Page 511 - Handbook of Thermal Analysis of Construction Materials

Section 9.0 - Industrial Applications 483

Kovler’s investigation of a blend of calcium sulfate hemihydrate (75% by
mass), portland cement (20%), and silica fume (5%) containing 2%
superplasticizer. The hydration of hemihydrate to form gypsum crystals in
the binary cement-gypsum system can be conveniently followed by DTA
methods (Fig. 27). Two characteristic peaks for gypsum and hemihydrate
(150° and 200°C, respectively) clearly indicate the growth of the gypsum
peak and the corresponding decrease in the hemihydrate peak. The analysis
permits estimation of the amount of gypsum growth in the paste up to about
2 hours (not shown in the figure). It is thus possible to estimate the degree
of reaction from DTA. Ten percent of the reaction occurs within 2 minutes,
50% within 5 minutes, and 90% within 10 minutes. Similar results are
obtained for the ternary cement system containing silica fume. The DTA
method is found to be a useful tool in providing the basis for the explanation
of setting and hydration mechanisms influenced by the complex sequence
of reactions in a ternary blend. It was apparent that strength development
of the ternary blend was similar to that of pure gypsum. Long term
improvement in the performance was the result of a different mechanism
associated with the cement and silica fume hydration process.

Figure 27. DTA thermograms for a gypsum-portland cement mix containing 2%
superplasticizer (1 to 10 min after mixing). [21]