## 1 Introduction  
Phosphogypsum is a solid industrial waste produced during the production of phosphate fertilizer, and under normal circumstances, every 1 ton of phosphormadite produced will generate about 5 tons of phosphogypsum. Guizhou, as one of the provinces with the largest production of phosphate fertilizer in China, generates about 5 million tons of phosphogypsum per year. At present, phosphogypsum treatment methods have mostly used land stacking and filling of rivers, lakes, and seas. These methods both encroach on the land and destroy vegetation, and leakage of acidic wastewater and some radioactive elements has caused pollution and endangered the health of human beings [1; 2; 3; 4]. Red clay is a special clay that exhibits many fissures, high shrinkage, and high water sensitivity, and these features affect the application of red clay in highway engineering [5; 6; 7; 8; 9]. Most of the current studies on phosphogypsum treatment problems have applied phosphogypsum in construction and road foundation projects [10; 11; 12; 13; 14; 15]. By mixing phosphogypsum with other substances to modify the treatment of different soil types, the soil properties can be enhanced and used for road base filling [16; 17; 18]. Applying phosphogypsum and red clay to subgrade pavement solves the problem of phosphogypsum pollution and improves the poor engineering characteristics of red clay, which has important scientific significance and applied engineering value. Many scholars have conducted some research on the application of phosphorus-containing solid waste on roads. [PERSON] et al. [19] proved that the lime-fly ash mixture with phosphogypsum cured for 28 days can be used as the base course and sub-base course of roads through unconfined compressive strength tests and indirect tensile strength tests. [PERSON] et al. [20] prepared lime-fly ash-phosphogypsum mixtures with different ages after three different curing treatments and applied unconfined compressive strength tests and unconsolidated undrained triaxial tests. According to the results of unconfined compressive strength, deviatoric stress, cohesion, friction angle, initial tangential modulus, and secant modulus obtained from the test, the lime-fly ash-phosphogypsum mixture can be applied to the subgrade. [PERSON] al. [21] mixed different proportions of gypsum in clay and conducted laboratory tests. They believed that the maximum dry bulk density decreased, the optimal water content increased, and the strength and CBR values increased significantly with increasing gypsum content. [PERSON] et al. [22] conducted indoor and field tests on the feasibility and mechanical properties of phosphogypsum used in roadbed and pavement engineering. After many indoor tests, they found that phosphogypsum exhibited the minimal expansion, good water stability, and high strength necessary for use in roads. [PERSON] et al. [23; 24; 25] discussed the drying and shrinkage characteristics of phosphogypsum, fly ash, lime, and clay mixtures used as base materials. From the perspective of drying and shrinkage characteristics, this mixture was combined with gypsum and deemed feasible for use as a pavement base material; the best mixing ratios for phosphogypsum-fly ash-lime-clay mixtures (phosphogypsum: fly ash: lime: clay) were 15:20:6:59 and 15:25:8:52. The above research showed that phosphogypsum has good prospects for applications in highway engineering. At present, there are few studies on the road performance and micromechanisms of phosphogypsum-stabilized red clay. In this paper, lime, phosphogypsum, or phosphogypsum and red clay is used to prepare mixtures in different proportions, the strength characteristics, water stability, swelling and shrinkage of phosphogypsum-stabilized red clay are probed through XRD quantitative analysis and scanning electron microscope (SEM), which revealed the mechanism for changes in the microstructure and provided technical support for applications of lime-phosphogypsum-stabilized red clay in highway engineering.