Page 453 - Handbook of Thermal Analysis of Construction Materials

428 Chapter 10 - Non-Portland Rapid Setting Cements

accelerates and citric acid retards the time of appearance of the correspond-
ing peaks. The second and third peaks shifted to earlier times in proportion
to the amount of sodium sulfate added. The formation of C AH was
3
6
suppressed by the addition of sodium sulfate. The hydration of C S (peak
3
at 14 to 35 hrs) in this series is retarded compared to pure C S, but it is
3
accelerated compared to the case without Na SO . The acceleration is
2 4
dependent on the added quantity of Na SO .
2
4
Uchikawa and Tsukiyama investigated the hydration of two Jet Set
cements at 20°C. [16] The cements contained about 60% alite, 20% calcium
fluoroaluminate, 4.5% ferrite phase, and 0.8–2.0% belite. One (A) con-
tained about 2% calcium carbonate, a citric acid based retarder (0–2%) and
1% sodium sulfate. The other (B) contained 1% sodium sulfate and 2.5%
2
hemihydrate. Both cements had surface areas of about 5000 cm /g. Pastes
were prepared at water/cement ratio = 0.40. DTA curves for these pastes
(hydration period ranging from 1 hour to 7 days) are presented in Fig. 20.
The endothermic peaks at about 130°C and 280°C correspond to the
dehydration of ettringite. A small peak at about 190°C corresponds to the
dehydration of monosulfate hydrate. The results suggest that the amount of
ettringite increases with hydration and subsequently decreases due to the
conversion to monosulfate hydrate. The maximum amount of ettringite is
formed in the samples at about 6 hours. The endothermic peak for Ca(OH) 2
(1 day), suggests the hydration of alite has progressed significantly.
Calorimetry curves for the two Jet Set cements described above are
shown in Fig. 21. There are four main peaks in the heat evolution curves.
The first peak appears immediately and is due to the following: dissolution
of free lime, hydration of anhydrite and hemihydrate, and the formation of
C-A-H and monosulfate hydrate. The second peak is attributed to the
formation of ettringite, the third to the formation of monosulfate hydrate,
and the fourth peak to the formation of C-S-H. The overlap of the second
and third peaks (cement B) and the larger third peak are attributed to active
conversion of ettringite to monosulfate hydrate. The broader fourth peak
(cement B) occurred later indicating a less active formation of C-S-H gel
than for cement A.
The differences between the two cements can be summarized as
follows. The initial rate of hydration (2 hrs) in the presence of hemihydrate
is greater than for citric acid. The reverse is the case at a later stage of
hydration. Mechanical strength using citric acid is greater than for a
hemihydrate addition. This corresponds to the following: a higher degree
of hydration corresponds to a larger amount of ettringite, a smaller average
pore size, and lower porosity.