Civil Engineering Reference
In-Depth Information
2.1
Effect of Calcium Chloride on Calcium Silicates
The silicate phases, C
3
S and C
2
S together constituting the major
portion of the components in portland cement, influence considerably its
hydration and strength development. The accelerating influence of CaCl
2
on the hydration of C
3
S is followed conveniently by estimating at different
times the amount of residual unhydrated C
3
S, the amount of Ca(OH)
2
, non-
evaporable water content, electrical conductivity, heat liberation, etc.
The amount of Ca(OH)
2
formed at different periods may be used to
follow the degree of hydration. The rate of hydration of C
3
S [determined by
estimating unhydrated C
3
S or Ca(OH)
2
] with 2% CaCl
2
shows the forma-
tion of an increased amount of C-S-H at all times up to 1 month.
[1][3]
Calcium chloride accelerates the hydration of C
3
S even at higher
temperatures and at different W/C ratios.
[4]-[6]
With the addition of 2%
calcium chloride, the degree of hydration at the same period at 22°C and
80°C is 60% and 70%, respectively.
[4][5]
Increasing the concentration of CaCl
2
even up to 20% with respect
to C
3
S has been found to accelerate intensely the hydration of the sili-
cate.
[1][3]
Increasing the amount of CaCl
2
not only accelerates the appear-
ance of the conduction calorimetric peak at earlier times, but also
intensifies the peak (Fig. 1). The addition of CaCl
2
to C
3
S not only alters
the rate of hydration, but also influences the chemical composition of the
C-S-H phase.
Because of the accelerated hydration, the silicate phases show
rapid setting characteristics in the presence of CaCl
2
. In a C
3
S:C
2
S mixture
containing 0, 1, and 2% CaCl
2,
the setting times have been found to occur
at 790, 525, and 105 minutes, respectively.
[7]
The addition of a small amount of calcium chloride is capable of
influencing the strength of tricalcium silicate. Figure 2 compares the rate of
strength development in C
3
S pastes with and without CaCl
2
.
[8]
At all times
up to 28 days, the strength in the paste containing CaCl
2
is higher than that
hydrated without the chloride. The increased strength is attributable to
increased degree of hydration.
Compared to the extensive investigations on the hydration of C
3
S
in the presence of CaCl
2
, only meager work has been done on the action of
CaCl
2
in the hydration of C
2
S.
[9][10]-[14]
Generally, all chlorides accelerate
the hydration of C
2
S, calcium chloride being more efficient than others at
nearly all ages.
