The crystallography has always been interesting to cement
and concrete researchers. In hydrated
cement, hydrated
products consist of various chemical compounds depending
on the hydration time. The shape and morphology of these
crystals, which is a means of their identification using
SEM, can be of hexagonal and prism form for example.
Here are some nice examples showing how beautiful the
cement can be at micro and nano levels! |
 Hydrated
Cement Paste
Depending on the time of
hydration and the Portland
cement composition, several
crystalline pictures can be
observed in hydrated cement
paste. A typical one
contains C-S-H, calcium
hydroxide and ettringite as
shown in this picture (photo
courtesy of Mr. Jim Margeson,
NRC-IRC).
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 Calcium Hydroxide
Calcium
hydroxide has a distinguishing hexagonal
shape.
The dimensions, of course, depends on
the concentration in aqueous solution and
the age of the crystal. This nice
crystalline shape has been produced on a
piece of glass
fibre in saturated
Ca(OH)2 solution.
Depending on the concentration of the
solution, other shapes of
the crystalline calcium
hydroxide can be formed. |
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Small 100nm hexagonal shape
Ca(OH)2 crystals
on the surface of the C3S
after 2.5 hours of hydration
at room temperature
(W/C=0.8). The hydration was
stopped by isopropanol and
subsequent vacuum drying. |
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 Calcium
Carbonate
Strange crystals appeared on
a compact of C-S-H after
unexpected long exposure to
the atmosphere. Their
"alien" and "snow
flake" shapes seem to be
similar to the calcium
carbonate crystals formed as
a result of the C-S-H
carbonation due to the
carbon dioxide in the air.
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 Calcite
rhombs mixed with the
Wollastonite needles.
Wollastonite is a form of
calcium silicates (CaSiO3)
that is not usually
carbonated unless through a
specific treatment. |
 Calcium
Silicate Hydrate (C-S-H)
The flaky C-S-H crystals
grown after 2 weeks of
hydration of C3S
(W/C=0.8). The
stoichiometric C/S ratio of
C-S-H and morphology of its
crystals in conventional
cement systems depend on the
curing condition.
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 The
growth of C-S-H and calcium
hydroxide crystals beside
each other. The dominating
phase in forming the shape
in the contact surface is
C-S-H probably due to its
significantly higher
strength.
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