Page 170 - Handbook of Thermal Analysis of Construction Materials

154 Chapter 4 - Introduction to Concrete Admixtures

The retarding effect on tricalcium silicate hydration is almost
completely eliminated in the presence of C A or NC A + gypsum. Adsorp-
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tion of TEA on ettringite may remove the admixture to facilitate the
hydration of tricalcium silicate. Figure 4 shows that the amount of TEA in
solution in contact with ettringite decreases with time. [44] Another possibil-
ity is that TEA chelates metallic ions in the highly alkaline medium. It can
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be speculated that the Fe in cement, precipitating during the hydration of
cement, coats the silicates and aluminates and slows down the reaction. [45]
The mono- and diethanolamines also affect the hydration of tricalcium
silicate.

Figure 4. Adsorption of TEA by ettringite.

The initial and final setting characteristics of portland cement
treated with 0–0.5% triethanolamine have been reported. [46] With up to
0.05% triethanolamine, the initial setting time is retarded slightly, and there
is an accompanying slight extension in the induction period of the hydration
of C S. At 0.1 and 0.5% triethanolamine, however, rapid setting occurs. The
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rapid setting may be associated with the accelerated formation of ettringite.