This chapter investigates the interfacial transition zones (ITZs) in modelled geopolymer concrete using micro/nanomechanical techniques and image analysis. The study compares fly ash-based geopolymer with Portland cement (PC) concrete, focusing on the properties and heterogeneity of ITZs. Modelled concrete specimens with polished aggregate and geopolymer mortar were prepared to provide neat aggregate-binder boundaries. Microstructural analysis using SEM and BSE imaging revealed that geopolymer forms a dense, gel-rich layer at the interface, while PC ITZs contain more pores and calcium hydroxide crystals. Nanoindentation and nanoscratch tests showed that geopolymer ITZs exhibit higher micromechanical properties than PC ITZs, with elastic modulus and hardness ratios of around 1.21 and 1.18, respectively. The study concludes that the strong bonding and dense microstructure of geopolymer paste contribute to its superior performance, highlighting the significance of ITZ properties in determining the overall behavior of concrete.