Refinement of waste phosphogypsum from Prahovo, Serbia: characterization and assessment of application in civil engineering

Two samples collected from the phosphogypsum deposits of the chemical products industry Elixir Prahovo (Serbia) were subjected to a recrystallization experiment performed over several repeated cycles. In these tests, phosphogypsum was separated into recrystallized (purified) gypsum, insoluble residue and supernatant. Both raw phosphogypsum and recrystallized gypsum were examined using inductively coupled plasma optical emission spectrometry, X-ray diffraction and scanning electron microscopy with energy-dispersive spectrometry. The activity concentrations of (238)u, (235)u, Ra-226, Pb-210,Th- 232 and 40 K were investigated using gamma-ray spectrometry. Based on the activity concentration results, a number of radiometric parameters were calculated for the original and recrystallized phosphogypsum (i.e. radium equivalent activity, y indices for construction materials, a index and external and internal hazard indices). Raw phosphogypsum samples showed greater mean activity concentration levels of U-238 and Ra-226 than the international recommended limits, while the recrystallized gypsum demonstrated notably lower activity concentrations for these two isotopes. The activity concentration of Ra-226 in recrystallized gypsum is similar to 6 times lower than in raw phosphogypsum. Therefore, recrystallized gypsum does not present a radiation hazard when used as a building material, while raw phosphogypsum meets the requirements only for road construction materials.