Slag-blended cement is a sustainable binder for building materials. However, when slag blends are mixed with ordinary Portland cement (OPC), the initial hydration reaction takes longer time, thus limiting its use. This research focuses on the characteristics of ground-granulated blast furnace slags from various sources based on their chemical and physical properties to assess their reactivity. Furthermore, to improve its early age reactivity, a mechanical activation was performed on slags to enhance its specific surface area and particle size distribution. The hydration and crystallinity degrees were investigated with calorimetry and X-ray diffraction tests. Moreover, the slag-cement binder strength development in the initial 72 h were assessed with the help of the ultrasonic measuring device. The results showed that the mechanical treatment significantly increased the reactivity of the slag already after 24 h, leading to a higher degree of hydration and modulus of elasticity. It was observed that mechanical activation of slag was also efficient from the point of view of strength development.