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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
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                              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
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                              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
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                                                                               [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
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                              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
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                                          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
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                              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
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