The rapid growth of wind power has led to an increasing production of wind turbine blades, which have a service life of 20–25 years. The main component of these blades is glass fiber reinforced polymer (GFRP), and repurposing GFRP in construction materials offers the most feasible large-scale recycling solution. However, the organic components in GFRP powder do not participate in pozzolanic reactions, making the powder harder to grind and resulting in larger particle sizes, which reduces its reactivity as a supplementary cementitious material. To address this, 550 ℃ activation effectively removes the inorganic components (550 GP), but the large particle size still limits its reactivity, remaining lower than fly ash. This study investigates the effect of mechanical grinding on enhancing the reactivity of 550 GP powder in mortar and concrete samples. The results show that mechanical grinding increases the reactivity of 550GP powder, and the reactivity improves with longer activation times. When mechanically activated for 15 min (550 GM3) or longer (550 GM4 and 550 GM5), the particle size becomes comparable to or smaller than that of fly ash, and its reactivity approaches or even exceeds that of fly ash. In C30, C40, and C50 concrete, replacing cement with mechanically activated 550 GM3 improves compressive strength at 3, 7, and 28 days compared to concrete with fly ash. These findings highlight that mechanically activated 550GP powder has the potential to be used as a supplementary cementitious material, providing a viable recycling pathway for waste wind turbine blades.