The reuse of waste concrete powder (WCP) in the synthesis of alkali-activated binder (AAB) composites has never been investigated. Herein, this paper reports the study about the preparation of ultrafine waste concrete powder (UWCP) and its utilization in alkali-activated slag/fly ash binders. The effects of UWCP on the mechanical properties and shrinkage behaviors of AAB were measured. With 6 wt% addition of UWCP, the compressive strengths of AMW6 sample were 45.9 MPa, 68.6 MPa, 83.1 MPa and 86.3 MPa at the ages of 1 d, 7 d, 28 d and 56 d, showing 23.05%, 17.87%, 18.21% and 15.22% higher than those of AWM0 sample at the same ages. Based on the heat release and setting time results, the roles of UWCP on the reaction kinetics were explored. Results indicate that the contact surface of solid precursors was reinforced due to the nano-size particles in UWCP, contributing to the promotion of reaction and shortening of setting time. The mineralogical composition determined by X-ray diffraction (XRD) and thermogravimetry (TG) analyses demonstrates that the UWCP did not affect the type of reaction products but change its quantity. It can be observed by mercury intrusion porosimetry (MIP) that the total porosity decreased to 8.04% and the proportion of large capillary pores with the size greater than 200 nm decreased apparently. This reflects that the pore structure of AAB paste was refined with the UWCP addition. Benefiting from these, the compressive strength was enhanced and shrinkage behaviors were mitigated significantly. This study is expected to offer a reference for the better reuse of WCP in AAB.