This research aims to study the morphological properties of fly ash based geopolymer paste as a repair material when applied on ordinary Portland cement (OPC) concrete substrate. The first part of this study investigates the optimal mix design of FA-based geopolymer paste with various NaOH concentrations of 8, 10, 12, and 14 molar in term of morphological analysis, which were used later as a repair material. The second part studies the morphology between the geopolymer repair material and OPC substrate concrete. The results showed that 12M revealed the best where there are no voids, cracks or unreacter fly ash occur on the sample. The results also shows that 12M sample highly irregular with no visible plane fracture surfaces revealed good bonding between geopolymer material and conrete substrate.