Characterization of Moroccan Phosphogypsum Toward a Road Engineering Application

Around 30 Mt of phosphogypsum are generated annually in Morocco. It is a by-product from the phosphoric acid industry where the majority is stockpiled by Jorf Lasfar and Safi plants. The purpose of this study is to characterize Moroccan phosphogypsum on the physico-chemical, mineralogical, morphological, geotechnical, and environmental level in order to assess its suitability as a road material. To this end, several techniques have been implemented, namely: X-ray fluorescence and X-ray diffraction (XRF/XRD), laser granulometry, inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy (SEM), proctor and oedometer tests as well as radioactivity and leaching tests. The results showed that Moroccan phosphogypsum has a pH of about 3.45, a fine grain size (< 200 μm) marked by the presence of parallelepipedic crystals. Chemical and mineralogical analyzes showed that phosphogypsum is mainly composed of dihydrate gypsum (86%), but contains impurities such as quartz, heavy metals, and radioelements from phosphate rock, in addition to some acid residues due to the transformation process. These impurities constitute the main source of pollution linked to the storage of phosphogypsum insofar as heavy metals can migrate from the heap to the groundwater and the radionuclides can decompose and pose risks to human health. From a geotechnical point of view, phosphogypsum is a low-density material with compressive behavior and low resistance to immersion due to the solubility of gypsum which further increases owing to its acidic nature. Although the radiological indices relating to phosphogypsum allow its use in road engineering, this is not possible given its sensitivity to water variations which could impact its bearing capacity as well as the risk of leaching of contaminating elements. Thus, for phosphogypsum to be valorized in road engineering, it requires a treatment in order to stabilize its behavior, in particular the improvement of its resistance to compression and immersion. Lime–fly ash treatment is a promising solution for a use as a road base material.