Page 166 - Handbook of Thermal Analysis of Construction Materials

150 Chapter 4 - Introduction to Concrete Admixtures

2.3 Effect of Calcium Chloride on Cement

The kinetics of hydration of cement may be followed by DTA,
TGA, XRD, conduction calorimetry, chemical estimation, etc. The non-
evaporable water content is a measure of the amount of hydrated cement
formed. In one study, a cement hydrated for 6 hours, 1, 3, 7, and 28 days
showed non-evaporable water contents of 7.0, 10.6, 13.6, 14.3, and 15.1%
respectively, and the corresponding values with 2% CaCl were 10.5, 14.9,
2
15.1, 15.6, and 16.3%. These results indicate that CaCl accelerates the
2
hydration of portland cement. [32]
The hydration of cement, being an exothermic reaction, produces
heat, and, if the hydration is accelerated, heat is produced at a faster rate.
The position of the peak corresponding to the maximum heat liberation
occurs at lower times as the amount of added CaCl is increased. [1][33]
2
Calcium chloride also accelerates the rate of hydration of Type II, IV, and
V cements, but only marginally that of Type III, a high early strength
cement.
Several mechanisms have been suggested for the action of calcium
chloride on cement. None of the theories can explain all the effects of CaCl 2
in concrete. It is possible that any one mechanism will be able to explain
only one or some of the observations. It is likely that an overall mechanism
should take into account the amount of the admixture used and the time of
hydration and the experimental conditions.
Calcium chloride increases compressive strength of cement pastes
especially at earlier times. The most significant effect on compressive
strength occurs with portland blast furnace cement and marginally with
portland-pozzolan cement. The compressive strength of cement pastes in
the presence of 2% CaCl improves by about 50, 41, 11, 9, and 8% over the
2
reference at 6 hours, 1, 3, 7, and 28 days, respectively. [34]
Ramachandran and Feldman [35] examined the strength develop-
ment in portland cement pastes hydrated in the presence of 0, 1, 2, and 3.5%
CaCl . At any particular degree of hydration, the sample with 3.5% CaCl
2 2
had the lowest strength; at lower degrees of hydration, a sample containing
0% CaCl was the strongest, although, with time, samples containing 1–2%
2
CaCl form stronger bodies than all others. Porosity, density, and bonding
2
affected these results. It is thus evident that addition of CaCl not only
2
changes the rate of hydration, but also the intrinsic nature of the hydration
products.