| Summary: | In Singapore, there is an urge to deal with the issue of heavy metal contamination in soils as it is detrimental to human beings and the environment, especially when it involves marine clays from surrounding waters. Stabilization/solidification using different binders has been developed as a low-cost and effective soil remedial technique. In spite of numerous studies on this topic, few of them discussed the specific treatment of zinc-contaminated clayey soils with lime (CaO)-activated and reactive magnesia (MgO)-activated ground granulated blast furnace slag (GGBS) and examined the immobilization mechanism in depth. Thus, this study aims to compare the effectiveness of these two binders in stabilizing zinc-contaminated clayey soils, in terms of unconfined compressive strength (UCS), leachability and microstructure.
The UCS results showed that the application of MgO-activated GGBS as a binder helped in strength development of the contaminated soils while the CaO-GGBS binder deteriorated the strength. The MgO-GGBS binder also achieved better UCS results as compared to the contaminated soils treated with ordinary Portland cement (OPC). Similar results were obtained in terms of leachability. The leachate zinc concentration of the contaminated soils treated with MgO-activated GGBS was below the limit for inert waste and the limit for non-hazardous waste, but the increasing CaO dosage facilitated the leachability of zinc. At a higher zinc concentration, the MgO-GGBS binder appeared to be more effective than the OPC in reducing the leachability of zinc. X-ray diffraction (XRD) and Thermogravimetric analysis (TGA) were conducted for microstructural analysis in order to clarify the immobilization mechanism and the results showed that a retarder, calcium zinc hydroxide was formed when the CaO-GGBS binder was added to the contaminated soils. Calcium zinc hydroxide hindered the hydration process and became soluble under a high pH environment, leading to the poor UCS and leachability results. In comparison, the soils treated by the MgO-GGBS binder were found to have a higher content of hydration products.
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