This work aims at bridging the efficiency of ball milling of granulated blast furnace slag (GBFS) to the structural and mechanical properties of slag cement pastes. Both conventional and high energy milling of GBFS are considered with a milling duration varied between 1 and 10 h. X-ray diffraction, infra-red spectroscopy, granulometry analysis and scanning electron microscopy are used to draw the main lines of structural and Morphological changes occurring during milling. Cement pastes formulated using 45% of GBFS in substitution are Characterized. Workability, X-ray diffraction analysis, differential scanning calorimetry and compressive testing ate performed to analyse main structural changes and reactions driven by the presence of milled GBFS as well as its direct consequence on the mechanical strength of slag cement pastes. Slag milling indicates the superior efficiency of high-energy milling, which allows a maximum slag finesse of 1.79 m(2)/g after 3 h of milling. Major structural changes occur during the first 3 h of high energy milling while conventional milling does not induce any remarkable trend. These changes concern amorphisation of the bulk structure in addition to the fracturing and agglomeration of slag particles. Workability of slag cement pastes is remarkably improved when using 1 h of high-energy slag milling. This result is consistent with slag finesse trend with respect to milling time and with the improvement of GBFS reactivity. The substitution of 45% of cement (CEM I 52.5) by GBFS is only beneficial at the condition of performing high-energy milling for at least 1 h. (C) 2016 Elsevier B.V. All rights reserved.