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Reutilising waste glass powder (WGP) could be a promising alternative to supplementary cementitious materials (SCMs) because of its availability and carbon footprint reduction, especially in Hong Kong. However, concerns about the adverse effects on hydration and early strength have been raised and might limit concrete application. A wet grinding method was applied to convert WGP into ultra-fine particles, which could address these concerns and promote the application of WG as an alternative local SCM with a low carbon footprint. This study investigated the effect of wet grinding on WGP's physicochemical properties and how wet-ground WGP modified early hydration kinetics, mechanical performance, and microstructure of cement paste. The results indicated that ultra-fine WG was produced with the D 50 of 500 nm and modified surface composition. Unlike micro-sized WGP, introducing ultra-fine WGP increased the 1-d strength by 50% and the 28-d strength by 5% of cement paste. The improvement of early strength could be attributed to three reasons. Firstly, the dissolution of ultra-fine WGP increased pH in the pore solution of the cement paste, which accelerated the hydration of C 3 A and C 3 S and promoted the precipitation of ettringite and the formation of C-S-H. Secondly, the submicron particles provided nucleation sites for C-S-H precipitation and increased the pozzolanic reactivity. Lastly, ultra-fine WGP could also effectively fill the pores in the cement paste, leading to a denser microstructure. According to the result, the reactivity of WG could be enhanced through wet-grinding and finely ground WGP could substitute reactive but costly binders, such as cement and silica fume. • Ultra-fine WG with the D 50 of 500 nm was produced through wet-grinding treatment, which modified surface composition of WGP. • Unlike micro-sized WGP, ultra-fine WGP increased the 1-d strength by 50% and the 28-d strength by 5% of cement paste. • Increased pH in the pore solution, nucleation sites for C-S-H precipitation and denser microstructure improved early strength. [ABSTRACT FROM AUTHOR]