Heavy metals migration during the preparation and hydration of an eco-friendly steel slag-based cementitious material

Abstract: Steel slag is a steel production byproduct and its utilization rate is only 29.5%. Creating sulfoaluminate cementitious material (SACM) using industrial solid wastes is an effective way to dispose steel slag on a large scale. However, the behavior of heavy metal during this process is still unclear. In this study, an eco-friendly steel slag-based sulfoaluminate cementitious material (ESS-CM) is prepared with up to 38.47 wt% steel slag. The migration behavior and immobilization mechanism of heavy metals in steel slag, raw material mixture, ESS-CM clinker and hydrated ESS-CM during the calcination-hydration process are investigate. The results indicate that leaching concentrations of eight heavy metals (Zn、Cu、Ni、Pb、Cr、As、Cd、Mn) in ESS-CM clinker and hydrated ESS-CM are far lower than the standard limits of the U.S. Environmental Protection Agency. It is noted that calcination and hydration could change the leaching behaviors of heavy metals. Due to the physical encapsulation effect and the chemical adsorption effect of ettringite, the leaching concentration of Mn decreases significantly after hydration. Sequential extraction analysis reveals that acid-soluble fraction of Cr increases with curing age, resulting in the high level of Cr leaching after hydration. Through electron probe microanalysis, it is found that most of Cr and Mn from steel slag are still existing in the mineral lattice of the divalent metal oxide solid solution (RO phase) in the clinker and it does not participate in the hydration reactions, which further reduces the migration of heavy metals. The findings of this study reveal that the steel slag-based cementitious material is environmentally safe and it provides a promising strategy to utilize steel slag on a large scale.