Page 127 - Handbook of Thermal Analysis of Construction Materials

Section 7.0 - Portland Cement 111

7.0 PORTLAND CEMENT

In the hydration of portland cement, several products can be
detected by applying DTA and TG techniques. Unreacted gypsum may be
identified by endothermal peaks in the temperature range of 140–170°C,
the C-S-H gel at temperatures in the range of 115–125°C, ettringite at
temperatures of 120–130°C, AFm phase at 180–200°C, calcium hydrox-
ide by an endotherm in the range of 450–550°C, and calcium carbonate at
750–850°C. Unhydrated cement may also exhibit a small endothermal peak
at about 485°C for CH formed during storage. It has to be recognized that
there are interference effects at low temperatures, depending on the drying
procedures and the state of the material. In some instances, a small
additional endotherm appears before the onset of the calcium hydroxide
endotherm and this is attributed to the dehydration effect of the chemi-
sorbed water on lime or to the finely divided form of lime. The lime formed
at different times may be estimated by determining the endothermal area of
the lime peak or weight loss. The amount of lime is nearly proportional to
the degree of hydration of cement.
The sequence of hydration products formed in the hydration of
cement at different periods has been followed by DTA (Fig. 30). [1][55] The
endothermal effect at 140 and 170°C characterize gypsum and the endother-
mal peaks below 500°C and 800°C are due respectively to the decomposi-
tion of Ca(OH) and CaCO . Within five minutes, an endothermic effect
2
3
appearing at 130°C is caused by the dehydration of ettringite. Gypsum is
partly consumed at this period as evident from the reduced intensity of the
peaks at 140°C and 170°C. At one hour, there is an increase in the intensity
of the ettringite peak at the expense of the gypsum peak. The onset of an
endotherm at about 500°C, after four hours of hydration, is due to the
Ca(OH) formed from the hydration of the C S phase. A small endotherm
2
3
registered below 500°C may be due to the chemisorbed water on free lime
or to the finely divided Ca(OH) . The endothermal effect at 800°C is caused
2
by the decomposition of calcium carbonate.
The DTA of low alkali and high alkali cements show interesting

features.Bensted [68] carried out DTA investigations of low and high alkali
cements hydrated from a few minutes to one year. The hydration sequence
(with respect to gypsum component) proceeded as follows: hemihydrate →
dihydrate of gypsum → ettringite → monosulfate/C AH . These reactions
4
13
did not proceed to completion at any one stage. Syngenite, appearing at very
early stages, became depleted through reaction with sulfate to form ettringite.