Geotechnical properties of hemihydrate and dihydrate phosphogypsum

Abstract

The article presents the results of analyses of more than 500 samples of phosphogypsum, which is the waste in the process of phosphorous fertilizers production, consisting of calcium sulfate with admixtures of sulfuric and phosphoric acids, silica and other substances. The characteristics of hemihydrate phosphogypsum (CaSO40,5H2O), in which cementation bonds form as the hydration process proceeds, were studied on undisturbed samples taken in 1 and in 6 months, 1, 5 and 10 years after the material was dumped. It is characterized by low strength and insignificant frost-resistance, which is typical for all building materials and products based on gypsum. The cementation bonds have not been formed in dihydrate phosphogypsum (CaSO4-2H2O) and it remains a non-cohesive granular substance in the dump, so its properties were determined by the methods used in a geotechnics for sands and silty soils. The artificially prepared samples with the relative compaction of 0.80, 0.90 and 0.95 were tested. While the obtained values of the internal friction angle, cohesion and hydraulic conductivity were characteristic for fine sands, the compressibility of this material due to the solubility of the particles was substantially higher. The utilization of phosphogypsum as ground material may have limited use, provided that water protection measures are taken and the acids contained in it are neutralized.