarding hydration products, SBC and ISSA promoted the formation of carboaluminate phases and highly polymerized C-(A)-S-H gels in the composite paste, which effectively filled the nanopores within the matrix.
