Page 121 - Handbook of Thermal Analysis of Construction Materials

104 Chapter 3 - Formation and Hydration

6.3 Calcium Aluminates Plus Gypsum

Tricalcium Aluminate Plus Gypsum. Several types of com-
pounds are formed in the hydration of aluminate phases with gypsum. The
peak effects are affected depending on the w/s ratio, particle size of the
aluminate, temperature, relative proportions of gypsum, etc. Compounds
detected in this system include, C AH , C AH , C AH , low sulfoaluminate
2 8 4 13 3 6
hydrate, high sulfoaluminate hydrate, and their solid solutions. Feldman
and Ramachandran [44] examined the sequence of products formed by
hydrating C A mixed with 0, 0.25, 2.5, 10, and 20% gypsum and hydrated
3
at 2, 12, 23, and 52°C for various periods. DTA, XRD, and length change
measurement techniques were also adopted. The endothermic peak ap-
pearing at about 170–200°C was assigned to the high sulfoaluminate
hydrate, the endotherms in the range 140–180°C and 240–285°C were
assigned to the hexagonal C AH and C AH compounds, respectively,
2 8 4 13
and two endothermal effects at 290–300°C and 460–500°C, to the cubic
C AH phase. The compound C AH may be associated with the low
3 6 4 13
sulfate aluminate solid solution. Thermograms revealed that the hexagonal
hydroaluminates are formed immediately on exposure of the C A + gypsum
3
to water. The formation of hexagonal hydroaluminates is retarded by
gypsum. The conversion of the hexagonal to the cubic form is also delayed.
In general the rate of hydration of C A and its reaction with gypsum are
3
increased with temperature. As the temperature is increased higher amounts
of gypsum are required to suppress the formation and prevention of
conversion of the hexagonal phases.
As a part of an investigation of the effect of calcium carbonate on
the C A-gypsum system, Ramachandran and Zhang Chun-Mei [45] studied
3
the hydration of the C A-gypsum system. In Fig. 25, DSC curves for the
3
C A + 12.5% gypsum mixture hydrated at 20°C are shown. Gypsum is
3
almost consumed in 30 minutes, as is evident from the intensity of the
peak at 130°C. Ettringite is characterized by an endothermic effect at
165–180°C. The intensity of the peak for ettringite increases after 30
minutes. The low sulfate hydrate, identifiable by an endothermal effect at
280–300°C, is present at 1 hour and later. With the addition of 25% gypsum,
ettringite is detected at 5 minutes and its amount increases with time
until 1–2 hours. Gypsum is mostly consumed within an hour, but at 12.5%
gypsum, it is consumed in 30 minutes (Fig. 26). [45]
Thermogravimetric technique has also been applied to follow the
formation of sulfoaluminate products hydrated in a mixture containing
tricalcium aluminate and gypsum. [17]