Cement-Based Solidi cation/Stabilization of Heavy Metal Contaminated Soils with the Objective of Achieving High Compressive Strength for the Final Matrix

Abstract. The Solidi cation/Stabilization (S/S) of heavy metal contaminated soils using Ordinary Pontlandite Cement (OPC) is studied as a hazardous waste treatment technology. The soil-binder ratio of 8% by weight is selected for treatment by setting a justi cation constraint on the solidi ed matrix. In order to consider the physical and chemical characteristics of the solidi ed soil-cement mixtures, Compressive Strength, Toxicity Characteristic Leaching Procedure (TCLP), X-Ray Di raction (XRD) and Scanning Electron Microscopy (SEM) are the tests done during this research. The results indicate that despite an intensive decrease in the compressive strength of solidi ed soil at early curing ages, the justi cation constraint set on the preparation of specimens enhances hydration reactions; and the solidi ed matrix achieves high compressive strength after 7 curing days. The leaching metal concentration in leachates is decreased gradually by aging from 1 to 28 curing days, indicating more metal stabilization in the form of metal hydroxide precipitation, which presents itself in the developed crystalline phases of hydrated cement, as observed in XRD patterns or by absorption on CSH gel or crystals. Leaching metal concentration in leachates is lower than regulatory limits, so there is no strength concern for environmentally safe disposal. Heavy metal contamination diverts the cement hydration reactions toward production of more subsidiary products, that bear less compressive strength, instead of main hydration products. Therefore, the main crystalline phases develop irregularly and coat the aggregates poorly, as observed in SEM micrographs. Keywords: Solidi cation/stabilization; Heavy metal contaminated soils; Portland cement; Leaching; XRD; SEM.