Civil Engineering Reference
In-Depth Information
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
11
A
7
•CaF
2
. At low SO
3
/Al
2
O
3
ratios the barrier
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
3
AH
6
.
In the ternary system, C
11
A
7
•CaF
2
- CaSO
4
•2H
2
O - Ca(OH)
2
acceleration occurs, as a result of the Ca(OH)
2
addition. Large amounts of
Ca(OH)
2
cause very rapid formation of ettringite. They coat the unhydrated
particles and significantly inhibit the hydration process. In general, an
increase in SO
3
concentration in the liquid phase decreases the handling
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
11
A
7
•CaF
2
and interferes with
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
11
A
7
•CaF
2
- C
3
S -
CaSO
4
in the presence of various additives has been reported. The concen-
tration of calcium ions in the liquid phase controls the rate of hydration of
C
3
S. Retardation of aluminate hydration is caused by excess hemihydrate
addition and C
3
S hydration is accelerated. Citric acid retards the hydration
of both C
11
A
7•
CaF
2
and C
3
S. Sodium sulfate suppresses the hydration of
C
11
A
7
•CaF
2
,
but accelerates the hydration of the C
3
S. Sodium carbonate
retards hydration of C
11
A
7
•CaF
2
, but, in the presence of sodium sulfate, acts
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
11
A
7
•CaF
2
.
Hydration proceeds gradually if calcium ion is supplied from calcium
hydroxide and carboxylic acid is sufficiently depleted. Surface active
agents, e.g.,
-naphthalene sulfonic acid, influence the morphology of
the hydrates, but do not appear to significantly affect the hydration
process.
β
