Page 163 - Handbook of Thermal Analysis of Construction Materials

146 Chapter 4 - Introduction to Concrete Admixtures

2.1 Effect of Calcium Chloride on Calcium Silicates

The silicate phases, C S and C S together constituting the major
3
2
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 S is followed conveniently by estimating at different
3
times the amount of residual unhydrated C S, the amount of Ca(OH) , non-
2
3
evaporable water content, electrical conductivity, heat liberation, etc.
The amount of Ca(OH) formed at different periods may be used to
2
follow the degree of hydration. The rate of hydration of C S [determined by
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estimating unhydrated C S or Ca(OH) ] with 2% CaCl shows the forma-
2
2
3
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 S even at higher
3
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 even up to 20% with respect
2
to C S has been found to accelerate intensely the hydration of the sili-
3
cate. [1][3] Increasing the amount of CaCl not only accelerates the appear-
2
ance of the conduction calorimetric peak at earlier times, but also
intensifies the peak (Fig. 1). The addition of CaCl to C S not only alters
2
3
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 . In a C S:C S mixture
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2
2
containing 0, 1, and 2% CaCl the setting times have been found to occur
2,
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
[8]
strength development in C S pastes with and without CaCl . At all times
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up to 28 days, the strength in the paste containing CaCl is higher than that
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hydrated without the chloride. The increased strength is attributable to
increased degree of hydration.
Compared to the extensive investigations on the hydration of C S
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in the presence of CaCl , only meager work has been done on the action of
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CaCl in the hydration of C S. [9][10]–[14] Generally, all chlorides accelerate
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2
the hydration of C S, calcium chloride being more efficient than others at
2
nearly all ages.