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 (C
3S)
or dicalcium silicate (C
2S) with water, the first step in the study is to produce pure
(C
3S) as it is more reactive than that of (C
2S). The high quality (C
3S) has been synthesized by calcination method at a temperature higher than 1250 °C. After that, the hydration
processes of C
3S 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 C
3S 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 C
3S 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, Ca
5Si
4O
13 , is proposed.
2) The formulation process, the morphology variation and space distribution of the
hydration products of C
3S has been clearly demonstrated by the experiments. It is
summarized that the hydration of C
3S includes calcium ions migration into solutions
with CH formed there and C-S-H formation on the surface of C
3S particle with the
distinguished outer to inner layers. The inner C-S-H layer and outer C-S-H layer
structure of the reacted C
3S particles is the first time observed in-situ with ethanol
immersion method. Based on such method, the unreacted C
3S 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 C
3S 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 SiO
2 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 C
3S 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 C
3S
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 C
3S 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.
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