Upcycling MSWI fly ash leachate as a novel modifier in low-carbon cement-free binders

Water-washing is an essential pretreatment approach for municipal solid waste incineration fly ash (MSWIFA) to remove chlorides and various soluble potential toxic elements (PTEs). This study proposed a novel approach to recycle MSWIFA water-washing leachate as a modifier to develop high-performance and cement-free binders. The influence mechanism of MSWIFA leachate on the hydration characteristics of binders was comprehensively evaluated. The hydration kinetics of the designed leachate-modified pastes were evaluated by isothermal calorimetric (ICC). The influence of leachate on the phase assemblage of binders was determined by the combination of X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The structure and chemical composition of C-(A)-S-H in leachate-modified pastes were evaluated by nuclear magnetic resonance (NMR) and energy-dispersive X-ray (EDS) points analysis, respectively. The pore structure of leachate-modified pastes was evaluated by mercury intrusion porosimetry (MIP). The results indicated that chlorides and sulfates enriched in leachate promoted the dissolution and reaction of precursor in the pastes. Aluminates in precursor reacted with chlorides and sulfates in leachate to Friedel's salt and ettringite, respectively. Besides, the high pH value of leachate was beneficial to the formation of C-(A)-S-H in leachate-modified pastes. Consequently, more hydration products were generated in the matrix of leachate-modified pastes, forming a more compact structure. The maximum 28-day compressive strength of leachate-modified pastes reached 45.7 MPa, fulfilling the requirements of ordinary concrete. These findings provided valuable guidance for the recycling of MSWIFA water-washing leachate.