Synergistic recycling of MSWI fly ash and incinerated sewage sludge ash into low-carbon binders

This study proposed a novel method for synergetic recycling of municipal solid waste incineration fly ash (IFA) and incinerated sewage sludge ash (ISSA) to produce low-carbon binders. The hydration mechanism, physical properties, and environmental risks associated with the designed binders were thoroughly evaluated. The results revealed a compositional complementary effect between IFA and ISSA, with ettringite being identified as the main crystal product resulting from the reaction between Ca²⁺ and SO₄^2- in IFA and reactive aluminates in ISSA. The chlorides in raw IFA were effectively immobilized by the formation of various Cl-containing products, and all potentially toxic elements were immobilized in the binder system, exhibiting low leachability. Besides, CaO was found to be a highly efficient activator, providing additional reactive Ca²⁺ for the formation of more abundant and larger ettringite crystals, which consequently led to the high compressive strength (> 40 MPa) of the designed binders. Overall, this study provided a feasible approach for the upcycling of IFA and ISSA, contributing to the realization of “Zero-waste city” and the advancement of sustainable construction materials.