## 3 Result and Discussion
### Pore Size Distribution  
According to the theory of NMR, the parameter T\({}_{2}\) is linearly related with the pores radii. Their relationship can be expressed as Equation (5) [15; 34; 35; 36].  
\[r=CT_{2} \tag{5}\]  
where \(r\) is pores radii--its unit is \(\upmu\)--C is the conversion coefficient between pores radii and T2. C, for most sandstones in China, is among 0.01-0.15 \(\upmu\)m/ms [35]; C in this paper is 0.01 [36].  
In terms of pore size classification, a uniform standard has not been formed, yet [PERSON] [35] classified pores into micro pores (\(<\)0.1 \(\upmu\)m), mini pores (0.1-1 \(\upmu\)m) meso pores (1-10 \(\upmu\)m) and macro pores (10-100 \(\upmu\)m). [PERSON] [37] divided pore sizes into small pores (\(<\)50 \(\upmu\)m) and large pores (\(\geq\)50 \(\upmu\)m). [PERSON] [36; 38] divides sandstone into macro pores (\(\geq\)1 \(\upmu\)m), meso pores (0.1-1 \(\upmu\)m) and mini pores (\(<\)0.1 \(\upmu\)m). In this paper, the pore sizes of sandstone are divided into macro pores (\(\geq\)1 \(\upmu\)m), meso pores (0.1-1 \(\upmu\)m) and micro pores (\(<\)0.1 \(\upmu\)m). The pore size distribution of sandstone is presented as Figure 4, below.  
Figure 4: Diagram presents pore size distribution on the T\({}_{2}\) spectrum curve.  
Figure 3: Porosity and pH after different F–T cycles: (**a**) porosity of sandstone after various F–T cycles under different solution, (**b**) pH varies with F–T cycles, (**c**) porosity increase of 10 F–T cycles treated sandstone immersed in solution with different pH.  
Figure 5 displays the variation trend of three type pores of sandstone under a coupled effect of freeze-thaw weathering and acid erosion. In Figure 5, micro-porosity, meso-porosity and macro-porosity represent the porosity of micro pores, meso pores and macro pores, respectively. Figure 5a shows that micro porosity fluctuates with F-T cycle increase, which is because the coupled effect of F-T weathering and acid erosion enlarged the pore sizes of micro pores, and new micro pores were generated in this period. In Figure 5b, meso porosity displays a steady increase trend; the meso porosity rises from 4.38% to 7.26% (pH 2.8, as an example). Meso porosity is affected by two factors. One is that micro pores expend to meso pores, which increases the meso porosity. The other is that some meso pores developed to macro pores, which decreased the meso porosity. Figure 5c reveals that the macro porosity gains most remarkable growth; the meso porosity rises from 0.62% to 5.59% (pH 2.8, as an example). In Figure 5c, the macro porosity experiences a drop after 20 F-T cycles; this phenomenon can be explained by the first peel off of sandstone on the surface of the sample. Because the F-T weathering and acid erosion effect is more serious on the surface, macro pores are more distributed on the surface; when the surface peeled off, the macro porosity faces a drop. Overall, meso porosity, micro porosity and macro porosity account for the largest, second largest and lowest ratio of total porosity after 40 F-T cycles. During the experiment, macro porosity, meso porosity and micro porosity account for the largest, second largest and lowest ratio of porosity growth. It indicates that F-T weathering mainly promoted the development of macro pores and meso pores.  
Figure 5d presents the porosity of samples under 40 F-T cycles in different acidic solution. It shows the ratios of porosity ranks, such as meso porosity, micro porosity and macro porosity in all acid solutions. The ratio of micro porosity decreases with the pH of solution decrease, while the ratio of meso porosity increases with the pH of solution decrease. It indicates that more micro pores are expanded to meso pores with an acidity of solution increase, and acid erosion plays major role in micro pores' development, which leads to the decrease in micro porosity and increase in meso porosity.  
Figure 5: _Cont._