Fly ash is a fine powdery particle collected from the unit operations of coal combustion furnaces in thermal power plants. Retained fly ash at bottom of hopper has been mixed with water and dumped in lagoons in form of slurry as pond ash (PA) or lagoon ash. Ground Granulated Blast Furnace Slag (GGBS) is a by-product obtained from steel industry. In this study, three phase of concrete specimens were prepared. In first phase, the specimens were prepared using 100% cement with various water-to-cementitious ratios. In second phase, specimens were prepared with varying water-tocementitious ratios and PA contents ranging from 0 to 20%. Finally, the third phase, specimens were prepared to determine the optimal PA content, with GGBS ranging from 0 to 25%. The mechanical and rheological properties of different proportions of PA and GGBS have been experimentally investigated at 28 days. In addition, the flow ability and packing density of different proportions of PA and GGBS various mixes were tested. The test results revealed that combination of PA and GGBS up to 27% would enhance the fresh and harden properties of cementitious material. The rheological behaviour of optimal PA and GGBS concrete were tested at 28 days using scanning electron microscope (SEM). The results confirmed that the addition of PA and GGBS resulting in a denser, less porous, and more compact CSH microstructure in concrete.