Comparison of limestone calcined clay cement and ordinary Portland cement for stabilization/solidification of Pb-Zn smelter residue

Abstract

Decreasing carbon emissions by replacing Portland cement (PC) with supplementary cementitious materials (SCMs), such as low-grade limestone (LS) and calcined clays (CC), has tremendous potential for stabilization/solidification (S/S) of industrial hazardous waste primarily with heavy metals. Recently, a low-carbon-based cementitious binder, namely, limestone calcined clay cement (LC), has emerged as an alternative for S/S treatment of wastes. However, comprehensive comparison between LC and PC application in solidifying/stabilizing wastes has not been conducted. This study aims to investigate the S/S efficiency of Pb-Zn smelter residue (LZSR) comprising heavy metals lead (Pb), zinc (Zn), and cadmium (Cd) at higher concentrations. LZSR is treated with LC and PC for capturing strength and leaching toxicity. The test results indicate that low-grade CC and LS in the LC binder can promote the alkaline environment, and act as fillers in solidifying heavy metals. The toxicity characteristic leaching procedure leaching concentrations of untreated (UT) LZSR were 503 mg/kg, 1266 mg/kg, and 251 mg/kg for Pb, Zn, and Cd, respectively. After a 28-day curing, the leaching concentrations in LC-treated LZSR reduced to 4.33 mg/kg, 189.68 mg/kg, and 0.46 mg/kg, while the leaching concentrations of PC-treated LZSR reduced to 29 mg/kg, 338 mg/kg, and 6 mg/kg for Pb, Zn, and Cd, respectively. The maximum immobilization efficiencies for Pb, Zn, and Cd reached 85%, 99%, and 99%, respectively. Moreover, the insoluble phases for Pb, Zn, and Cd obtained from the sequential extraction test results were 63.5%, 72.1%, and 42.4% for LC-treated LZSR and 35.7%, 38%, and 43% for PC-treated LZSR with binder content of 8% binder and curing time of 28 days. Increasing curing time and binder content reduced leaching concentrations, and the underneath mechanisms were interpreted by XRD, SEM-EDS, and FTIR analyses. Overall, the results indicate that Pb, Zn, and Cd can be successfully immobilized using 8% LC binder by transforming soluble heavy metals to insoluble hydroxides and their complexes.

Keywords: Chemical speciation; Heavy metals; Immobilization efficiency; Limestone calcined clay cement (LC); Portland cement (PC).