Using of the ground granulated blast furnace slag as the substitutes of cement mixtures is increasing. This is used about 40% as the substitutes commercially but recently there are several tests used the maximum 80%. When used as the substitutes of cement mixtures which can improve the durability, the watertightness and the sulfate resistance but there is reduced the concrete quality due to early strength reduction and the drying shrinkage increase. Therefore it is very important to make the curing environment which helps early hydration activation vigorously. In this study, the hydrate characteristics replacing the maximum 80% of ordinary portland cement with blast furnace slag powder and using the alkali activator to increase early hydration activation were considered and analyzed by the XRD, TG-DTA, SEM, From these tests results, the use of the alkali activator had the increase of the calcium hydroxide according to ages and the reach time reduced about 1 min. and 9 min. for the first and second peak respectively. The effects on the hydrate characteristics of using the alkali activator were not significant.