In order to investigate the contact-hardening mechanism of the mortar component of demolished concrete, the mortars were separated and ground. One group of the power was directly molded at a certain compression, and the other group was molded after hydro-thermal synthesis at the temperature of 120 ℃ with a water solid ration of 5 by adding ground lime and quartz sand to adjust Ca/Si morel ratio. The apparent density, compressive strength, flexural strength and softening coefficient of the molded specimens were measured immediately. The results have shown that the ground mortar has a contact-hardening property to some extent and after hydro-thermal reaction, this property is obviously improved. The reacting production is amorphous silicate calcium hydration by XRD analysis. The maximum flexible strength and compressive strength are up to 5.7 MPa and 16.8 MPa respectively, and the specimen has a certain water resistance property, of which the softening coefficient can be 0.7. The property of contact-hardening is affected by the styles of the coarse aggregates of the demolished concretes and the constant time of temperature.