Page 414 - Handbook of Thermal Analysis of Construction Materials

Section 8.0 - Regulated-Set Cement 391

The addition of calcium sulfate hemihydrate (the solubility is greater
than that of the anhydrite and dihydrate) is conducive to the co-existence of
ettringite and gypsum. The ettringite and gypsum crystals are larger than
those produced in the systems initially containing anhydrite and dihydrate.
The degree of hydration is about 60% at six hours. This is maintained for an
extended period due to formation of a dense barrier layer of ettringite
surrounding unreacted C A •CaF . At low SO /Al O ratios the barrier
2
3
3
2
11
7
layer is disrupted by the partial transition from ettringite to monosulfate
hydrate several hours after mixing. This resulted in further formation of
monosulfate hydrate and cubic C AH .
6
3
In the ternary system, C A •CaF – CaSO •2H O – Ca(OH) 2
4
11
7
2
2
acceleration occurs, as a result of the Ca(OH) addition. Large amounts of
2
Ca(OH) cause very rapid formation of ettringite. They coat the unhydrated
2
particles and significantly inhibit the hydration process. In general, an
increase in SO concentration in the liquid phase decreases the handling
3
time of regulated set cement concrete. The greater the solubility and rate of
dissolution of calcium sulfate, the greater is the effect on handling time in
the presence of a suitable amount of calcium hydroxide. Excess addition of
calcium hydroxide retards the hydration of C A •CaF and interferes with
7
2
11
the hardening process.
Admixtures can be used to control the rate of hardening in regulated
set cement products. [75] The hydration of the system C A •CaF – C S –
11 7 2 3
CaSO in the presence of various additives has been reported. The concen-
4
tration of calcium ions in the liquid phase controls the rate of hydration of
C S. Retardation of aluminate hydration is caused by excess hemihydrate
3
addition and C S hydration is accelerated. Citric acid retards the hydration
3
of both C A CaF and C S. Sodium sulfate suppresses the hydration of
7•
2
11
3

C A •CaF , but accelerates the hydration of the C S. Sodium carbonate
11 7 2 3
retards hydration of C A •CaF , but, in the presence of sodium sulfate, acts
7
11
2
as an accelerator.
The addition of carboxylic acid lowers the solubility of the calcium
ion in the liquid phase and severely retards the hydration of C A •CaF .
11
2
7
Hydration proceeds gradually if calcium ion is supplied from calcium
hydroxide and carboxylic acid is sufficiently depleted. Surface active
agents, e.g., β-naphthalene sulfonic acid, influence the morphology of
the hydrates, but do not appear to significantly affect the hydration
process.