Page 281 - Handbook of Thermal Analysis of Construction Materials
P. 281
262 Chapter 7 - Superplasticizing Admixtures
2.0 TRICALCIUM ALUMINATE
There is general agreement that the superplasticizers based on
sulfonated melamine formaldehyde (SMF) and sulfonated naphthalene
formaldehyde (SNF) retard the hydration of tricalcium aluminate. [1]–[3] The
influence of a superplasticizer depends on its dosage, molecular weight, the
type of cation associated with it, the water/cement ratio, and the tempera-
ture. Figure 1 compares the conduction calorimetric curves of C A hydrated
3
in the presence of 2% SMF and that without the admixture. The results show
that, within a few seconds of contact with water, a rapid rate of heat
development occurs with a peak at about 8–9 minutes for the sample with
no admixture. [4][5] In the specimen containing SMF, the total amount of heat
generated in the first 30 minutes and the amplitude of the peak are lower in
comparison with that without SMF.
DSC curves demonstrate that C A hydration is retarded in the
3
presence of SMF. It is also seen from the DSC curves (Fig. 2) that the
hydration of C A to the hexagonal phases, as well as the conversion of the
3
hexagonal to the cubic phase, are retarded at SMF dosages of 1, 2, and 4%. [1]
At 1 day, C A containing 0 or 4% SMF shows a large endothermal peak at
3
about 320–340°C for the presence of the cubic phase. The corresponding
peak for the samples containing 2 or 4% SMF is of lower amplitude. The
endothermic peak at about 170–185°C is indicative of the presence of the
hexagonal phase. It is evident that higher amounts of SMF retard the
hydration of C A and the conversion of the hexagonal to the cubic phase
3
[6]
more efficiently than the lower dosages of this admixture. There has been
some evidence that the SNF admixture does not retard the hydration of C A
3
as efficiently as SMF.
[7]
Collepardi, et al., applied DTA and XRD to the study of the
hydration of C A + Ca(OH) mixtures containing 0.6% SNF. There were no
2
3
substantial changes in the rate of hydration of this mixture in the presence
of SNF.
The adsorption and dispersion effects of superplasticizers on the
aluminate phase have been studied. [1][5] Adsorption of SMF on C A occurs
3
as soon as the solution comes into contact with the solid. The rate and the
amount of adsorption on C A far exceeds that on the C S phase or cement
3
3
(Fig. 3). [1][5] Even the hexagonal phase adsorbs large amounts of SMF.
Adsorption is irreversible, indicating that a chemical interaction occurs
between the hydrating C A and SMF. Thus, the retardation of C A may be
3 3
explained by adsorption of SMF on the hydrating C A surface. Similar
3