Civil Engineering Reference
In-Depth Information
The crystal structures of the individual tricalcium silicate phases are similar, and differ
mainly in the spatial orientations of the SiO
4
tetrahedra.
Tricalcium silicate is thermodynamically stable only between about 1250°C and 2150°
C. It melts incongruently above 2150°C, and becomes thermodynamically unstable with
respect to dicalcium silicate and calcium oxide below 1250°C. Thus, to produce a clinker
containing tricalcium silicate, the raw mix has to be heated to a temperature that is higher
than the lower limit of thermodynamic stability of this phase, and has to be cooled rapidly
enough to prevent a noticeable decomposition of tricalcium silicate in the course of
cooling. Such a conversion of C
3
S to C
2
S and C may take place especially at
temperatures slightly below 1250°C, whereas at ambient or only moderately elevated
temperatures tricalcium silicate remains preserved for an indefinite time.
In industrial clinkers the existing tricalcium silicate is doped with foreign ions, also
present in the raw mix. These dopants prevent a conversion of tricalcium silicate to its T
1
modification, which is the one that is “metastable” at ambient temperature, and stabilize
one of the high-temperature modifications of this compound, usually M
1
or M
3
. Such a
doped form of tricalcium silicate is usually called
alite
.
Upon contact with water, tricalcium silicate undergoes hydration, yielding an
amorphous calcium silicate hydrate phase called the
C-S-H phase
(or just C-S-H) and
calcium hydroxide as products of hydration. The rate of hydration will depend on the
quality and quantity of dopants incorporated within the crystalline lattice, on the cooling
rate in the production of the clinker, on the fineness of the cement, and on other factors.
The C-S-H phase is an amorphous or nearly amorphous material of the general formula
CaO
x
.SiO
2
.H
2
O
y
, where both
x
and
y
may vary over a wide range. On the nanometer
scale the C-S-H phase is structurally related to the crystalline phases 1.4 nm tobermorite
and jennite. In cement pastes limited amounts of foreign ions may be incorporated into
the C-S-H phase. On the micrometer scale the C-S-H phase appears either as a dense
amorphous mass or as a microcrystalline material with an acicular or platelet-like
morphology. The material contains pores with radii between about 1 and 10
4
nm, and
exhibits a specific surface area exceeding 100 m
2
/g.
C
alcium hydroxide is formed—besides C-S-H—as the second product of C
3
S
hydration. This happens because the C/S molar ratio within the C-S-H phase is always
distinctly lower than that of the original C
3
S. In hydrated tricalcium silicate or Portland
cement pastes calcium hydroxide is present in the form of crystals up to about 10-30 µm
large, and in this crystalline form is called portlandite.
