Page 122 - Handbook of Thermal Analysis of Construction Materials

106 Chapter 3 - Formation and Hydration

In the conduction calorimetric curves of the aluminate-gypsum sys-
tem, the first stage is characterized by a peak at 30 minutes for the formation
of ettringite. In the second stage, the peak occurs at 24–48 hours when the
monosulfate phase forms by the reaction between ettringite and aluminate.
[1]
Kalousek, et al., in an extensive work on the thermal analysis of
cements concluded that both low and high aluminate sulfate forms ulti-
mately disappear and are incorporated into the C-S-H gel.
Collepardi, et al., [46] studied the effect of gypsum and calcium

hydroxide on the hydration of C A by applying DTA and isothermal
3
calorimetry. It was found that the retardation of C A hydration occurred
3
only when ettringite was formed. Almost no retardation resulted when
sodium sulfate was substituted for gypsum. With sodium sulfate addition,
no ettringite was formed. Largest retardation was observed when both
gypsum and lime were present. This effect was attributed the formation of
a colloidal sized ettringite crystals on the aluminate surface. Odler and
Abdul-Maula [47] compared the DTA and XRD methods for estimating the
low and high sulfoaluminates in hydrating systems.
A calcium fluoroaluminate of formula 11CaO•7Al O •CaX , where
3
2
2
X is a halogen, preferably fluorine, is used in combination with portland
cement to produce a regulated set cement. This cement is useful where
quick setting and early strength would be needed. DTA has been applied to
study the hydration behavior of the halogenated compound hydrated for
different times and temperatures. [54] The peak obtained at 90°C was
attributed to the loss of adsorbed water and water from alumina gel. A peak
at 300°C denoted the presence of C AH and one at 260–270°C was caused
6
3
by the conversion of AH (gibbsite) to AH. Transformation of C AH to
8
2
3
C AH and AH was characterized by a peak at 250°C. At 15°C of
3
3
6
hydration, initially CAH formed and after 2 hours C AH and C AH
10 2 8 3 6
appeared. At 3 days, hydration was completed. At 25°C, the main hydration
products were C AH , C AH , and alumina gel. The C AH phase disap-
2 8 3 6 2 8
peared after 7 days and C AH was well developed. At 30, 40, and 60°C, the
6
3
only hydrates that existed were C AH and alumina gel. In the presence of
3
6
cement, ettringite and monosulfate hydrate were identified. The rate of
strength development was rapid during the first 6 hours. Further strength
was attained at later ages. The strength in the earlier period was due mainly
to ettringite. Later, the C-S-H phase contributed to strength. It was con-
cluded that in the presence of fluroaluminate, hydration of alite in portland
cement is accelerated.
Attempts have been made to apply an evolved gas analysis tech-
nique (EGA) to investigate cement systems. In the EGA technique, a