## 3 Result and Discussion
### Discussion  
The coupled effect of F-T weathering and acid erosion plays a complex role in developing the microstructure of sandstone. On the one hand, frost heaving caused by F-T weathering exerts frost heave force on pores and cracks, which drives pores and cracks to initiate and develop. On the other hand, acid erosion plays important role in developing the microstructure of sandstone. As mentioned before, sandstone adopted in this paper is composed of Quartz (95.1%), Illite (1.48%), Montmorillonite (2.32%) and Ferric component (1.1%). Ferric components are mainly made up of Fe\({}_{2}\)O\({}_{3}\). Among these components, Illite, Montmorillonite and Fe\({}_{2}\)O\({}_{3}\) are soluble in sulfuric acid solution--the chemical reaction is shown as follows [27]:  
\[\mathrm{K_{x}(H_{2}O)_{4}(Al_{2-x}Mg_{x})[Si_{4}O_{10}](OH)_{2}+H_{2}O+H^{+} \to K^{+}+Mg^{2+}+Al_{2}O_{3}\cdot 2 SiO_{2}\cdot 2H_{2}O+H_{4}SiO_{4}} \tag{6}\]  
\[\mathrm{K_{1-x}(H_{2}O)_{x}\Big{\{}Al_{2}[AlSi_{3}O_{10}](OH)_{2-x}(H_{2}O)_{ x}\Big{\}}+H_{2}O+H^{+}\to K^{+}+Al^{3+}+Al_{2}O_{3}\cdot 2 SiO_{2}\cdot 2H_{2}O+H_{4}SiO_{4}} \tag{7}\]  
\[\mathrm{Al_{2}O_{3}\cdot 2 SiO_{2}\cdot 2H_{2}O+6H^{+}=2 Al^{3+}+2H_{2}SiO_{3}+3H _{2}O} \tag{8}\]  
\[\mathrm{Fe_{2}O_{3}+6H^{+}=6 Fe^{3+}+3H_{2}O} \tag{9}\]  
As soluble components dissolved in the acid solution, pores and cracks expanded, which offer more space to contain water to exert the frost heave force on the microstructure. Therefore, freeze-thaw weathering and acid erosion jointly promote the development of the microstructures of sandstone.  
The damage of freeze-thaw weathering accumulates as time goes on, while the damage of acid erosion declines when acid solution is consumed. As shown in Figure 3b, acid solutions are mainly consumed in the first 10 F-T cycles as the concentration of acidic solution is greatest during this period, which indicates that acid erosion mainly worked during the first 10 F-T cycles. This explains the remarkable increase in porosity and permeability during the first 10 F-T cycles, which are shown in Figures 3 and 5 and Table 1.  
In terms of the effect of pH on porosity increase, the phenomenon of porosity increase with pH decrease occurs during the first 10 F-T cycles. Solutions with low pH are more concentrated. Soluble components are dissolved more easily and, therefore, porosity increase grows with pH decrease during 10 F-T cycles, as shown in Figure 3c. However, acid erosion weakens with acid solutions consumed, while F-T weathering strengthen with time goes on. Therefore, porosity increase caused by F-T weathering exceeded and covered that caused by acid erosion. As a consequence, the meaningful effect of PH on the microstructure is hard to find.  
Figure 10 displays the view of sandstone after different F-T cycles. As can be seen from Figure 10b, the acid erosion during the first 10 F-T cycles made the surface of sandstone coarser. When the F-T cycles come to 20, as shown in Figure 10c, the end of the sample peeled off, which may explain the drop in macro-porosity in Figure 5c. Then, the damage of sandstone deteriorated with F-T cycles adds up, as shown in Figure 10d,e. Figure 10 intuitively shows that F-T weathering contributes more to the damage and microstructure development of sandstone than acid erosion in this experiment.  
However, the influence of pH on the damage and microstructure development is unclear in this paper. Based on the analysis above, it is easy to find that the influences of F-T cycles on porosity, permeability and fractal dimensions are clear, while the influence of pH on them is not that clear. This is the weak point of this paper. On the one hand, it is believed that the effect of F-T weathering covered the effect of acid erosion with time add-up. On the other hand, acid solutions are mainly consumed within the first 10 F-T cycles, while acid solutions are not supplemented. This aspect deserves further experimental investigation in the future.