Page 451 - Handbook of Thermal Analysis of Construction Materials

426 Chapter 10 - Non-Portland Rapid Setting Cements

acid. The rate of hydration of C S increased dramatically as the rate for
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C A •CaF decreased. An excess of hemihydrate results in the excessive
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retardation of C A •CaF and the acceleration of C S hydration. Citric acid
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retards the hydration of both C A •CaF and C S. Sodium sulfate retards
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the hydration of C A •CaF and accelerates the hydration of C S. Sodium
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carbonate retards the hydration of C A •CaF , but in combination with
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sodium sulfate, the opposite effect occurs. Calcium carbonate and
superplasticizer additions have little effect on the hydration process.
Conduction calorimetry curves for the above systems are shown in
Fig. 19 (a–i). Figures 19a and b are for the C S and the C S-sulfate systems.
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For systems containing C A •CaF , there are up to five characteristic
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peaks observed as depicted in Fig. 19c. The first (immediate) is due to the
rapid hydration of C A •CaF and the formation of various calcium
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aluminate compounds. The second peak (1 to 5 hrs) is due to the formation
of hexagonal and cubic C-A-H. The third and fourth peaks are assigned to
the formation of monosulfoaluminate hydrate and ettringite. The active
hydration of C S is retarded in the presence of anhydrite (peak 5), and it
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occurs at 50 hours.
In the quaternary system containing both hemihydrate and anhy-
drite, the first peak is due to the dissolution of C S, the hydration of the
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hemihydrate, and the formation of ettringite immediately after mixing with
water. The second peak is attributed to the formation of C-A-H and
specifically C AH . The third and fourth peaks are due to the formation of
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monosulfate hydrate and newly formed ettringite. The hydration of C S
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begins after 40 hours (peak at 70 hrs). For the curve for the system
containing the largest amount of hemihydrate, the second peak corresponds
to the hydration of C S.
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The effect of citric acid addition is depicted in Fig. 19e. The first
and second peaks, corresponding to the dissolution of mineral compounds
and the formation of C-A-H, were less intense and shifted to a later stage.
The third peak, due to monosulfoaluminate and ettringite, was shifted to an
earlier stage proportionally to the amount of citric acid addition. The fourth
peak, corresponding to active C S hydration, was shifted to a later stage.
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The shift was dependent on the amount of citric acid added.
The effect of Na SO addition is illustrated in Figs. 19f, g, and h.
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Heat evolution, due to the formation of ettringite, was observed immedi-
ately in the samples containing hemihydrate. In the absence of hemihy-
drate, no heat evolution peak was observed immediately after mixing.
There is no significant difference in the heat evolution curve (after the first
peak) for hydration in the presence of Na SO except that hemihydrate
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