Application of Phosphogypsum as Filling Materials in Roadbed Engineering

Using modified phosphogypsum (PG) as a filler material may be the most effective approach for large-scale utilization. To verify the feasibility of this method in subgrade engineering, a series of theoretical and experimental investigations were carried out. First, a comparative analysis of the physical properties of raw and modified PG was conducted via direct shear and compaction tests. Subsequently, the modification mechanism was analyzed qualitatively, and the optimal proportion of the modifier was determined. Finally, the impact of compactness on the strength of modified PG was evaluated quantitatively. The research shows that the mechanical properties of raw PG—characterized by an optimal moisture content of 15.3% and a maximum dry density of 1.65 g/cm^3—are significantly influenced by moisture content. Mixing phosphogypsum with Portland cement is an effective modification method. The 7-day compressive strength of compacted modified PG with a 5% cement content reaches 2.5 MPa. Compactness serves as a key control index for determining the engineering performance of PG as a filling material. Once compactness drops below 80%, modified PG fails to develop effective strength. Based on reasonable construction procedures, the application of modified PG as a subgrade filling material is technically feasible.