## 1 Introduction  
The area of the Chinese territory covered by frozen soil is the third largest in the world, of which the seasonally frozen area is \(5.137\times 10^{6}\) km\({}^{2}\), accounting for 53.5% of the country's land area [1; 2], widely distributed in the northeast and northwest. The freezing and thawing cause the strength of the subgrade soil to decrease during the spring-thaw period, resulting in the part or all of the bearing capacity being lost, thereby reducing the pavement quality. The silty soil as a widely distributed subgrade material in the northeast of China is exposed to at least one freeze-thaw (F-T) cycle each year. Because it has low natural moisture content, a small plasticity index, and weak shear strength, these make it sensitive to frost heave and water stabilities [3; 4]. In general, the subgrade material in the seasonally frozen area filled with silty soil has the problems of large settlement deformation and difficult compaction, so it is necessary to modify it before using as the subgrade material [5; 6; 7; 8; 9]. At present, most of the excellent fillers are used to improve the quality of the subgrade material. However, due to the lack of high-quality fillers along the road, the transportation costs will cause a significant increase in engineering costs. Therefore, the use of a modifier to improve the soil is a very effective method. The modifiers for improved soil are still mainly cement, lime, fly ash, and so on, in domestic and foreign applications [10]. According to the relevant literature, the macro-mechanical properties and frost resistance of cement modified soil are better than those of the other modified soils: the cohesion of cement modified soil was 2-3 times that of lime modified soil at the same F-T times and cooling temperature [11]. The cohesion, shear strength, and California Bearing Ratio (CBR) values of cement modified soil were significantly higher than those of lime modified soil at an equal blend ratio [12; 13; 14; 15]. The improvement of shear strength in Libyan silt soil by lime was only within a certain range, and the improvement in compressibility with cement was more obvious [16]. The unconfined compressive strength of modified soil increased as the cement content and cement age increased, while it increased first and then decreased with the increasing amount of fly ash [17]. The strength of cement modified soil was high in the early stage and increased obviously in the later stage, which could effectively reduce the subgrade maintenance costs and prolong its service life under the long-term F-T cycles. Hence, the cement modified silty soil was chosen as the research objective. The microstructure properties of soil include the size, shape, and surface characteristics and the connection and arrangement of the particles. To study the intrinsic nature of the physical and mechanical behaviors, the variation of the soil microstructure, which largely affects the engineering properties of the soil, must be grasped. The study methods for the microstructure mainly include optical microscopy, X-ray diffraction (XRD), mercury intrusion, gas adsorption, and scanning electron microscopy (SEM). Among these methods, SEM is widely used because it can precisely obtain the size, shape, arrangement, and pore development inside the microstructure unit [18; 19; 20]. Currently, many researchers are purely analyzing the factors affecting the microstructure while the effect of F-T cycles and the modifier are little involved, as well as the microscopic mechanism of the mechanical properties is rarely demonstrated. Only the characteristics of lime modified soil with larger pores could be influenced by the water content and compaction of the soil [21]. The physicochemical factors that made up the micro level and how it affected soil behaviors have been discussed [22].