## 1 Introduction  
Red clay is a distinctive clay type primarily composed of minerals like kaolinite, illite, and chlorite, displaying characteristics such as swelling-shrinkage behavior, high fissuring propensity, and heightened sensitivity to changes in moisture content [1; 2]. Red clay is widely distributed in regions of China, including Yunnan, Guizhou, and Guangxi. However, due to its unique properties, it cannot be directly utilized as roadbed material [3; 4]. Consequently, researchers both domedically and internationally have undertaken extensive investigations into the enhancement of red clay properties. Among the various approaches, the common and effective method involves amending red clay with materials such as lime, cement, and fly ash [5; 6; 7]. Nevertheless, the production of lime and cement consumes substantial resources, and lime-treated and cement-treated soils often exhibit early-age strength deficiencies and poor moisture stability. Additionally, the escalating problem of environmental pollution has exacerbated the issue of solid waste disposal. In response to these challenges, some researchers have begun to explore the utilization of solid waste materials for the improvement of red clay, with the incorporation of phosphogypsum into red clay being a frequently employed and efficacious technique [8; 9]. Phosphogypsum is a solid waste generated during the wet-process production of phosphoric acid, containing hazardous components such as diphosphorus pentoxide, fluoride, and free acids. It is worth noting that the production of 1 ton of phosphoric acid yields approximately 4 to 5 tons of phosphogypsum [10; 11]. Due to the lack of appropriate disposal methods, a significant amount of phosphogypsum in China is currently disposed of through stacking, with only a small fraction being effectively recycled. This not only occupies valuable land resources but also poses potential environmental hazards [12]. In response to these challenges, both domestic and international researchers have conducted extensive studies on phosphogypsum treatment and have made notable progress in soil improvement. [PERSON] et al. [13], for instance, employed scanning electron microscopy (SEM) analysis to investigate the influence of cement and phosphogypsum on the microstructure of loess, revealing that those changes in mechanical properties primarily stem from alterations in the internal bonding system. [PERSON] et al. [14] conducted experiments by incorporating phosphogypsum into lime-treated soil, finding a direct proportionality between the strength and swelling rate of the improved soil and the content of lime and gypsum. [PERSON] et al. [15] utilized phosphogypsum to modify lime-treated and cement-treated soils, conducting a series of laboratory tests to confirm the feasibility of phosphogypsum-modified soil for roadbed construction and providing recommended mix ratios. [PERSON] et al. [16], through a high phosphogypsum content modification of red clay, observed that the improved soil met road use requirements in terms of 7-day unconfined compressive strength (UCS), albeit with increased swelling deformation. In summary, many scholars at home and abroad have conducted extensive research on the application of phosphogypsum in road engineering, but the research on phosphogypsum-improved diabase-stabilized red clay is still imperfect. The optimal phosphogypsum content ratio remains undefined, and there is a scarcity of studies combining macroscopic and microscopic analyses in this field. Therefore, in order to solve the problems of low early strength and poor water stability of lime-fly ash-stabilized red clay, the recycling of solid waste phosphogypsum was also considered. The present study conducted a series of indoor experiments by manipulating the dosage of phosphogypsum and the curing age, ultimately culminating in an analysis of the microstructure of the improved lime-ash soil using SEM (Scanning Electron Microscopy) and XRD (X-Ray Diffraction) techniques.