A clinker-free binder for making sustainable concrete was developed using cement kiln dust (CKD) and Class F fly ash. The CKD-activated fly ash binder developed a compressive strength of approximately 30 MPa after 48 h of elevated temperature curing. The mineralogical composition of the developed clinker-free binder was determined with the help of thermogravimetric and X-ray analysis. Microstructure of the hardened binder was examined to identify phases using a scanning electron microscope equipped with an energy dispersive X-ray detector. Various reaction products, such as calcium aluminosilicate hydrate (C-A-S-H), ettringite (Aft), and calcium silicate hydrate, were identified in the hardened clinker-free binder. The major contribution to strength development is attributed to the C-A-S-H gel, which was found extensively as a ground mass in the hardened CKD-activated fly ash system.