As a major hydration product, calcium silicate hydrate (C-S-H) determines the properties of cementitious materials from every perspective including strength development, shrinkage, creep, and durability. Thus C-S-H is often considered as the "gene" of cementitious materials. Therefore, a comprehensive knowledge of the compositions, structures and performances of C-S-H would advance our understanding of the intrinsic properties and microstructure development of cementitious materials, thereby the macro performances of cement-based materials can be better controlled and even designed from the bottom up accordingly. In recent years, the development of the advanced characterization and analytical technology provides novel and inspiring methods to the investigation of the C-S-H. Great achievements have been made through an increa-sing number of literatures, which have reported the theories on the nucleation and growth, the improvement on mechanical properties together with morphology controlling of C-S-H. Synthetic C-S-H which is relatively pure and with higher crystallinity is usually favored by researchers than the amorphous C-S-H gel in cementitious materials. It is demonstrated that the globular C-S-H is observed in the nucleating and growing course of C-S-H synthesized by double decomposition reaction which later on transforms to foil-like C-S-H as the final product, in line with the non-classical nucleation and growth pathway. At sub-particle level, more and more evidences suggest that C-S-H structure is closer to that of tobermorite rather than jennite. At particle level, CM colloidal models are widely used to explain the physical properties, such as specific surface area, and behaviors of the shrinkage and creep of cement-based materials. The extensive investigations on the characteristic parameters of C-S-H, including Ca/Si, the content of water, silicate chain structure and morphology provide more information for the establishment of the relationship between features at the micro/nano level and properties at macroscopic scale. Additionally, the controlling of the intrinsic C-S-H structure provides a promising way to improve the toughness of cementitious materials. Some achievements have been reached compassing the refinement of the pore structure, development of organic-inorga-nic composites and optimization of the microstructure by using nano materials. The methods of synthesizing C-S-H, nucleation and growth theories of C-S-H as well as structural models are summarized systemically in this paper, through combining the classical theories and the latest investigations. The characteristics of C-S-H micro/nano structure are reviewed mainly from its Ca/Si, H2O/Si, the degree of polymerization of silicate chain and morphologies. The approaches for controlling and designing C-S-H in terms of morphologies and mechanical properties are introduced. The limitations of the current researches and perspectives for the future on the investigation of C-S-H are provided in the end. © 2021, Materials Review Magazine. All right reserved.
