Eco-cement produced from waste concrete was proved to be feasible in early research. The seed crystal of ground granulated blast furnace slag (GGBS) was utilized in this research to lower the sintering temperature of eco-cement clinker. The mineral compositions of clinker with GGBS seed crystal was analyzed by X-ray diffraction (XRD), and the mechanical properties of eco-cement with GGBS seed crystal was also tested. Four main cement minerals were all observed in eco-cement clinker and the compressive strength of the eco-cement pastes can approach to about 66 MPa at 28 curing days. The results showed that GGBS seed crystal was favourable for the formation of cement minerals at a lower temperature. It can help reduce by about 50~100°C for the sintering process of cement clinker. Content of GGBS seed crystal should better be in the rage of 5%~8%, and the suitable sintering temperature should be 1350°C.