Page 211 - Handbook of Thermal Analysis of Construction Materials

194 Chapter 5 - Accelerating Admixtures

In the presence of 1% CaCl the thermograms of hydrating C S
3
2,
show significant differences from those hydrated without calcium chloride
addition (Fig. 2). The endothermal effects below 300°C in the presence of
CaCl are much larger than those obtained in samples without the addition
2
of calcium chloride (Figs. 1 and 2). An endotherm at 550°C appearing up
to two hours in the presence of 1% CaCl is absent in C S hydrated without
2 3
CaCl . There is also evidence that the endothermal effect due to Ca(OH) 2
2
is more intense in samples containing 1% CaCl than without it. Of the total
2
amount of Ca(OH) formed at 30 days, 33% is formed within 8 hours of
2
hydration. A remarkable feature of these thermograms is the onset of an
intense exothermic peak at four hours at a temperature of 690°C. This peak
is always followed by a large endothermal dip at about 800–840°C. There
[5]
is some evidence that it may be caused by the chemisorbed chloride
on the C-S-H surface and possibly also by chloride ions in the interlayer
positions.
In the presence of 4% CaCl , some thermal effects become more
2
intense at earlier times than the corresponding ones hydrated in the presence
[4]
of 1% CaCl (Fig. 3). Exothermal peaks are also evident at temperatures
2
above 600°C at three hours and beyond. The possibility of a surface
complex of chloride on the hydrating silicate phase is suggested by an
endothermal effect in the range 570–590°C. A larger effect at 810–850°C
following an exothermal effect is present from three hours to thirty days.
If the rate of hydration of C S is determined in terms of the amount
3
of Ca(OH) formed at different times, at six hours the sample containing 4%
2
chloride will have the largest amounts of calcium hydroxide. At 24 hours
and 30 days, the sample containing 1% will have higher amounts of calcium
hydroxide. If the hydration is determined by the disappearance of C S, then
3
at 30 days C S with 4% CaCl is hydrated to the maximum extent followed
3 2
by that containing 1% CaCl . The apparent discrepancy is due the differ-
2
ences in the CaO/SiO ratios of the C-S-H products formed during the
2
hydration.
Calcium chloride accelerates the hydration of C S even at higher
3
temperatures. The effect is particularly greater at earlier periods of hydra-
tion. Heat evolution curves show that at temperatures of 25, 35, and 45°C,
the addition of 2% CaCl not only influences the total heat developed at
2
early periods but also the time at which the maximum heat evolution peak
[6]
occurs. Increasing the concentration of CaCl up to 20% with respect to
2
C S has been found to influence the conduction calorimetric curves. [7] In
3
Fig. 4, conduction calorimetric curves of C S containing 0–20% calcium
3
chloride are given. The sample containing no chloride shows a hump with