| The magnesite was calcined at 800℃, 900℃and 1000℃for an hour respectively to prepare undercalcined magnesia. The difference between undercalcined and heavy burnt magnesia was carried out. The comparative study of undercalcined magnesia and ettringite as expansive agents to compensate shrinkage of cement paste has been mainly made in the present work. The expansive behavior of neat cement paste and cement mortar added with the two agents was experimentally examined. XRD and SEM were applied to explore the transition of ettringite to calcium monosulphoaluminate. The pore structure of cement paste and mortar added with the two agents was characterized using mercury intrusion porosimeter.The experimental results are shown following:1 The expansive properties of magnesia is greatly affected by calcination temperature. The activity of undercalcined magnesia is much higher than that of heavy-burnt one, as shown by citric acid examination. The decomposition of magnesite fired at 800℃, 900℃and 1000℃for an hour is completed.2 The early expansion of undercalcined magnesia is greater than that of heavy-burnt one. Comparable early expansion to ettringite may be achieved by addition of appropriate amount of magnesia.3 The rate of hydration and expansion of undercalcined magnesia is greatly affected by curing temperature. The expansion of undercalcined magnesia is noticeably enhanced with increasing temperature. The expansion of magnesia under restraining condition may be adjusted to be similar to that of ettringite.4 The expansion of ettringite is sensitive to curing condition, especially at early age. Lack of water may induce failure of shrinkage compensation. The drying shrinkage assistance of magnesia is better than that of ettringite.5 It is shown by XRD and SEM that the transition of AFt to AFm may occur at higher curing temperature, indicating ettringite is not suitable for using at elevated temperature.
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