Experimental study on solidification/stabilization of heavy metal lead and chromium sludge solidified by cement-based materials

To explore the strength change law and solidification mechanism of cement solidified heavy metal lead and chromium contaminated soil, ordinary Portland cement (OPC) and magnesium phosphate cement (MPC) were used for solidification/stabilization treatment. Through the unconfined compressive strength (UCS) test, scanning electron microscopy (SEM) and the leaching toxicity test, the microscopic characteristics of the cement type, the cement content, the ion type, the ion concentration, and the age and their influence law on the strength characteristics of heavy metal contaminated soil were obtained. The UCS test shows that the compressive strength increases with the cement content and age. When OPC and MPC were used to solidify lead and chromium heavy metal contaminated soil, the compressive strength of OPC was greater than that of MPC, and when the curing age was 28 days, 20% of OPC and MPC satisfied the landfill requirements. The leaching toxicity test showed that after the solidification/stabilization of heavy metal lead and chromium contaminated soil by OPC and MPC, the leaching content of lead ions in the leaching solution conforms to the national limit. For chromium contaminated soil, the concentration was less than or equal to 5000mg/kg, which conform to the limit standard. When the concentration was 10000mg/kg, the content exceeded the limit. Therefore, heavy metal lead contaminated soil is more easily stabilized than chromium contaminated soil. Microscopic tests show that OPC solidified heavy metal contaminated soil is solidified/stabilized by heavy metal ions through hydration products, namely, ettringite and amorphous cementing materials. MPC is obtained by the hydration reaction of magnesium oxide and ammonium dihydrogen phosphate to form struvite, and heavy metal ions are encapsulated in the lattice to achieve solidification/stabilization.