This paper reports on the generation, properties, and spatial variability of the graded layer in functionally graded concrete (FGC) made using 20, 35, and 50% ground granulated blast furnace slag (GGBFS) as partial replacement of ordinary Portland cement (OPC). Two different layers of concrete, one being OPC based normal concrete and the other being GGBFS blended concrete were used to prepare the FGC specimens. The control concrete specimens were made with a single layer OPC or GGBFS concrete. Excluding the control concrete samples, three types of double-layered cube samples of 150 mm thick were prepared in various combinations of the thickness of constituent layers. These double-layered combinations were 25 mm + 125 mm, 75 mm + 75 mm, and 125 mm + 25 mm thick containing OPC concrete and GGBFS blended concrete respectively. The properties like compressive strength, the interfacial bond between two constituent layers, modulus of elasticity, micro-structure, and graded layer were examined. The formation, thickness, and properties of the graded layer at the interface of constituent layers were evaluated using elastic equations and microstructural studies like scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The investigation was conducted by applying the load parallel and perpendicular to the interface plane between constituent layers. The results showed that the properties of concrete improved in FGC compared to control. Formation of graded layer added value to the FGC. Among the specimens of various thickness combinations, the samples with 75 mm OPC concrete and 75 mm GGBFS concrete showed the best results when the load was applied parallel to the graded layer. Finally, environmental benefit in terms of embodied CO₂, energy and cost benefit of the different mix proportion concrete are analysed. Further the effectiveness of FGC is also reported which has huge potential to be used in concrete structure elements.