This work explored the effect of mineralogical changes of a modified basic-oxygen furnace slag (BOFS) on the properties of alkali-activated binders. The study quantified the amorphous-crystalline proportions of particles from three different size ranges. These size fractions were finely milled and activated with NaOH solutions. The effects on the reaction kinetics and the mechanical strength of the produced binders were established. The early-stage reaction kinetics were tracked, and the significance of the precursor fraction, the addition of admixtures (gypsum and plasticizer), and the liquid-to-solid ratio (L/S) on the reaction kinetics were identified. The relative variation in the amorphous content between fine and coarse fractions was 45%. These mineralogical differences interacted with the presence of admixtures and the L/S, which impacted the kinetics of the cementitious reactions and the rate of strength development of the binder. Nevertheless, the 30-day strength was similar for all specimens (58 MPa). •The granulation of BOFS altered the mineralogical proportions found in slag particles of different sizes.•The mineralogical variations impacted the kinetics and the strength development rate of alkali-activated binders.•The strength of the binders after 30 days was similar and independent of the size fraction of slag used as precursor.•The kinetics of the alkali activation can be customized by using specific size fractions of the modified BOFS as precursors.