Page 450 - Handbook of Thermal Analysis of Construction Materials

Section 3.0 - Jet Set (Regulated Set) Cement 425

The addition of CH (Fig. 18) shifts the second and third peak to an
earlier stage. The formation of the calcium aluminate hydrates are acceler-
ated, as is the conversion of the hexagonal hydrate into C AH .
3
6
Anhydrite addition (the rate of dissolution is slower than that of
other calcium sulfates) shifts the second peak slightly toward a later stage
(Fig. 18). The third peak shifts to an earlier stage as the SO /Al O ratio
3 2 3
increases. The third peak corresponds to the formation of
monosulfoaluminate and partly to the continued hydration of C A •CaF 2
7
11
forming ettringite on fresh surfaces of C A •CaF . The conversion of
2
7
11
calcium aluminate hydrate provides a source of water for the hydration.
In the case of the gypsum addition (Fig. 18), the heat evolution
curves are significantly different. The second peak appears at about 15
minutes and the third peak at about six hours after mixing with water. The
third peak shifts to a later stage as the amount of gypsum increases. The
second peak is due to the formation of ettringite (XRD evidence supports).
Hydration of the remaining C A •CaF is accelerated when the
2
11
7
third peak appears. Monosulfoaluminate and C AH form during this
6
3
period at a low SO /Al O ratio (0.5). At higher SO /Al O ratios, the third
3
3
2
3
2
3
peak becomes smaller and the monosulfoaluminate phase appears later. At
7 days, ettringite and the monosulfate hydrate coexist.
The rate of heat evolution for the hemihydrate addition is large, and
the peak positions correspond to very early times (Fig. 18). This is a result
of the relatively high solubility and rate of dissolution of calcium sulfate
hemihydrate. In the case of the high SO /Al O ratio (1.5), the second peak
2
3
3
corresponding to the formation of ettringite is higher; the third peak is not
observed. XRD results confirm the co-existence of ettringite and gypsum.
The handling time of regulated set cement concrete depends on
several factors. The addition of carboxylic acid, for example, lowers the
++
solubility of the Ca ion in the liquid phase, and effectively retards
hydration of C A •CaF . The thermal analysis results show that increasing
11 7 2
the SO concentration in the liquid phase retards the handling time of
3
regulated set cement concrete. The solubility and rate of dissolution of
calcium sulfate, therefore, affects the regulation of the handling time.
Excess addition of calcium hydroxide retards the hydration of C A •CaF 2
11
7
and interferes with the hardening process.
The hydration of mixtures of C A •CaF , C S, and CaSO with
11 7 2 3 4
various additives has been investigated. [15] The molar ratio of C A •CaF 2
11
7
to C S was 1.0 to 15.4. The SO /Al O molar ratio was 1.0. Hydration
3 3 2 3
occurred at 20°C with a water/solid ratio = 0.60. The additives included
calcium sulfate hemihydrate, sodium sulfate, sodium carbonate, and citric