Abstract: Residues of co-combustion are not addressed in Taiwan standards for fly ash use in concrete. This paper presents feasibility of re-utilizing CFB co-fired fly ash (CFFA) as an alkali activator for the eco-binder of CFFA and GGBS. The optimal mixture of the binder is specimen F3S7, which reaches 31.43 MPa that is 72.4% compressive strength of OPM at 56 days. XRD and SEM showed that CFFA activated the GGBS to generate CaO, which reacted with H2O, SiO2, and Al2O3 to form C–S–H and C–A–S–H pastes. Compressive strength reduction trend for F4S6 to F7S3 started after 28 days because high SO3 content.