Page 207 - Handbook of Thermal Analysis of Construction Materials

190 Chapter 5 - Accelerating Admixtures

In this chapter, typical examples of the application of various
thermal techniques to the study of the effect of admixtures on the hydration
of cement and cement compounds is emphasized. Where relevant, the
results obtained with these techniques are compared with those derived
from other tools.

2.0 CALCIUM CHLORIDE

Calcium chloride is a unique accelerator in the sense that, of the
-
+2
various cation-anion combinations, the Ca and Cl combination ranks as
the best accelerator for cements. The accelerating influence of CaCl on the
2
hydration of calcium silicates was observed more than sixty years ago by
[1]
[2]
Sloane, et al., and Haegerman, and has been confirmed by subsequent
[3]
work. The accelerating influence of calcium chloride on the hydration of
C S is conveniently followed by adopting any of the following methods,
3
viz., estimating at different times the amount of residual unhydrated C S,
3
the amount of Ca(OH) , loss on ignition, electrical conductivity, heat
2
liberation, etc.
The techniques of DTA, TG, and Conduction Calorimetry have
proven to be valuable to follow the hydration of C S in the presence of
3
[3]
calcium chloride. Typical results of the application of DTA are illustrated
[4]
in Figs. 1, 2, and 3. Thermograms of C S hydrated to different times in the
3
presence of 0, 1, or 4% CaCl enable a study of the progress of hydration,
2
identification, and estimation of some of the products. Unhydrated C S
3
exhibits endothermal effects at about 680, 930, and 970–980°C represent-
ing crystalline transitions (Fig. 1). Onset of hydration is indicated by the
endothermal effects below 300°C. They are caused by expulsion of loosely,
as well as firmly, bound water from the C-S-H gel. The increase in the
intensity of this effect with time is indicative of the formation of larger
amounts of C-S-H product. A very small endothermal effect at about 480°C,
which appears within a few minutes becoming more evident at one hour and
after, may be attributed to the dehydration effect of Ca(OH) . In the first
2
eight hours, the amount of Ca(OH) produced is about 25% of all the
2
calcium hydroxide that is formed in thirty days.