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 eement 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% CaCl2 were 10.5,14.9, 15.1, 15.6, and 16.3%. These results indicate that CaCl2 accelerates the hydration of portland cement. 

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 CaCl2 is increased.l lP3] 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 CaCl2 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 meehanism 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% CaCl2 improves by about 50,41,11,9, and 8% overthe reference at 6 hours, 1, 3, 7, and 28 days, respectively.P ] 

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