## 5 Conclusions  
In this paper, through a series of indoor experiments, the effects of phosphogypsum on the strength, water stability, expansion, and contraction of diash-stabilized red clay were discussed, and the microscopic mechanism of phosphogypsum-improved diash-stabilized red clay was analyzed. The conclusions of this study are as follows:  
(1) Phosphogypsum can significantly enhance the early strength of lime-stabilized red clay. With an increasing phosphogypsum content, the UCS of lime-stabilized clay first increases and then decreases. When the phosphogypsum content is 4%, the strength reaches its maximum. The 7-day UCS increases by 67.2%, and the 28-day UCS is enhanced by a factor of 3. Notably, the lime-stabilized clay shows a substantial increase in UCS during the 7-day to 28-day curing period, while the rate of increase slows down after 60 days of curing. Similarly, the splitting strength follows a similar trend of initially increasing and then decreasing with phosphogypsum content. The maximum splitting strength is achieved at 4% phosphogypsum content, resulting in a 4.3-fold increase at 28 days. With an increase in curing age, the splitting strength of lime-stabilized clay gradually increases without a significant decrease in the rate of improvement. Furthermore, the CBR value of lime-stabilized clay at 4% phosphogypsum content increases by 46%, meeting the road performance requirements. (2) Phosphogypsum significantly enhances the moisture stability of lime-stabilized red clay. The 7-day moisture stability coefficient of samples with 4% phosphogypsum increases by 43%, reaching 0.91 at 28 days. Across various curing durations, lime-stabilized clay samples with phosphogypsum show good overall integrity after 24 h of soaking. However, when the phosphogypsum content exceeds 4%, the moisture stability of lime-stabilized clay gradually diminishes. (3) High phosphogypsum content leads to increased expansion and shrinkage deformations in lime-stabilized clay. The shrinkage and expansion deformations of lime-stabilized clay are relatively small when the phosphogypsum content is below 4%. However, when the phosphogypsum content exceeds 4%, the deformation amplitude increases significantly. In addition, the higher the initial moisture content of lime-stabilized clay, the greater the shrinkage and expansion deformations. (4) Microscopic analysis reveals that the calcium aluminate produced by adding phosphogypsum to lime-stabilized clay can effectively fill the internal pores and cracks of lime-stabilized clay, bonding the laminar and granular soil particles together and forming an overall gel-like network structure adhered to the surface of the soil particles. This significantly enhances the overall integrity and strength of lime-stabilized clay. However, excessive phosphogypsum can lead to the overaccumulation of calcium aluminate within the soil, causing increased expansion deformation, which disrupts the overall integrity of lime-stabilized clay and leads to reduced strength and moisture stability. Experimental results indicate that the addition of phosphogypsum effectively enhances the early strength and moisture stability of lime-stabilized red clay. lime-stabilized clay modified with 4% phosphogypsum (8% lime and 16% fly ash) can be used as a road subbase material. This not only offers an environmentally friendly solution for addressing phosphogypsum waste but also contributes to improved engineering performance. However, this study focused on phosphogypsum content greater than 4%, and future research could consider reducing the content for further investigation. **Author Contributions:** Conceptualization, S.H. ; methodology, S.H., Y.M. and T.C. ; software, Y.M. ; validation, S.H. ; formal analysis, Y.M. ; writing--original draft preparation, S.H. ; writing--review and editing, S.H., J.W. and Z.L. ; supervision, Z.L. ; project administration, J.W. All authors have read and agreed to the published version of the manuscript.