The results of a study on the early hydration process of pastes composed of ground granulated blast furnace slag and fly ash from fluidized bed combustion of brown coal are presented here. The results of the tests concerning setting time, hydration processes (spectroscopy, calorimetry and thermogravimetry) and mechanical strength confirm that the hydration process occurs and solid microstructures and hydration products (hydrated calcium silicates and aluminosilicates, sulphoaluminates and calcium hydroxide) are formed. The presence of calcium carbonate was confirmed. Increasing the amount of fly ash in pastes intensifies the process of early hydration, thereby accelerating the initial setting time and increasing the amount of water bound in hydration products. The early compressive strength is also improved. For example, the specimen containing the highest amount of fly ash showed a decrease in the initial setting time by about 20% and an increase in the 2-day compressive strength by 22%, compared to the specimen containing the lowest amount of fly ash.