## 5 Conclusions  
In the present study, dry-wet alternation tests were conducted on jointed sandstone specimens. The pore structure, throat features, mesoscopic damage evolution and mineral composition of jointed sandstone were measured by NMR and SAXS. Subsequently, the mesoscopic damage evolution mechanism of jointed sandstone during the dry-wet alternation process was investigated. The main conclusions are summarized as follows:  
1. The dry-wet alternation action can cause the development of the porosity channel, leading to cumulative damage in sandstone. Therefore, the accumulated porosity and permeability of sandstone increase as the wet-dry cycle goes on, and the cumulative porosity goes through the slow accumulation stage, the rapid accumulation stage and the steady development stage.
2. During the dry-wet alternation process, the dissolution soluble minerals, hydration reaction and uneven shrink increase the size of pores and pore throats. In detail, micropores transform to medium and macropores, while medium pores transform to macropores gradually. The more micropores exist in the sandstone, the more obvious is the transformation from micropores to medium pores. Besides, the proportion of small pore throats decreases gradually, while the proportion of medium and large pore throats increases gradually.  
3. The contents of feldspar (potassium feldspar and albite), calcite and clay minerals (TCCM) in the test rock samples decreased as the dry-wet cycle went on, which led to the continuous development of pores and pore throats. 4. In the alternating dry-wet environment, the combined effects of extrusion crush, tensile fracture, chemical reaction and dissolution action, etc., of clastic particles inside the jointed sandstone led to the continual development of mesoscopic pore structures, which ultimately caused the porosity and permeability to increase. The damage process was the essential reason for the weakening of rock strength.  
L.Z.: conceptualization, data curation, funding acquisition, investigation, methodology, writing -original draft. G.W.: supervision, funding acquisition. R.W.: investigation, visualization, writing-review and editing. B.L.: investigation. K.W.: investigation. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Postdoctoral Science Foundation of China (No. 2022 MD723831), National Natural Science Foundation of China (No. 42407270), Natural Science Basic Research Program of Shaanxi Province (No. 2024)C-YBQN-0029), National Natural Science Foundation of China (No. 51978106), Chongqing Postdoctoral Innovative Talent Support Program (No. CQBX202208), Natural Science Foundation of Chongqing (CSTB2023 NSCQ-BHX0194), and Graduate Student Research Innovation Project (CYB240039).  
This study did not require ethical approval.  
Not applicable.  
Some or all data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request.  
This work was supported by Postdoctoral Science Foundation of China, National Natural Science Foundation of China and Natural Science Basic Research Program of Shaanxi Province.  
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.