Geotechnical Experimental Study of Phosphogypsum for Use in Quarry Reclamation

This study presents the geotechnical evaluation of phosphogypsum, a byproduct of phosphate fertilizer production. The objective is to assess the suitability of phosphogypsum or its mixtures with natural materials as a technically viable and environmentally responsible backfill material for the restoration of closed and abandoned quarries. This study adds to the scarce existing literature on the use of phosphogypsum for quarry reclamation and further investigates the behavior of phosphogypsum mixtures incorporating clay and marble dust. A comprehensive experimental program was conducted to evaluate typical geotechnical properties, i.e., grain size distribution, Atterberg limits, compaction characteristics, permeability, compressibility, and shear strength. The results indicate that phosphogypsum is fine grained, low in plasticity, and exhibits relatively high permeability and compressibility, which limits its application as a deep fill material. The addition of clay increased the liquid and plastic limits but had a limited positive effect on strength and compressibility. In contrast, mixtures with marble dust improved particle gradation, reduced permeability, and enhanced compaction behavior without significantly increasing plasticity or settlements. Notably, the most promising mixture of phosphogypsum with a modest proportion of marble dust demonstrates improved shear strength and reduced hydraulic conductivity, making it suitable for use in the upper layers of quarry fills.