To reuse concrete returned from job sites, the method to remain the fresh properties of the returned concrete for a long time was studied by stopping cement hydration temporarily and adding an activator to the concrete next day to reuse in the same manner as an ordinary concrete. At the same time of adding an stabilizer, a superplasticizer was added to obtain the consistency with slump of 22cm or so. As a result, the slump of approximately 18cm was obtained next day. The reactivated concrete exhibits equal bleeding, compressive strength, freeze resistance, and change in length to ordinary concrete. The setting time of returned concrete reactivated retards as stabilized concrete retards. However, the setting time of the reactivated concrete mixed with returned concrete and new concrete becomes equal to that of ordinary concrete.
