Page 288 - Handbook of Thermal Analysis of Construction Materials

Section 3.0 - Tricalcium Aluminate-Gypsum System 267

presence of SMF, but, at 6 hours, there is more ettringite in this mixture than
in the reference sample. More monosulfate is found at 6 hours in the
reference than in the sample with 4% SMF. The rates of these reactions are
dependent on the dosages of the admixture. The lower amounts of
monosulfate formed at 6 hours and 48 hours in the presence of SMF versus
reference samples suggest that the conversion of ettringite to the monosulfate
form is retarded by SMF.

Table 1. Amounts of Ettringite and Monosulfate Hydrate Formed in the
C A-CaSO •2H O-H O System in the Presence of SMF Superplasticizer
3 4 2 2

Ettringite Monosulfate Hydrate
Hydration
Time (Hrs) Reference SMF (4%) Reference SMF (4%)

2 28.3 —— 0 ——
6 17.6 24.4 5.5 3.5

24 12.3 —— 13.2 ——
48 10.0 18.8 15.0 9.9

Massazza and Costa [11] applied the DSC, XRD, and conduction
calorimetric techniques to investigate the effect of a SNF type of
superplasticizer on the hydration of C A-gypsum mixtures. The results
3
indicated that SNF retards the formation of ettringite and its conversion to
the monosulfate form.
DTG technique has been adopted for the investigation of the C A-
3
Ca(OH) -gypsum system containing SNF hydrated for different periods. [7]
2
It was found that at a dosage of 0.6% SNF by weight of C A, the rate of
3
ettringite formation and its conversion to monosulfate was not affected
(Fig. 5). The peak at 130°C is attributed to ettringite and that at 160°C to
gypsum. It can be seen that the intensities of these peaks are of the same
magnitude in the reference as well as in the mixtures containing SNF.
The conversion of ettringite to monosulfate is identified by a peak at
220–270°C. The intensity of the peak remains the same in the reference and
that containing the superplasticizer. Conduction calorimetric studies con-
firmed these observations.