Immobilization of Zn(â ¡) and Cu(â ¡) in basic magnesium-sulfate-cementitious material system: Properties and mechanism.

To solve the environmental problems caused by heavy metal pollution, a new cementitious material (basic magnesium sulfate cement, BMSC) was developed for the solidification of Cu2+/Zn2+. First, the effects of different amounts of Cu2+/Zn2+ on the properties (compressive strength, setting time, pH, and leaching toxicity) of the BMSC were investigated. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) were used to investigate the effects of different amounts of Cu2+/Zn2+ on the phase and microstructure of BMSC. The results showed that Cu2+/Zn2+ inhibited the hydration of BMSC, reduced compressive strength, and prolonged the setting time. The results of the leaching tests showed that the BMSC system exhibited high immobilization efficiency (up to 99%) for Cu2+/Zn2+. Further, the BMSC solidification matrix exhibited excellent acid resistance (compressive strength >40 MPa after 28 days of immersion). The physical phase analysis showed that the main phases of BMSC were the 5Mg(OH)2-MgSO4-7 H2O (5-1-7) phase and Mg(OH)2, and the crystal structure refinement analysis suggested that Cu2+/Zn2+ ions were substituted with Mg2+ in the 5-1-7 phase. It was confirmed that the solidification mechanism of BMSC on Cu2+/Zn2+ is mainly performed by chemical complexation and ionic substitution.