Page 284 - Handbook of Thermal Analysis of Construction Materials

Section 3.0 - Tricalcium Aluminate-Gypsum System 265

The rate of hydration of C AF is retarded in the presence of
4
[8]
superplasticizers. In an investigation involving conduction calorimetry,
2-
Gagne [10] found that, in the presence of SNF and SNF + SO , the hydration
4
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 , C AH , and C AH 8
10
13
2
4
which eventually convert to the stable cubic aluminum hydrate, C AH . In
3
6
the presence of 2–4% SMF, SNF, or modified lignosulfonate, the DTA
results have shown that the degree of conversion of CAH and C AH 8
2
10
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 A-
3
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 A and gypsum, the type of sulfate, and the temperature. There is,
3
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 A + 25%
3
[1]
gypsum may be followed by conduction calorimetry (Fig. 4). 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 A-ettringite
3
reaction to produce low sulfoaluminate may be caused by the adsorption of
SMF on the hydrating C A surface.
3
[3]
Slanicka used XRD and DTA techniques to follow the hydration
of C A-gypsum mixtures containing 1–4% SMF. The mixture was hy-
3
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