Civil Engineering Reference
In-Depth Information
The rate of hydration of C
4
AF is retarded in the presence of
superplasticizers.
[8]
In an investigation involving conduction calorimetry,
Gagne
[10]
found that, in the presence of SNF and SNF + SO
4
2-
, the hydration
of tetracalcium aluminoferrite was retarded during the 60 minutes of
hydration.
Monocalcium aluminate, a major component of high alumina
cement, hydrates to metastable products CAH
10
, C
4
AH
13
, and C
2
AH
8
which eventually convert to the stable cubic aluminum hydrate, C
3
AH
6
. In
the presence of 2-4% SMF, SNF, or modified lignosulfonate, the DTA
results have shown that the degree of conversion of CAH
10
and C
2
AH
8
phases to the cubic phase is marginally retarded.
[9]
3.0
TRICALCIUM ALUMINATE-GYPSUM
SYSTEM
The role of the superplasticizer on the rate of hydration of C
3
A-
gypsum mixtures is not clear. All the possibilities, viz., retardation, accel-
eration, and ineffectiveness have been reported. The variation in results
reported from different sources may be due to the differing conditions of
hydration such as the w/s ratio, the type of superplasticizer, its molecular
weight, the type of cation associated with it, the proportion and particle size
of the C
3
A and gypsum, the type of sulfate, and the temperature. There is,
however, general agreement that the conversion of ettringite to monosulfate
is retarded in the presence of a superplasticizer.
The effect of 0, 1, 2, and 4% SMF on the hydration of C
3
A + 25%
gypsum may be followed by conduction calorimetry (Fig. 4).
[1]
An exother-
mic peak appears at 20 hours with 1% SMF, at about 23 hours with 2% SMF,
and at about 15 hours with 4% SMF. A continuous hump is registered for
the sample containing no admixture. The retardation effect of C
3
A-ettringite
reaction to produce low sulfoaluminate may be caused by the adsorption of
SMF on the hydrating C
3
A surface.
Slanicka
[3]
used XRD and DTA techniques to follow the hydration
of C
3
A-gypsum mixtures containing 1-4% SMF. The mixture was hy-
drated for 2 hours to 24 hours, and the products were estimated by DTA and
XRD. Table 1 shows the amounts of ettringite and monosulfate hydrate
formed at different times of hydration in the presence of 4% SMF. It is
obvious that the formation of ettringite is almost nil at 2 hours in the
