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 Diraction (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.