Page 117 - Handbook of Thermal Analysis of Construction Materials

100 Chapter 3 - Formation and Hydration

Variations in the peak temperatures of thermograms reported in the
literature may be due to the differences in the purity of the sample,
temperature of hydration, water:solid ratio, the type of equipment, rate of
heating, etc.
Hydration characteristics of C A exposed to 20 or 80°C (w/s ratios
3
of 0.12 and 1.0) have been studied applying DTA, TG, scanning electron
microscopy, and microhardness techniques. [42] The DTA results are shown
in Fig. 23. Whereas it takes 6 hrs before C AH forms in considerable
6
3
amounts at 20°C, it is evident even at 15 secs at 80°C. A similar trend was
also obvious at higher water:solid ratios. The results indicate that at 80°C
the cubic phase forms almost immediately on contact with water; hexagonal
phases could not be detected at this temperature. At 20°C, the rate of
hydration is much slower, the hexagonal phases forming initially and
gradually converting to the cubic phase. Electronmicrographs show that at

20°C the hydrated product consists of irregular as well as spherically
shaped particles in the form of disconnected chunks. The product at 80°C
has spherical particles connected or welded into a continuous network. This
was caused by the direct bonding of C AH products formed mainly on the
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3
original sites of C A. Microhardness values of C A hydrated at 80°C are
3
3
more than fourfold those of the samples hydrated at 20°C. Feldman and
Ramachandran. [43] applied DTA technique to follow the hydration of C A
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cured at 2, 12, 23, 52, and 80°C from a few seconds to 10 days. The rate of
conversion of the hexagonal phases to the cubic phase increased with
temperature. It was concluded that the reaction occurred at the surface of the
aluminate phase, and the passage of water through the hexagonal alumi-
nates controls the overall rate of reaction. In the system containing calcium
hydroxide:cubic aluminate hydrate (5:1) the stability of the hexagonal
hydrate formed is maintained even up to 28 days. [1]
The alkali in clinker is combined as a solid solution with the C A
3
phase. The crystalline structure changes from cubic to orthorhombic or
monoclininc structure, depending on the content of Na in the C A phase.
3
Shin and Han [50] studied the effect of different forms of tricalcium alumi-
nate on the hydration of tricalcium silicate by applying DTA, TG, and
conduction calorimetry. It was concluded that the hydration of tricalcium
silicate is accelerated when orthorhombic, monoclinic, or melt C A was
3
present in the mixture. The cubic form of tricalcium aluminate was least
effective for accelerating the hydration of the silicate phase.