Mechanisms of solidification of cadmium in municipal solid waste incineration fly ash usinga slag cemented backfill agent

In cement production, the synergistic use of municipal solid waste incineration (MSWI) fly ash provides a novel approach to solve the problem of the increasing productions of fly ash and heavy metal ions. In this study, slag-steel slag based cementitious material (referred to as metallurgical slag cementitious materials) was combined with four types of MSWI fly ash for preparing cementitious materials, all of which use sand as a binder. According to the determination of fluidity, compressive strength and Cd²⁺ leaching concentration of filler samples, the following are found:(1) the flow rate of filler material prepared from metallurgical slag and fly ash is 240-265 mm, which fulfills the pumping requirement of mine filling; (2) the 28-day compressive strength values are>8.88 MPa, which meets the general mine filling strength requirements (1-6.5 MPa); and (3) the Cd²⁺ leachate concentration is lower than the drinking water standard of 5 μg·L^-1 limit. The results of X-ray diffraction, infrared spectroscopy, and thermogravimetry-differential scanning calorimetry show that the leading hydration products of gelling materials are ettringite, Friedel, and C-S-H. Furthermore, X-ray photoelectron spectroscopy results reveal that Cd²⁺ has a great effect on the binding energy of Al³⁺. Ettringite and Friedel salts are found to have a curing effect on cadmium ions.