Atomistic Simulation studies of the Cement Paste Components

Atomistic Simulation studies of the Cement Paste Components
⎡
4 ⋅⎢
⎣
⎤
Ca2 nH wSi(3n−1) O(9n− 2)
OH Ca ⋅
w+ n( y−2)
n y
m H
2O⎥
2 ⎦
{ }( )
{ }
n= 1; w= 0; y = 2; m= 0 →4⋅⎡⎣
Ca2Si2O7 ⋅Ca1⎤⎦
(3.3.3)
The relaxed structure after the optimization (figure 3.2.3) has a very poor symmetry,
in comparison with the low C/S ratio case. Consequently, the total dipolar moment of
the particle is very high, 13.72 D. It can be suggested that the energy gain of the
multiple bond formation primes over the dipole cancellation in the present case. Each
precursor can be still distinguished, although their structure is distorted. The added
calcium atoms are coordinated not only to the terminal oxygen atoms of the silicate
chains, but also to those of the Ca-O layer of each precursor. Thus, the silicate dimers
are now linked to two different Ca-O layers, within the precursors and between them.
The mean charge of the calcium ion of the layer formed between the units is slightly
smaller than the one within the precursor, just about 0.1 e - lower. This small decrease
indicates that the iono-covalent nature of both layers is similar.
3.5. Discussion of the assembly mechanism
The formed structures after the assembly of precursors can be related to the crystalline
species tobermorite 14 Å and jennite. A schematic representation is presented in figure
3.2.4. When the dipole-dipole interaction is the responsible of the system organization, a
tobermorite-like structure is formed. The configuration can be identified as a precursor
assembly in the perpendicular direction of the silicate chains, with the Ca +2 ion acting as
a linker between layers. However, when the formation of multiple Ca-O bonds primes,
the silicate chains rearrange themselves and are coordinated to two Ca-O layers. This
would be equivalent to a jennite-like structure, growing in the laminar plane and
perpendicular to the silicate chain direction. Thus, starting from a single precursor,
tobermorite or jennite-like structures can be reached, depending on the mechanism,
which in its turn depends on the calcium to silicon ratio. The growth mechanisms are
consistent, therefore, with the calcium content of the crystalline species.
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