:Profiles of Ca2＋ ion concentration changes in the capillary water of the low-cement castable containing organic deflocculant during the setting show tree peculiarities, which were（1）initial concentration value,（2）dissolution rates and（3）thresholds of the setting. The author considered that they determine the setting time. Hence, the Ca2＋ ion concentration changes were analyzed and investigated relationship between the tree peculiarities and the setting time, when some well-known factors for setting time variation, temperature, kinds of the alumina cement, retarder and deflocculant, were varied. The setting time variations was able to explain by the three peculiarities. By temperature change, the dissolution rate mainly altered. By cement bland modifying, the initial value and the dissolution rates changed. Retarder and deflocculant influenced to both the dissolution rate and the threshold. A dissolution rate was not constant and delayed at middle stage conceivable by nation of alumina cement.

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Hydrate of high alumina cement with silica fume was cured for 22 days at room temperature (23℃±0.5℃). XRD analyses revealed the existence of stratlingite (C2ASH8), CAH10 and gibbsite (AH3). DTA analyses showed an endothermic peak of C2ASH8 and AH3-gel. As disclosed by EPMA analyses of the hydrated products, in most situations, they were chemically heterogeneous with a considerable excess of SiO2 content as compared with theoretical composition of C2ASH8.

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