Abstract
The false setting of cement caused by the aeration was studied from the analysis of the cement characters, its hydraulic reactivity, its hydration reaction, the paste rheological properties and the relationship among them. The aeration of cement was simulated by the treatment of cement exposing under an environment of temperature, 20°C and relative humidity, 80%. With a longer treatment, cement showed larger amounts of ignition loss. Hydration products with particle sizes under 0.3 μ m were formed on cement particles and the amount was increased with the increase of treatment time. Although the treatment did not change the Blaine specific surface area, it changed the BET specific surface area as much as 1.4 times higher during seven days of treatment. The treatment significantly decreased the hydration reactivity of interstitial phases of cements. The treatment longer than certain period of time brought a false setting in cements, which was not occurred in fresh cement paste. When aerated cements was mixed with water, sulfate ion was supplied to liquid phase from alkaline sulfates or calcium sulfate hemihydrate. However, the sulfate ion was not consumed by the ettringite formation reaction because of the low reactivity of interstitial phases. As the results of poor consumption of sulfate ion, gypsum crystals deposited from liquid phase to form large automorphic plate crystals and finally cement paste was thought to show the false setting, i.e. plaster set.
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