The concrete, which is the most common material used after water, is associated with emissions of large amounts of CO2 related to the Portland cement which is responsible for a total 5–8% of the global CO2. The manufacture of hybrid concrete elements allows the use of the cement to be optimized and minimized by replacing it with secondary cementitious materials. Replacing the cement with secondary cementitious materials can help in reduction of CO2 footprint but unfortunately it may result in slower strength development and low durability of the produced concrete. This study investigates the potential of casting simultaneously vertically layered prismatic elements composed of an external durability layer and an internal ecological concrete layer to minimize the cement usage. Interfacial transition zone (ITZ) formed between the two casted concrete is the key factor that determines the bond strength and durability of the structure. In this study bond behavior and micro – properties of the wet-on-wet casting interface of ultra–high–performance fiber reinforcement concrete (UHPFRC) – blast furnace slag concrete (BFSC) is investigated. The investigation includes flexural bond strength test and backscattered electron microscope (BSE) for analyses of micro – properties. The flexural strength test showed a good bond formed between UHPFRC and BFSC casted on wet-on-wet. The microstructural investigation confirmed a dense zone in the interface where the porosity and phase composition change gradually from the inner layer to the outer layer of the hybrid concrete.