## 2 Methods and Materials  
The test rock came from Jinhua City, Zhejiang Province. It was mostly made of tuff. All rocks are carved into 100 mm in height and 50 mm in diameter cylindrical specimens. Two distinct series of tests were conducted to examine the pore changes and evolution that emerges from the salt and frost weathering processes in tuff samples. The first batch of tuff samples were immersed in saturated sodium or magnesium sulfate solutions for 60 days to simulate salt weathering. The experimental tuff samples were treated as follows: tuff samples were soaked in one of two saturated salt solutions or distilled water at 25 \({}^{\circ}\)C and oven-dried at 120 \({}^{\circ}\)C for 24 h. The second group of tuff samples were immersed for two days in saturated solutions of sodium sulfate, magnesium sulfate, and distilled water. Then, each sample was placed in a cabinet set to \(-\)20 \({}^{\circ}\)C. After 12 h, the samples were taken out of the cabinet and left to defrost for an additional 12 h at room temperature (25 \({}^{\circ}\)C). The freeze-thaw cycle was repeated until sixty cycles had been completed. All of the samples in the first and second group were weighted and X-rayed every 10 days or freeze-thaw cycles. (Figure 2).  
A high-precision (5 \(\upmu\)m) micro-CT scanning system (GE manufactures) including an X-ray detector, carrier rotating stage, two ray emitters, and a data processing system was used to scan tuff samples (Figure 3). Each sample group was divided into 1200 cross-sections for a 45-minute scan to assure 3D reconstruction accuracy. VG Studio Max, one of the most advanced CT image post-processing software platforms, was used to precisely measure, analyze, and calculate 3D volume data sets from the 1200 images captured by the scanner [25]. For the analysis of the evolution of the 3D spatial structure, we only selected the middle portion of the specimen (20 mm in diameter, 60 mm in height) to reduce the influence of the specimen boundary on the results (Figure 3). Since the CT scanner has a spatial resolution of 0.03 mm \(\times\) 0.03 mm and a scanning layer thickness of 0.08 mm, each pixel cell in the image represents the porosity of 0.03 \(\times\) 0.03 \(\times\) 0.08 mm\({}^{3}\) cube cells. All of the scanned images were analyzed with the use of the software VG Studio Max, which performed operations such as binaryzation, segmentation, and normalization [25]. Pores were represented in the scanned images by gray values that were more than eight, while particle skeletons were represented by the other values. To quantitatively investigate the change and evolution in the pores, it was necessary to define the porosity of the samples. The calculation of porosity involved the division of the volume of identified pores by the overall volume of the selected region within the three-dimensional model of the specimen.  
Figure 2: Schematic diagram of test system of frost and salt weathering of tuff.  
The determination of the porosity of a sample's cross-section involves calculating the ratio of the total area occupied by recognized pores to the entire area of the specimen section. In our paper, we only focused on the microscopic scale of the internal pores of pumice [28]. In total, four experimental runs were conducted with diverse salt and freeze-thaw cycles to examine the pore change in tuff during frost and salt weathering processes (Table 1).