Na2SO4 modified low-carbon cementitious binder containing commercial low-reactivity metakaolin for heavy metal immobilization: Mechanism of physical encapsulation and chemical binding

Abstract

In this study, a Na2SO4 modified low-carbon cementitious binder based on commercial low-reactivity metakaolin (MK) and cement was utilized for the immobilization heavy metals (Cu2+, Ni2+, Cd2+ and Zn2+). The compressive strengths and leaching concentrations of the immobilization systems were investigated to evaluate the immobilization effect for heavy metals. Meanwhile, the pore structure and chemical compositions of the immobilization systems were characterized to study the immobilization mechanisms. Results demonstrate that the MK-blended systems exhibit excellent heavy metal immobilization effect and low releasing risk under the effect of Na2SO4. The incorporation of Na2SO4 significantly improve the compressive strength of the MK-blended systems due to the promotion effect on the early hydration kinetics of cement, the formation of ettringite and the pozzolanic reaction of MK. Moreover, the incorporation of Na2SO4 decreases the capillary porosity and/or increases the tortuosity of the MK-blended systems, which enhances the physical encapsulation to heavy metals. The MK-blended systems have higher chemical binding capacities to heavy metals than pure cement immobilization systems. This is due to the increase in the Si/Ca and Al/Ca atom ratios of calcium aluminosilicate hydrate that introduces more negative charged silicon and aluminium sites to bind heavy metal cations.