## 1 Introduction  
Porous materials are widespread in human living environments, including rock materials of slopes, tunneling, and artificial materials of roads and bridges. Pores and cracks in porous rocks are of great influence, which affect their physical and mechanical properties, such as porosity, permeability, pore size distribution and static and dynamic properties. They have quite an important impact on the stability and durability of these materials. Freeze-thaw (F-T) is a kind of special natural environment caused by temperature change, whose covering area accounts for over 60% [1] of China's territory. F-T weathering causes remarkable damage to the inner structure of porous material. As F-T weathering is caused by the temperature fluctuates at the freezing point of water, frost heaving and osmotic effects caused by water-ice phase transition inside the porous medium exert frost heaving force and penetration force to porous material [2; 3; 4], eventually changing the microstructural parameters of the material, resulting in the damage of the material. Therefore, F-T weathering is a major factor that damages porous materials. Scholars have conducted many studies on the material damage caused by F-T weathering. A number of researchers adopted the deterioration of mechanical properties, including uniaxial static compression [5; 6; 7], uniaxial split strength [8; 9], and dynamic strength [10; 11] to characterize the damage of material. Although mechanical properties are closer to engineering, this method is indirect to characterize material damage. [PERSON] [12] studied micro-structure evolution by collecting and analyzing the acoustic emission of rock samples under F-T cycles. [PERSON] [13] and [PERSON] [14] adopted Scanning Electron Microscopy (SEM) and Computerized Tomography (CT) to reveal the external and internal structure of porous materials. [PERSON] et al. [15; 16] characterized microstructure evolution of F-T treated rock samples by porosity and pore size distribution. Chemical erosion is another important factor affecting rock inner structure, especially when porous rocks are soluble in chemical environment. Among chemical erosion, acid erosion caused by acid rain caused great damage to the geotechnical engineering and environment. Take Sichuan as an example, it underwent acid rain with an extreme low pH of 3.05 and 2.80 in 2004 and 2012, respectively, which caused irreversible damage to the environment. On this background, research on rocks under coupled the effect of freeze-thaw weathering and chemical erosion became a hot issue. [PERSON] [1] and [PERSON] [17] investigated the mechanical properties of rock samples under the coupled effect of F-T and chemical erosion. [PERSON] et al. [18; 19] revealed the structure evolution of sandstone under the coupled treatment of F-T cycles and various chemicals reacting from the perspective of porosity and pore size distribution. Overall, only little attention has been drawn to the microstructure of rocks under coupled treatment of F-T process and chemical erosion, while it is vital to rock engineering in cold regions. Currently, CT [20; 21], Mercury injection porosimetry (MIP) [22], SEM [14] and Nuclear Magnetic Resonance (NMR) [23; 24; 25; 26] are commonly adopted as microstructure detecting methods. CT detects the microstructures of materials based on the theory that X-ray attenuation of pores is different from aggregates. MIP detects the microstructures of materials based on the theory that mercury injection pore size is negatively correlated with mercury injection pressure. SEM detects the microstructures of materials by magnified electron microscope image directly. NMR detects the microstructures of materials by monitoring the H\({}^{+}\) of water inside pores. Among these detecting methods, MIP damaged microstructure during microstructure detection, SEM acquires the local and surface structure of the sample only, CT is ineffective in describing the characteristics of a wide range of pore size distributions compared to NMR.