Migration and solidification behavior of the semivolatile heavy metal elements Zn and Cd during high ferrite cement clinker (HFCC) calcination

Abstract

The use of solid waste to produce low carbon, high ferrite cement clinker (HFCC) with favorable durability delivers potential application prospects. However, the semivolatile heavy metals Zn and Cd in solid waste not only restrict its utilization ratio but also cause potential environmental pollution. In this study, the migration and solidification behavior of Zn and Cd during calcination and their action mechanisms on phase reconstruction are systematically studied. The reaction sequence and conditions of Zn were revealed by experimental tests and phase diagram simulation. Zn2+ will firstly solidify in the C2S phase by the solid-phase reaction. After liquid phase forms, Zn2+ is more inclined to substitute Fe3+ in the C4AF phase with 4-coordination in the [FeO4]. Only a small amount of Zn2+ enters the C3A and C3S phases in the form of interstitial solid solution. Zn promotes the formation of the liquid phase and C3S phase. And HFCCs indicate a favorable solidification effect on Zn. While due to low volatilization temperature and high volatility of Cd, even at a low sintering temperature, the HFCC cannot greatly reduce the volatilization of Cd. Cd2+ mainly exists in HFCC by substituting Ca2+ in the C3S, C2S and C4AF phases with a 6-coordination number in [CaO6] to form the spatial configuration of octahedral complexes.