The binding chemistry and leaching mechanisms of zinc-bearing sludges in cementitious solidification/stabilization systems

Conference · Thu Jul 01 04:00:00 EDT 1999

OSTI ID:20014804

Lo, I M.C.; Tang, C I; Li, X D; Poon, C S

This paper aims at studying the binding chemistry and leaching mechanisms of cement-based solidification of industrial Zn-bearing sludge. Two cement-based solidification systems were considered: one with cement alone and the other with cement and fly ash. Toxicity characteristic leaching procedure (TCLP), dynamic leaching test (DLT), and sequential chemical extraction (SCE) were employed in this study. TCLP and DLT results show that the sample with cement alone has lower leaching of zinc and calcium compared to the sample with cement and fly ash. SCE results show that zinc has the highest leaching in the oxide phase. Microstructures and crystalline phases of the solidified samples were examined by scanned electron microscopy (SEM) and x-ray diffraction (ORD), respectively, SEM images of solidified samples suggest that the presence of zinc might retard the hydration of cement and fly ash because identifiable hydration products of cement were no longer observed. For the solidified samples with the presence of zinc sludge, only calcium silicate (CS) was detected by XRD while calcium silicate hydrate (CSH) and calcium hydroxide crystal (CH) were not found. Moreover, zincite (ZnO) was found in those solidified samples by XRD. It is believed that zinc may have replaced calcium from calcium hydroxide to form zincite. The high leaching in the oxide phase from SCE, and the high leaching of calcium from TCLP also supported this finding. For the sludge sample solidified by OPC alone, the results from SEM and XRD indicate that zinc might react with calcium hydroxide and is likely to be trapped in the solidified cement matrix. This leads to low leaching of zinc from both TCLP and DLT. Although zincite was also observed in the sample with both OPC and PFA, the high leaching of zinc from this sample implies that OPC alone is more effective than a mixture of OPC and PFA in the solidification/stabilization process.

• Research Organization: Hong Kong Univ. of Science and Technology, Kowloon (HK)
• OSTI ID: 20014804
• Country of Publication: United States
• Language: English

Related Subjects

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATION
36 MATERIALS SCIENCE
BINDING ENERGY
CEMENTS
FLY ASH
LEACHING
MICROSTRUCTURE
SLUDGES
SOLIDIFICATION
WASTE FORMS
WASTE PRODUCT UTILIZATION
ZINC COMPOUNDS