Calcium silicate hydrate (C-S-H) is the most important hydration product in concrete due to its unique function in binding all the individual particles together and determining the mechanical strength of the composites. Understanding the nature and materials structure of the C-S-H is very important as it can reveal the origin of the mechanical properties of concrete and provide methodology to improve the performance of the concrete structure. Unfortunately, owing to the intrinsic complicated structure of C-S-H and the incapable research measures for the study on the nanometer scaled structure of C-S-H, the knowledge on the characteristics of C-S-H is still very limited at this moment. This study is aimed at revealing the intrinsic characteristics of C-S-H through the experiment...[ Read more ]

Calcium silicate hydrate (C-S-H) is the most important hydration product in concrete due to its unique function in binding all the individual particles together and determining the mechanical strength of the composites. Understanding the nature and materials structure of the C-S-H is very important as it can reveal the origin of the mechanical properties of concrete and provide methodology to improve the performance of the concrete structure. Unfortunately, owing to the intrinsic complicated structure of C-S-H and the incapable research measures for the study on the nanometer scaled structure of C-S-H, the knowledge on the characteristics of C-S-H is still very limited at this moment. This study is aimed at revealing the intrinsic characteristics of C-S-H through the experimental works carried out with the most advanced equipment.

As the C-S-H is usually formed through the hydration of the tricalcium silicate (C₃S) or dicalcium silicate (C₂S) with water, the first step in the study is to produce pure (C₃S) as it is more reactive than that of (C₂S). The high quality (C₃S) has been synthesized by calcination method at a temperature higher than 1250 °C. After that, the hydration processes of C₃S under both high and low w/s ratios have been investigated. The materials structure and mechanical properties of the hydration products have been characterized by various methods such as TEM, SEM, NMR, XRD and nanoindentation. Finally, the characteristics of the C-S-H formulated from the hydration of C₃S have been compared with that from the C-S-H obtained by direct synthesis method. The main findings of the current study are:

1) A more realistic C-S-H chemical formula has been revealed relying on the experimental observation of the silicate structure of the hydrated C₃S with water. At a mature stage with high hydration degree, for all different w/s ratios, similar C/S values of 1.2 to 1.3 have been identified. Based on the observation, a new chemical formula of C-S-H, Ca₅Si₄O₁₃ , is proposed.

2) The formulation process, the morphology variation and space distribution of the hydration products of C₃S has been clearly demonstrated by the experiments. It is summarized that the hydration of C₃S includes calcium ions migration into solutions with CH formed there and C-S-H formation on the surface of C₃S particle with the distinguished outer to inner layers. The inner C-S-H layer and outer C-S-H layer structure of the reacted C₃S particles is the first time observed in-situ with ethanol immersion method. Based on such method, the unreacted C₃S can be clearly separated from the formed product.

3) More intrinsic C-S-H composition, structure and crystallinity have been identified from the samples of high w/s ratio under observation of high resolution TEM. The C-S-H product possessed wrinkled cluster morphology by comprising of certain amount of small round shape particles packed together. The final C-S-H with crystallined properties, the structure of which have been located at foil-like proportion as well as edge of the formed wrinkles, which locations are observed for the first time in the field. The size of such crystallined proportion is found to be quite large dimensions of tens of nanometers.

4) For the first time, CH morphology of hydrated C₃S has been systematically studied by various characterization methods. In additional the hexagonal structure traditionally known for CH, there are various morphologies related to CH formed under influence of C-S-H, such as round, spindle, fibrillar, gel particles and crystal inserted colloidal particles.

5) The systematic diagram of synthesized C-S-H compound obtained by direct reaction of CaO and SiO₂ has been built up by considering various influencing factors. The nature of the C-S-H (similar to the secondary C-S-H in cementitious material) is ill-crystallined tobermorite at ambient temperature. Other kind of crystals formed within synthesized C-S-H is xonotlite which is appeared only at high temperature.

6) The nanoindentation test have been conducted on the pure C₃S hydrated products for the first time in the field of cement and concrete research and the modulus values have been obtained for the outer, inner C-S-H layers as well as the unhydrated C₃S residual, which fills in the vacancy in this area.

The experimental results obtained in this study reveal the more realistic chemical composition, morphology appearance, and space arrangement of the hydration products of C₃S and identify the nature and characteristics of C-S-H. Hence, the research provides solid base for the C-S-H model development at atomic scale or nanoscale as well as the useful methodology for optimization of C-S-H materials structure to achieve better mechanical performance of concrete.