In the field of construction materials and particularly in concrete, cement is considered as a key element since it generates a strong and durable material through a simple hydration process. However, for many reasons (mainly economic and environmental) researchers are trying to find a new material that could replace cement or at least part of it as a binding agent in concrete. Regarding this issue, cement replacement materials like fly ash and slag have taken the lead during the last few decades. These materials have characteristics similar to cement but when it comes to the hydration they first need to be activated. For that, many studies were performed to find a way to activate these materials without using cement to produce what is called Geopolymer Concrete. The activation is simply based on preparing an ambient similar to the one produced by cement hydration. Hence, alkali activation is widely used to activate the polymerization reaction. On the other hand, when compared to ordinary concrete, producing geopolymer concrete is still complicated since it has no standard and needs several activation steps. In this work, fly ashes that are taken from three different sources in Turkey are used to produce geopolymer concrete. The study aims to compare the effect of fly ash characteristics, particularly their chemical composition and fineness, on the activation process.