The structure of silicate ions in C-S-H discussed from chemical composition

Calcium silicate hydrate (C-S-H) is a main product of Portland cement hydration. Although C-S-H is treated as an amorphous phase in macro scale, the atomic-scale structure of C-S-H is considered to be nanocrystalline and short-ordered. Various studies have been carried out and many models have been proposed in the past few decades to describe the structure of C-S-H at nano scale. Tobermorite and jennite are considered as two naturally existing crystalline solids sharing similar structures with C-S-H, especially tobermorite. As a result, tobermorite-based models are predominant in the literature. Besides, combination models with both tobermorite and jennite as well as exceptive models have also been developed in the same period. In this paper, the C-S-H models developed in the past decades are reviewed systematically. The reliability of the models is discussed based on the fundamental requirements of the C-S-H structure from consideration of experimental observations and structural chemistry, including charge balance, Ca/Si (C/S) ratio and Qn distribution. Finally, some considerations on the understanding of C-S-H are put forward for possible development of C-S-H structure in the future.