Page 217 - Handbook of Thermal Analysis of Construction Materials

200 Chapter 5 - Accelerating Admixtures

The setting and hardening of concrete are accelerated in the
presence of calcium chloride. These are related to the effect of CaCl on the
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rate of hydration of cement in concrete. The hydration of cement is
exothermic, resulting in the production of heat, and if the heat is produced
at a faster rate, larger amounts of the hydrates are formed at earlier times in
the presence of accelerators. This is particularly significant in the first
10–12 hours. The influence of different amounts of calcium chlo-
ride on the rate of heat development is depicted by conduction calorimet-
ric curves (Fig. 8). [14] The position of the peak corresponding to the
maximum liberation of heat moves towards shorter times as the amount of
CaCl is increased. It occurs at about eight hours for the reference cement
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and at about three hours with 2% CaCl . Calcium chloride also increases the
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amount of heat liberated during the first few hours. Their reaction can also
be accelerated by hydration at higher temperatures. The addition of 2%
CaCl was found to have the same accelerating effect on hydration as an
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increase of 11°C in the hydration temperature. [14]

Figure 8. The effect of CaCl upon the rate of heat development in cement.
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