Civil Engineering Reference
In-Depth Information
In the presence of 1% CaCl
2,
the thermograms of hydrating C
3
S
show significant differences from those hydrated without calcium chloride
addition (Fig. 2). The endothermal effects below 300°C in the presence of
CaCl
2
are much larger than those obtained in samples without the addition
of calcium chloride (Figs. 1 and 2). An endotherm at 550°C appearing up
to two hours in the presence of 1% CaCl
2
is absent in C
3
S hydrated without
CaCl
2
. There is also evidence that the endothermal effect due to Ca(OH)
2
is more intense in samples containing 1% CaCl
2
than without it. Of the total
amount of Ca(OH)
2
formed at 30 days, 33% is formed within 8 hours of
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
is some evidence
[5]
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
2
, some thermal effects become more
intense at earlier times than the corresponding ones hydrated in the presence
of 1% CaCl
2
(Fig. 3).
[4]
Exothermal peaks are also evident at temperatures
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
3
S is determined in terms of the amount
of Ca(OH)
2
formed at different times, at six hours the sample containing 4%
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
3
S, then
at 30 days C
3
S with 4% CaCl
2
is hydrated to the maximum extent followed
by that containing 1% CaCl
2
. The apparent discrepancy is due the differ-
ences in the CaO/SiO
2
ratios of the C-S-H products formed during the
hydration.
Calcium chloride accelerates the hydration of C
3
S even at higher
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
2
not only influences the total heat developed at
early periods but also the time at which the maximum heat evolution peak
occurs.
[6]
Increasing the concentration of CaCl
2
up to 20% with respect to
C
3
S has been found to influence the conduction calorimetric curves.
[7]
In
Fig. 4, conduction calorimetric curves of C
3
S containing 0-20% calcium
chloride are given. The sample containing no chloride shows a hump with
