Page 218 - Handbook of Thermal Analysis of Construction Materials

202 Chapter 5 - Accelerating Admixtures

3.0 NON-CHLORIDE ACCELERATORS

One of the limitations to the wider use of calcium chloride in
reinforced concrete is that, if present in larger amounts, it promotes
corrosion of the reinforcement unless suitable precautions are taken. The
use of calcium chloride is banned in many countries. There is, hence, a
continuing attempt to find an alternative to calcium chloride, one equally
effective and economical, but without its limitations. A number of organic
and inorganic compounds including aluminates, sulfates, formates, thiosul-
fates, nitrates, silicates, alkali hydroxides, carbonates, halides, nitrites,
calcium salts of acetic acid, propionic acid, butyric acid, oxalic acid and
lactic acid, urea, glyoxal, triethanolamine, and formaldehyde have been
suggested. However, practical experience and research on these admixtures
are limited. The effect of these compounds on the hydration of individual
cement compounds and cement has been widely studied by thermal analysis
techniques.
Triethanolamine of formula N(C H OH) (TEA for short) is an oily
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water-soluble liquid having a fishy odor and is produced by reacting
ammonia with ethylene oxide. Normally it is used in combination with other
chemicals in admixture formulations.
Ramachandran [16] followed the hydration of C A (with and without
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gypsum) containing triethanolamine. Figure 10 refers to thermal curves of
C A hydrated for 0, 1, 5, 10, 30, and 60 minutes in the presence of 0, 0.5,
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1.0, and 5% TEA. At 1 minute, the C A sample containing no TEA exhibits
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endothermal peaks at 150 and 250°C typical of a mixture of hexagonal
aluminate hydrates. At about 5 minutes, all samples show endotherms
between 300 and 400°C. They are typical of cubic tricalcium aluminate
hexahydrate. At 1 minute, C A samples containing 0.5 and 1% TEA show
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the formation of larger amounts of hexagonal hydrates, as evidenced by the
greater intensity of the peak at about 150°C. At 5% TEA, the existence of
a larger amount of C AH even at 1 minute indicates that the hydration of
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C A is enhanced by increased amounts of TEA. An exotherm (400–500°C)
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in samples containing 5% TEA is caused by a complex that forms between
hydrated aluminate and TEA.
DTA results of C A + 25% gypsum mixtures hydrated with 0% and
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1% TEA are shown in Fig. 11. The unhydrated mixture exhibits an
endothermal doublet between 100 and 150°C due to a stepwise dehydration
of gypsum. Hydration seems to start from the first minute of contact of water
with the sample. At 5 minutes, a larger amount of gypsum disappears from