Characterization of cementitious granular materials produced from contaminated soils

In the present paper, an improved solidification/stabilization (S/S) process for the production of cement-based granular materials from contaminated soils is described. The presented method is based on the use of Portland cement as a binder and superplasticizers (SPs). The effectiveness of this process on the immobilization of inorganic hazardous wastes has been tested on a contaminated soil from an industrial site, showing high levels of several heavy metals and metalloids such as copper, zinc, arsenic and lead. At first, a characterization of the contamination was carried out. The nature and distribution of contaminants have been investigated in detail by means of SEM-EDS (scanning electron microscopy with energy dispersive spectroscopy) and micro-PIXE (particle-induced X-ray emission) chemical mapping, both in the original soil and in the cement grains after the S/S treatment. The coupling of the high imaging capabilities of SEM with the excellent detection limits of micro-PIXE, allowed the identification of several metal-bearing phases in the investigated samples. The analyses showed that the main source of pollution is related to the presence of μm- to mm-sized particles of inorganic compounds, employed as pigments or additives by the glass production plant formerly operating at the studied area. In a second stage, the physico-chemical properties of the granular S/S materials were evaluated by means of mechanical and leaching tests. In particular, the attention was focused on the role played by superplasticizers in the S/S process. For this purpose, the performances of samples produced following the presented method have been compared with those of similar grains prepared without the addition of superplasticizers. Due to the lower demand of mixing water, the samples produced using superplasticizers showed a general improvement of performances in terms of decreased porosity, reduced leaching of contaminants and improved mechanical properties. The laboratory tests that were carried out showed that the granular materials produced with this improved S/S technique may be suitable for an in-situ re-use as filler or concrete aggregate and may be employed in several other large scale applications.