Page 299 - Handbook of Thermal Analysis of Construction Materials

Section 5.0 - Cement 277

addition to the dispersion effect, the acceleration may also be related to the
higher amounts of alkali sulfates present in the cement. In another cement
containing a negligible amount of sulfate, the formation of ettringite was
retarded.

Figure 10. DTA curves for cement hydrated for different periods in the presence of
superplasticizers.

An extensive work was carried out by Uchikawa [28] on the conduc-
tion calorimetry of superplasticizers, such as SNF (NS in the figure),
lignosulfonate (LS), a co-polymer of acrylic acid with acrylic ester (PC),
and a three dimensional polycondensate product of aromatic aminosulfonic
acid with trimethyl phenol (AS) (Fig. 11). The first peak in the calorimetry
corresponds to the heat of dissolution of alite, the heat of formation of the
AFt phase, and the calcium hydroxide formation from free lime. The second
peak corresponds to the heat of hydration of alite. The admixtures were
found to accelerate the formation of the ettringite phase. At w/c ratios of 0.3
and 0.5 and a later addition of the admixture, the appearance of the second
peak was significantly delayed and the peaks were of lower intensity. Most
retardation occurred with polycarboxylic acid and amninosulfonic acid-
based admixtures (Fig. 11). DSC was used to determine the amount of lime
formed at different times. The DSC results show that the addition of
admixtures at different w/c ratios generally decreases the amounts of lime
in the presence of superplasticizers (Fig. 12).