The effect of metakaolin on the properties of MSWI fly ash-slag binder: Compressive strength and chloride ions...

Municipal solid waste incineration fly ash (MSWIFA) can be used to activate granulated blast furnace slag (GBFS). However, the potential chloride ions limit the valorization of MSWIFA-GBFS binder. In this study, metakaolin (MK) was employed to improve the mechanical properties and chloride ions immobilization of this binder. The compressive strength and chloride ions immobilization ratio were measured; XRD, TGA, ICP, SEM, and MIP were used to characterize the hydration process and immobilization approach. Results show that MK enhances later strength and chloride ions immobilization ratio of the pastes: 10% MK increases 28-day compressive strength by 21% and improves the chloride ions immobilization ratio by 15% at 56 days. In the early period, owing to the dilution effect of MK, the formation of ettringite is accelerated, while the formation of Friedel's salt (Fs) is inhibited. As the continuous hydration of MK, the generation of hydration products, mainly including Fs and C-(A)-S-H, is accelerated, and the harmless pore volume is increased by MK. The chloride ions immobilization is attributed to the combination of chemical binding and physical adsorption. The content of Fs is increased due to the dissolution of MK, exhibiting the greater chemical binding; the generation of C-(A)-S-H and the volume of pores less than 50 nm is enhanced by MK, showing the stronger physical adsorption. This research offers a safety route to valorize MSWIFA in cementitious materials.