Application of phosphogypsum to solidification of silty soil: Mechanical properties and microstructure

As a type of common industrial waste, phosphogypsum (PG) has raised concerns about environmental pollution and its comprehensive usage. However, scholars need to deepen the research on the application of PG in the solidification of silty soils. The impacts of PG on the microstructure and mechanical properties of solidified soil remain unknown under complex environmental conditions. Firstly, combined with several consolidation tests, this study analyses the impact of different PG contents on the compression modulus of silty soil with a cement-lime curing agent. Secondly, a scanning electron microscopy (SEM) test was used to take microscopic photos, and Image-Pro Plus 6.0 (IPP 6.0) software was used to process microscopic photos. Then, three microstructure parameters (Dp, Db and e) of solidified silty soil were calculated. Finally, the connection between the compression modulus and the microstructure parameters was assessed quantitatively. Results revealed that PG can stimulate the formation of calcium silicate hydrate (C–S–H) gels and react with calcium aluminate to produce ettringite. As a result, the compression modulus of solidified silty soil increased from approximately 7.39% to 51.3%. Given the excessive volume expansion produced by ettringite, the compression modulus with 4% of PG declined from 4.76% to 6.8% after reaching its maximum 24.98 MPa at 14 days of curing. A linear relationship was observed between the compression modulus and the three microstructure parameters at the declining stage of the compression modulus. The correlation coefficient R2 of fitting lines were all greater than 0.9, thus representing the connection between the macroscopic mechanical properties and the microstructure of the solidified silty soil. This study seeks to provide theoretical basis and practical experiences for the efficient utilization of PG in the field of soil solidification.