Characterization of naturally aged cement-solidified MSWI fly ash

Solidification/stabilization (S/S) is the most common treatment for municipal solid waste incineration fly ash (MSWI-FA), and is widely applied in developed countries but has a history barely longer than 10 years in China. However, our understanding of the physicochemical characteristics of the solidified FA body after long-term natural aging is comparatively poor. Focusing on cement-solidified FA that was naturally aged for 6 years (hereafter referred to as FA-6), the physicochemical characteristics including elemental composition, mineral composition, microstructure, thermogravimetry, distribution of heavy metals in mineral phases, and leaching characteristics of inorganic salts (Na, K, Ca), anions (Cl and SO4) and heavy metals (Cd, Cr, Cu, Pb, Zn) were investigated in this study. By combining pH-dependent leaching results with the geochemical model LeachXS, the chemical forms of heavy metals in the FA solid phase was determined. The main conclusion was as follows: (1) soluble salts of FA-6 decreased by more than 92% compared with fresh FA. (2) In FA-6, the proportions of Pb, Cd and Zn in the non-mineral phase were 100%, 100% and 58%, respectively, which may cause potential environmental risk of heavy metal release. The leaching concentration of Pb was 4007.37 μg/L according to compliance batch test of HJ300, which was far higher than the landfill requirement of 250 μg/L. (3) The controlling phase for Pb in FA-6 was Pb5(PO4)3Cl (pH 2-12) and Pb2(OH)3Cl (pH > 12). (4) Carbonates, hydrous Fe oxides (HFO) and dissolved organic carbon (DOC) in FA-6 also affected the phase-controlled leaching of heavy metals. The carbonate fraction partly controlled the leaching of Cd, Cu and Zn. For example, smithsonite (ZnCO3) controlled the release of Zn (pH 2-13). Adsorption to solid humic acid (SHA) controlled the Cr leaching at pH < 7 and the Cu leaching except pH > 12.