Portland cement is one of the basic building materials for economic development, with high frequency of use, large quantity and wide application range. Calcium silicate hydrate (C-S-H) gel is a main hydration product that affects the macroscopic properties of Portland cement-based materials, and it is important to clarify the relationship between its structure and performance for the improvement of cement industry. However, the C-S-H gel produced by hydration of cement-based materials is difficult to extract, and its composition, morphology and structure are affected by different factors, which further affects its application performance. Therefore, this review analyzed the relationship between the microstructure and performance of C-S-H gel, discussed its mechanism of action on cement hydration, and provided a reference for its large-scale preparation and application.C-S-H gel is an amorphous substance, its structure and composition are generally complex and variable. The structure of C-S-H gel has a certain regularity. Some work proposed three related theoretical models. The first model is an atomic structure model, which can provide a theoretical basis for the differences in phase composition and tetrahedral structure, and lay a reference for the mechanism of adsorption performance. The second model is a nanostructure model, which can make a reasonable explanation for the morphological changes of C-S-H gel and provide a theoretical support for improving the seed effect. The third model is an intermediary structure model, which can provide a more profound and comprehensive understanding of its internal microstructure. These models are an important basis for understanding the characteristics of cement as well as a key to analyzing the microstructure of C-S-H gel and applying its characteristics.The proposal and verification of the three models above usually rely on the research of single-phase C-S-H gel, and the hydration products of cement affect each other, so it is difficult to extract C-S-H gel from the hardened matrix alone. Therefore, some researchers usually use artificial methods to prepare and control the composition and structure of C-S-H gels, especially hydrothermal synthesis. Firstly, keeping the reaction temperature and time (i.e., 70 ℃ and 7 h), choosing a low Ca/Si ratio and adding Al3+ or Na+ can reduce the generation of impurities. Secondly, increasing the reaction temperature and time improves the degree of polymerization of hydration products, and changing the Ca/Si ratio and ion species within a certain range also affects the chain length. In addition, the temperature increases, the reaction time prolongs, and the C-S-H surface gradually changes from the loose state to the dense state as the Ca/Si ratio decreases in the presence of Mg2+. Therefore, the precise control of C-S-H gel structure and characteristics can be achieved via a reasonable selection of technical parameters and applied to improve cement strength, thus avoiding ion penetration and other fields.More importantly, the effective regulation of C-S-H gel can further improve the performance in different fields. The fiber-like morphology can be synthesized by properly reducing the Ca/Si ratio, adding Mg2+, increasing the temperature and extending the time, and the seeding effect of C-S-H gel can be improved to the greatest extent. Meanwhile, increasing Ca/Si ratio, mixing K+ and Na+, reducing reaction temperature, and shortening reaction time can increase the adsorption on the surface and accelerate the curing effect of C-S-H gel.Summary and prospects As an important source of strength in cement-based materials, the structural characteristics of C-S-H gel in microscale can determine its macroscopic applications in different fields. This review represented the typical structures of C-S-H gel at different stages and introduced the inherent connection between structure and performance. The precise control of different C-S-H structure using hydrothermal synthesis was introduced, and the influence of synthesis factors on its composition, morphology, and structure was described. The basic research on the synthesis of C-S-H gel in relevant fields was summarized. This review summarized the relationship between the structure and performance of C-S-H gel as well as the exploration of the prospects for the engineering application of C-S-H gel. This review provided a theoretical reference for fully understanding the hydration mechanism of cement and developing high-performance, environmentally friendly building materials.At present, the research on the essence and mechanism of C-S-H gel is still relatively weak, and there are still many problems to be solved and further explored:1. This review mainly summarized the influence of synthetic C-S-H gel doped with cement-based materials on the overall performance, but could not realize an integrated study of "structure-performance-application" of C-S-H gel. It is necessary to deeply analyze the influence of the structure and performance of cement hydrated C-S-H gel on the matrix, and further improve the growth regulation and performance application of C-S-H gel in cement concrete.2. In the process of hydrothermal synthesis of C-S-H gel, the influence and law of key variables such as Ca/Si ratio, alkali ion, reaction time and temperature on the particle size are not determined yet. In the future, it is necessary to continuously debug the experimental environment and parameters to explore a relationship between the particle size and properties.