Page 216 - Handbook of Thermal Analysis of Construction Materials

Section 2.0 - Calcium Chloride 199

The thermogravimetric method may also be used to estimate the
amount of Ca(OH) formed by the hydration of cement. Determination of
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the weight loss between 450 and 550°C caused by the decomposition of
Ca(OH) , and that between 550 and 900°C caused by the decomposition of
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CaCO permits the estimation of the amount of lime formed during the
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hydration of cement with 0, 1, and 2% CaCl . Table 1 gives the results for
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cement samples hydrated by adding the chloride to the mixing water or by
adding it a few minutes after the cement is mixed with water. [13] The lime
formed is reported in terms of CaO. The amount of CaO at 1 day in the
presence of calcium chloride is almost twice that formed in the absence of
the admixture. The degree of hydration is generally higher in samples
hydrated with 2% CaCl . It is also observed that even at 90 days the samples
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with calcium chloride are hydrated to a greater extent than those without it.
Hydration is promoted by delayed addition of the chloride. This is ex-
plained as follows—in the absence of chloride, the reaction that occurs
initially involves the aluminate and ferrite phases, and gypsum. In the
presence of CaCl , where the aluminate and ferrite phases preferentially
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react with the chloride, less chloride is available for accelerating the
hydration of C S. If, however, chloride is added a few minutes after the
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cement is mixed with water, the sulfate would have already reacted with the
aluminate and ferrite phases, and there is more chloride available to
accelerate the silicate reaction.

Table 1. Effect of CaCl on the Amount of CaO Formed in Portland Cement
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Pastes

Calcium Chloride Method of Addition Percentage CaO formed at (days)
Addition of CaCl 1 3 9 28 90
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0% ———— 3.5 6.8 8.0 7.9 8.1
1% with water 6.7 7.0 8.0 8.3 8.4
1% 2 minutes later 6.9 7.7 8.1 8.4 8.5
1% 4 minutes later 7.0 7.7 8.3 8.4 8.5
1% 8 minutes later 6.5 7.2 8.1 8.0 8.4
2% with water 7.1 7.7 8.2 8.5 8.6
2% 2 minutes later 7.8 8.0 8.3 8.6 8.8
2% 4 minutes later 7.9 8.0 8.3 8.6 8.8
2% 8 minutes later 7.3 7.8 7.9 8.0 8.3