The presence of titanium in Ground Granulated Blast-furnace Slags (GGBS) has been suspected to modify cement properties. This study provides the first evidence of a relation between the TiO 2 content of slags and the mechanical properties of mortars based on slag cements. It is observed that only the slags containing less than 1%TiO 2 show a compressive strength at 28 days that remains within the 52.5 MPa norm with CEM III cements complying with the European Standard NF EN 197-1. The structural origin of this chemical dependence of the performance of cements is investigated by determining directly the titanium speciation in various European slags by spectroscopic methods. Electron Paramagnetic Resonance indicates that about 76% of Ti in slag occurs as Ti 4+. The atomic structure around Ti was determined by Ti K-edge X-ray Absorption Near Edge Spectroscopy, which shows that Ti is mainly 5-fold coordinated in square-based pyramid geometry. Five coordinated Ti acts as network-stabilizer of the silicate network as it increases the polymerization. Requiring Ca 2+ for charge-compensation of the titanyl bond, it reduces the availability of Ca 2+ during glass alteration in a modified random model of glass structure, where Ca 2+ atoms are clustered in percolating cationic domains. As a consequence, the presence of 5-coordinated Ti results in a slower dissolution of the slag. These peculiar structural properties of titanium may explain the detrimental role of Ti above a 1% concentration, for many physical and chemical slag properties. This work provides a scientific ground for the technological acceptability of the upper limit of the Ti-content of GGBS.