Dehydration ettringite

Fig. 9.23 Dehydration of ettringite in isothermal conditions in air under normal pressure, (according to [103])...

An addition of 0.1% CLS may extend the initial and final setting times of cement mortar by two and three hours, respectively. The influence of 0.3% CLS on the hydration of cement is shown in Fig. 9. Thermograms indicate that the reference cement containing no admixture exhibits a broad endothermal peak below 200°C, representing the formation of both ettringite and C-S-H phase. These peaks increase in intensity as the hydration period is increased. The effect between 450 and 500°C is caused by the dehydration of Ca(OH)2 and its intensity indicates the extent to which the hydration of the C3S component has progressed. The cement hydration, in the presence of lignosulfonate, is retarded as seen by the lower intensity of the Ca(OH)2 decomposition peak. The low temperature effect below 300°C in the presence of CLS is not sharp as that obtained in the reference sample. 

In normal portland cement, hydration leads to the formation of ettringite. In many instances, the formation of ettringite leads to expansion. To prevent the formation of ettringite, a gypsum-based material with 75% hemihydrate, 20% portland cement, 5% silica fume, and a superplasticizer was fabricated. The pastes were cured in water for 1 to 10 minutes and subjected to DTA. In Fig. 10, thermograms show the stepwise dehydration endothermal peaks (shown upwards) at 150° and 200°C as is typical of gypsum. There was no indication of ettringite or monosulfate, which normally are identified by peaks at 125-130° and 190-195°C. 

Figure 8.25 shows an example of a calcium sulfate cement blended with gypsum. The hydrates, ettringite and amorphous AH3 are similar to the CAC-gypsum system (Figure 8.24) but more finely intermixed. Cracks again arise from the dehydration of ettringite in the vacuum of the SEM. 

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