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The influence of calcium sulfate on the hydration of XaO. A&03 in the presence of Ca(OH), was studied using conduction calorimetry, differential thermogravimetry, and X-ray diffraction. Sodium sulfate was also used instead of calcium sulfate. A substantial retardation of tricalcium aluminate hydration in the presence of sulfate occurs only when calcium sulfate is used and enough ettringite is formed. When ettringite disappears due to the consumption of gypsum, tricalcium aluminate hydration is renewed. Sodium sulfate does not signiticantly retard this hydration. The results confirm the hypothesis that ettringk formation is essential for coating 3 C a 0 .~, 0 3 grains and then retarding their hydration.

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