## 3 Results and Discussion  
### Mechanical Properties of LCMs under Sodium Sulphate Corrosion  
After 28 days of curing in Na\({}_{2}\)SO\({}_{4}\) solution of different concentrations, the appearance of LCMs with high-sulphur fly ash contents of 50%, 66.7%, 75%, 80% (1:1, 1:2, 1:3, 1:4) is shown in Figure 3. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Number** & **Cement** & **High-Sulphur** & **Water** & **Early Strength** & **Foam** & **Dry Density of** \\  & **(kg/m3)** & **Fly Ash (kg/m3)** & **(kg/m3)** & **Agent (kg/m3)** & **(I/m3)** & **Mixture (kg/m3)** \\ \hline A1-1 & 465 & 465 & 650 & 18.6 & 1200 & 816 \\ A1-2 & 310 & 620 & 650 & 18.6 & 1200 & 874 \\ A1-3 & 235 & 695 & 650 & 18.6 & 1200 & 860 \\ A1-4 & 190 & 740 & 650 & 18.6 & 1200 & 758 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Mix proportions. Figure 2: Electrochemical impedance spectroscopy measuring device. Each group of pictures shows the appearance of specimens when cured in water and 5%, 10%, and 15% sodium sulphate solution. The appearance of specimens with 50% high-sulphur fly ash under different environmental curing conditions changes little, and there is no obvious pitting seen on the surface, suggesting a good integrity. At 66.7% high-sulphur fly ash content, the specimens did not crack, but with the increase in the sodium sulphate concentration, the surface of the specimens gradually became rougher, and the edge of the specimens fragmented. At 75% high-sulphur fly ash, the specimens show different degrees of damage. With the increase in the concentration of sodium sulphate, the surface of the specimens exhibited small cracks, accompanied by the precipitation of white paste substances and even the corrosion and exfoliation of the epidermis. When the content of high-sulphur fly ash increases to 80%, the specimens remain intact only under water curing. The unconfined compressive strength of specimens cured in different environments is shown in Figure 4; the unconfined compressive strength of LCMs has certain regularity with the change in fly ash content. The unconfined compressive strength of the specimens decreased the with increasing fly ash content. Among them, in low-concentration sodium sulphate environments (0% and 5%), the change in the strength of the samples is small when the fly ash content is less than 75%, and the strength decreases significantly when the content is higher than 75%; for the high-concentration sodium sulphate environments (10% and 15%), the strength of the specimens always decreased rapidly. Figure 4: Effect of a high-sulphur fly ash content on the compressive strength under different curing environments. Figure 3: Corrosion morphology. (**a**) 50% high-sulphur fly ash, (**b**) 66.7% high-sulphur fly ash, (**c**) 75% high-sulphur fly ash, (**d**) 80% high-sulphur fly ash. The change in specimen strength with the fly ash content results from the change in the content of the cementing material, which plays a connecting role between the components; when the material is hydrated, the formation of CH causes the environment to be alkaline. These substances surround the active substances present in high-sulphur fly ash, and exchange ions through the water membrane, which causes the pozzolanic effect. Pozzolanic products such as C-S-H and CAH are produced; these bind the particles together through cementation to strengthen the structure. In addition, the fly ash particles are very small. In the manufacturing process of specimens, the fly ash particles readily play the role of filler within the micro-voids in the material, and also play a certain role in strengthening it; therefore, the strength decreases slowly at low contents of high-sulphur fly ash. With the increase in the high-sulphur fly ash content, the activity of the active substances therein is not fully stimulated, and cannot continue to cause the development of the strength of the specimens in this regard; the strength of such specimens is mainly provided by the cementing action of the hydration product C-S-H.