## 4 Conclusions  
This paper studies the water absorption evolution behaviors of sandstone in Mogao Grottoes in China using real-time experimental monitoring and using DIC methods to analyze the deformation characteristics of sandstone hydration absorption, and the conclusions can be summarized as follows:  
1. Through the real-time monitoring system, the pressure-less hydration absorption experiment of Mogao Grottoes sandstone under capillary action was carried out, and the water absorption curve of sandstone was obtained. The curve results show that the hydration absorption of sandstone can be divided into three stages: the initial stage increases rapidly, the intermediate stage increases slowly, and the final stage tends to be stable. The actual hydration absorption curve of sandstone can be well fitted using the exponential function. 2. The deformation characteristics of sandstone in the process of hydration absorption were analyzed using the DIC method, and the relationship between the pressure-less hydration absorption and deformation of sandstone was obtained. The deformation of sandstone increases rapidly in the initial stage, and the deformation of sandstone gradually slows down in the middle stage. When the water absorption reaches saturation, the interaction between water and sandstone still exists, and the sample continues to deform for a period of time before stopping. The bedding structure and particle composition of sandstone will affect the deformation of sandstone, resulting in uneven changes in sandstone. 3. The sandstone samples are divided into sparse and dense areas. The results of the mercury intrusion test and the XRD test show that the porosity of the sparse area is larger than that of the dense area, and the particle content of the dense area is lower. When the sandstone is saturated with water, the liquid is immersed in the pores between the solid particles, which makes the sparse area more prone to stress concentration, and the deformation in the sparse area is larger. Therefore, when analyzing the water absorption deformation of sandstone, the porosity should be considered. In this paper, the pressure-less hydration absorption experiment of sandstone is carried out to describe the hydration absorption process of sandstone. The deformation characteristics of sandstone in the process of hydration absorption are analyzed using the DIC method, and the harmful behavior related to water migration is analyzed from the perspective of the hydration absorption deformation of rock. On the basis of this paper, more accurate water-rock interaction and mechanism results can be obtained by considering temperature, humidity, and multi-field coupling. Therefore, the experimental system and procedure used in this study can also be extended to the analysis of water-rock interaction. However, the microstructure of the sample was not measured using scanning electron microscopy in this paper. The speckles actually did not reach the ideal conditions, and the quality of the speckles determined the success or failure of the deformation experiment. The hydration of surrounding rock and salt damage of murals caused by water-salt migration are very complex processes. The water-rock interaction behavior and deformation characteristics under different temperature, humidity, and multi-field coupling conditions are important research topics in the future. N.H.: Conceptualization, Methodology, Resources, Funding acquisition. Y.W. : Conceptualization, Methodology, Resources, Writing--Reviewing and Editing, Supervision, Project administration, Funding acquisition. Y.Z. and W.J. : Methodology, Software, Formal analysis, Investigation, Data curation, Writing--Original draft preparation, Visualization. P.L. and N.L. : Formal analysis, Investigation, Data curation. All authors have read and agreed to the published version of the manuscript.