## 3. Results
### Microstructure Analysis  
To observe the structure of phosphogypsum-stabilized red clay particles and analyze the reasons for the increased strength of the mixture, a Hitachi S-4800 Scanning Electron Microscope at 20,000\(\times\) magnification was used to scan plain red clay and a phosphogypsum mixture with 8% lime, and analyzed using a Rigaku SmartLab SEX-ray Diffractometer. SEM results are shown in Figure 14. XRD quantitative analyses are shown in Table 8 and Figure 15.  
Figure 15: Quantitative analysis of the compound composition of the mixture. (**a**) 0%L + 0%P + 100%T; (**b**) 8%L + 8%P + 84%T; (**c**) 8%L + 16%P + 76%T; (**d**) 8%L + 24%P + 68%T; (**e**) 8%L + 31%P + 61%T; (**f**) 8%L + 46%P + 46%T. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{1}{c}{**Mineral Phase Content**} & \multicolumn{5}{c}{**Phosphogypsum Content (\%)**} \\ \multicolumn{1}{c}{**(WT\%)**} & \multicolumn{1}{c}{**Chemical Formula**} & **0** & **8** & **16** & **24** & **31** & **46** \\ \hline Quartz & SiO2 & 42.8 & 29.0 & 21.3 & 15.6 & 12.6 & 10.5 \\ Calcite & CaCO3 & 0 & 9.8 & 7.6 & 6.9 & 3.9 & 5.7 \\ Ettingrite & Ca6 Al2(SO4)3(OH)32.26H2O & 0 & 16.8 & 13.2 & 12.4 & 9.5 & 6.6 \\ Gypsum & Ca5O4.2H2O & 0 & 9.5 & 28.5 & 45.6 & 61.1 & 69.6 \\ Kaolinite & Al2(OH)5 Si2O5 & 18.7 & 0.3 & 0.2 & 0.2 & 0.1 & 0.1 \\ Goethite & FeOOH & 8.8 & 15.6 & 13.4 & 8.0 & 4.5 & 2.2 \\ Musocvite & (K,Na)Al2(Si,Al)O100(OH)2 & 29.7 & 17.9 & 15.4 & 11.4 & 8.3 & 5.4 \\ Montmorillonite & (Na,Ca)0\({}_{\text{-}}\)3(Al,Mg)3 Si2O10(OH)2 & 0 & 1.0 & 0.5 & 0 & 0 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Quantitative analysis table for compositions of plain red clay and the mixture (L = 8%). Under certain compaction conditions, a series of physical, physicochemical, and chemical changes occurred in the mixture. The main chemical reaction was alkaline lime reacting with activated silica and activated alumina in the red clay to produce hydrated calcium silicate (3 Ca-SiO2-nH2O) and hydrated calcium aluminate (3 CaO-Al2O3-nH2O). The chemical reactions are as follows:  
\[3\;\mathrm{Ca(OH)_{2}+SiO_{2}+(n-3)\;H_{2}O\to 3\;CaSiO_{2}\cdot nH_{2}O}, \tag{3}\]  
\[3\;\mathrm{Ca(OH)_{2}+Al_{2}O_{3}+(n-3)\;H2O\to 3\;CaAl_{2}O_{3}\cdot nH2O}, \tag{4}\]  
This is the reason for the early strength of lime-phosphogypsum-stabilized red clay. The generated hydrated calcium silicate and hydrated calcium aluminate continued to react with phosphogypsum in the mixture to form (3 CaO-Al2O3-3 CaSO4-32H2O), which is what we often call ettrinigite. The chemical reaction is as follows:  
\[3\;\mathrm{CaO\cdot Al_{2}O_{3}\cdot nH_{2}O}+3\;\mathrm{CaSO_{4}\cdot 2H_{2}O}+(26 -n)\;\mathrm{H_{2}O}\to 3\;\mathrm{CaO\cdot Al_{2}O_{3}\cdot 3 CaSO_{4}\cdot 32H_{2}O}, \tag{5}\]  
Figures 14 and 15 and Table 8 show that the shapes of plain soil granules were obvious. With the addition of lime, the reaction produced cemented calcium silicate hydrate and calcium aluminate hydrate, and the soil particles gathered together and provided some strength. Continued addition of phosphogypsum generated ettrinigite, which filled the pores. In addition, ettrinigite can be cemented with gelling substances to form a skeleton; the structure is more compact, the pore size is reduced, and the porosity is reduced, which reduces the shrinkage rate of the stabilized soil mixed material and improves its crack resistance. However, excessive ettrinigite causes breaking of the originally formed skeleton due to excessive expansion, and this results in decreased strength. When L:P = 1:1, that is, P = 8%, the moisture content of the mixture was 30.24%. Compared with other groups, the moisture content was the largest, and the internal voids were the largest. Based on these results, ettrinigite is most abundant at this time. After filling a large number of pores, there was still a small amount of excess ettrinigite that broke the originally formed framework structure, which resulted in a decrease in strength and a small absolute expansion rate.