Abstract To promote the efficient and high‐quality use of steel slag (SS), a geopolymer was innovatively developed from ground granulated blast furnace slag (GGBFS) and SS using an optimized alkali equivalent method. As SS‐based geopolymer content increases, trends in compressive strength and total admittance modulus of the GGBFS‒SS‐based geopolymer reveal distinct synergistic effects. To investigate these effects, electrochemical impedance spectroscopy, a relatively novel technique, was employed alongside other analytical methods. From a physical perspective, the synergistic effects manifest through the encapsulation of C‐S‐H gels by C‐A‐S‐H and N‐A‐S‐H gels, the filling of microdefects by C‐S‐H gels, microdefects truncation, and reinforcement by SS microaggregates. Chemically, the effects include the microaggregation of inert components and competitive activation induced by hydrated Ca(OH) 2 . However, these chemical effects have a dual nature, making it unclear whether they positively or negatively impact the binary geopolymer. Thus, the physical synergistic effects are considered predominant in the GGBFS‒SS‐based geopolymer.