Self-Solidification/Stabilization of Heavy Metal Wastes of Class C Fly Ash–Based Geopolymers

The goal of this paper is to study the feasibility of using geopolymers based on Class C fly ash (CFA) to self-solidify/stabilize (S/S) heavy metal wastes (HMWs). The S/S effectiveness and mechanism of Pb(II), Cr(VI), and Hg(II) by CFA-based geopolymers were studied by compressive strength test, Toxicity characteristic leaching procedure (TCLP), X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR), and scanning electron microscope–energy dispersive X-ray analysis (SEM-EDXA). The results show that with HMWs the compressive strength has a slight decrease, and the lowest 28 days strength of the CFA-based geopolymers was 51.1 MPa. All leaching concentrations of heavy metals from geopolymers are far lower than the maximum concentration limits, and S/S ratios of these heavy metal ions are very high. The XRD patterns show no significant changes in the mineral compositions when heavy metals are present. Compared to the FT-IR spectrum of the blank specimen without heavy metals, the peaks of $\mathrm{Si}\text{-}\mathrm{O}\text{-}\mathrm{Si}/\mathrm{Si}\text{-}\mathrm{O}\text{-}\mathrm{Al}$, Si-O, and $\mathrm{OH}/\mathrm{H}\text{-}\mathrm{O}\text{-}\mathrm{H}$ are shifted in geopolymers with HMWs. The SEM-EDXA data reveal other shapes of products coexist within the amorphous geopolymeric gels. It indicates that heavy-metal ions are physically encapsulated and chemically bonded in the three-dimensional geopolymeric network. This work is beneficial to the understanding and future application of CFA-based geopolymers as S/S materials to make HMWs less available as environmental pollutants.