Civil Engineering Reference
In-Depth Information
The dicalcium silicate phase hydrates at a slower rate than the C
3
S
phase.
[15]
The C/S ratio of the C-S-H product in the hydrated C
3
S is slightly
higher than that formed in the hydrated C
3
S. Table 1 shows the effect of
different percentages of CaCl
2
on the rate of hydration of C
2
S.
[16]
The
hydration rate is increased as the amount of chloride is increased.
Addition of CaCl
2
to C
2
S results in an increase in the rate of strength
development. An addition of 3% CaCl
2
increases the bending strength of
C
2
S by 26% at 2 months, about 34% at 3 months, and about 60% at 6
months.
[17]
Large additions of CaCl
2
are, however, detrimental to strength
development. The strength development also depends on the fineness of
C
2
S showing that as the fineness increases, the strength also increases at any
particular dosage of CaCl
2
.
[18]
Table 1.
Degree of Hydration of
β
-C
2
S with Different Amounts of Calcium
Chloride
Sample
Degree of Hydration
1 day
7 days
28 days
0.0% CaCl
2
16.1
24.3
33.0
0.5% CaCl
2
21.3
29.2
47.0
2.0% CaCl
2
21.6
34.1
56.1
5.0% CaCl
2
26.8
35.9
54.9
Mechanism of Acceleration.
Several mechanisms have been sug-
gested for the accelerating influence of calcium chloride on the silicate
phases. They include: complex formation, catalytic action, instability of
C-S-H phases, nucleation, reduced alkalinity, polymerized silicates, ionic
radii effects, chloride diffusion, etc. It appears that no single mechanism can
explain all the effects of CaCl
2
. Possibly, a combination of mechanisms
may be operating depending on the experimental conditions and the period
of hydration.
