Page 438 - Handbook of Thermal Analysis of Construction Materials

414 Chapter 10 - Non-Portland Rapid Setting Cements

Monocalcium aluminate (CA) is the principal binding mineral in
HAC. Ramachandran and Feldman have studied the hydration of CA at low
[8]
water/solid ratio using DSC methods. Samples were in the form of
compacted powders formed at 415 MPa. Disks 31.75 mm in diameter by
1.27 mm thick were formed yielding an effective water:aluminate ratio of
0.15. The product at 80°C shows a much higher strength than that hydrated
at 20°C. The main initial hydration products are 2CaO•Al O •8H O and
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alumina gel. The data indicate that it is possible to obtain durable high
alumina cement products by using a low water/cement ratio and hydrating
at higher temperatures. The formation of the cubic C AH is initiated within
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an hour in the sample hydrated at 80°C. A C AH -C AH bond is favored
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under these conditions. Thermograms (DSC) of CA hydrated to different
periods at 20°C and 80°C are shown in Figs. 11 and 12.
At 20°C, an endothermal valley with a peak at about 100°C appears
along with a small endothermal doublet (in the range 175° to 225°C) at 10
hours (Fig. 11). At 1 day, a sharp endothermal peak appears at about 175°C
and increases in intensity as hydration progresses. Additionally, a small
endothermic peak at about 260°C appears at 1 day and continues to grow in
intensity. Also, an endothermal peak appears at about 300°C at 5 days and
increases in intensity up to 60 days. The large endotherm around 100°C may
be attributed to the removal of water from alumina gel. The endotherm at
125°C (not apparent in some curves) may be ascribed to the presence of
C AH . The endotherm appearing at about 175°C is due to the presence of
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CAH . The dual peaks occurring in the temperature range 200° to 325°C
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represent dehydration reactions involving gibbsite and C AH .
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The thermal behavior of CA hydrated at 80°C is significantly
different from that hydrated at 20°C (Fig. 12). Large endothermal effects
appear after 30 minutes at about 100, 145, 210, and 280°C. The first effect
is caused by alumina gel and is practically absent after 2 days of hydration.
The peak at about 150°C, present in all samples up to 1 day, is attributable
to the presence of C AH . The endothermal effect at 280°C is due to C AH 6
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and gibbsite; it increases in intensity with hydration. The endotherm at
225°C which emerges at 2 days may be due to dehydration of CAH and
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possibly gibbsite. The resolution of the large endothermal effect into two
effects at about 300°C and 340°C at 5 days confirms the formation of
gibbsite and C AH . The endothermal peak at about 500°C represents the
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typical stepwise dehydration effect of C AH . The degree and rate of
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conversion of the hexagonal phases and alumina gel to the cubic and
gibbsite phases, respectively, are also enhanced at a higher temperature. It
is also suggested that conversion to C AH and gibbsite phases occurs
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