## 3 Results and Discussion
### Electrochemical Impedance Spectroscopy Analysis  
For cement-based materials, the internal structure is more complex than the general electrochemical system, which is composed of solid, liquid and gas phases [53; 54; 55; 56]. Generally, the conductive path in cement-treated soil can be divided into three types, as shown in Figure 7.  
The first kind of conductive path exists only in solids. Because of the insulation of the material itself, the impedance in the circuit is mainly provided by the solution resistance of the surrounding micro-pore; the second conductive path exists between solids and pores. For cement-based materials, many free charges are gathered in the C-S-H gel, which leads to the formation of capacitance under the action of alternating current. The impedance in the circuit is supplied by the transfer resistance inside the material and the capacitative reactance generated by the pore capacitance; the third conduction path is formed by the interface between the solid medium, the pores, and the pore water. Along this path, ions in the pore fluid will enter the diffusion zone at the solid liquid interface through ion exchange, and diffusion impedance is generated in the process of ion diffusion; The impedance in the circuit is provided by the pore solution resistance, the transfer resistance inside the material, and the diffusion impedance. In addition, under the three paths all ion exchange occurs at the electrode material interface, so it is necessary to consider the impedance at the electrode; the equivalent circuit is shown in Figure 8.  
Figure 6: XRD patterns under a 75% high-sulphur fly ash content. E: Ettringite; H: Calcium sulphate hemihydrate; G: Gypsum; Q: Quartz; CC: Calcite. Figure 7: Conductive path. increase in the sodium sulphate concentration, the semicircle diameter in the Nyquist diagram of each group increased. The diameter of the high-frequency arc essentially reflects the process of charge transfer. As the concentration of sodium sulphate increases, ettringtite crystals, which are difficult to dissolve in water, are generated. Corrosion products gradually fill the pores, while the decrease in pore diameter and porosity is not conducive to the migration of ions at the solid liquid interface. Thus, the charge transfer in the electrochemical system is blocked and the radius of the high-frequency arc is increased. With the increase in the proportion of high-sulphur fly ash in the mixture, the arc radius in the high-frequency band also increases. This is attributed to the adsorption of ions by fly ash, which increases the diffusion resistance of ions in porous solution and at the solid liquid interface. In addition, due to the large specific surface area of fly ash, it can fill small holes in the material, which renders the structure more compact and reduces its porosity. Therefore, the resistance increases with the increase in the high-sulphur fly ash content. The electrochemical impedance spectroscopy of each group was further studied. Figure 10 demonstrates the variation of components \(R_{\rm s}\) and \(R_{\rm cI1}\) with the concentration of sodium sulphate in each group. Without sodium sulphate corrosion, the value of the \(R_{\rm S}\) of high proportional high-sulphur fly ash content is larger than that of specimens containing lower amounts thereof. With the increase in the sodium sulphate concentration, the value of \(R_{\rm S}\) of specimens with different high-sulphur fly ash contents decreases to varying degrees, especially at high proportional concentrations of high-sulphur fly ash. The material ion transfer resistance \(R_{\rm cI1}\) increases with the increasing sodium sulphate concentration and the higher the high-sulphur fly ash content, the faster the increase. Figure 9: Nyquist at different high-sulphur fly ash contents. (**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. This phenomenon can be explained as the filling and adsorption effect of fly ash on the internal structure.