Evaluation of red mud based binder for the immobilization of copper, lead and zinc

In this study, by products such as red mud, phosphorus gypsum and fly ash were used as binders, and are compared with Portland cement (PC) in immobilizing Cu, Pb and Zn. Cu, Pb and Zn -doped pastes and mortars were prepared with a metal to binder ratio at 1%. Samples were cured for 7d, 14d and 28d. The unconfined compressive strength (UCS) test, a batch leaching test along with scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) were applied for the testing of Cu, Pb and Zn-doped pastes and mortars. The UCS results show that red mud-phosphorus gypsum treated samples produce higher strength than these treated by red mud-fly ash, or PC. The results of leaching test revealed that the immobilization degree of heavy metals from these pastes depends on the leachate pH of these pastes. With the aid of the cement, red mud-phosphorus gypsum-cement pastes leached less metals compared to that of red mud-phosphorus gypsum pastes. The leachate concentrations of Cu, Pb and Zn from red mud-phosphorus gypsum-cement pastes are 1.5 mg/L, 1 mg/L, and 3 mg/L respectively. They are able to meet the China Ministry of Environment Protection (MEP) regulatory limit. With the increase of the curing time, the unconfined compressive strength and the leaching concentrations of these pastes showed a slightly increasing trend. In addition, SEM and TGA analyses show that the major hydration product is ettringite.