The present study assessed the effects of grinding on the rice husk ash (RHA) microstructure and specific surface area (SSA). The pozzolanic reactivity of RHA subjected to different grinding periods and of silica fume (SF) has been evaluated by determination of the accelerated pozzolanic strength reactivity index in mortar, the portlandite content in cement pastes and the change of electrical conductivity of saturated portlandite solution at 40°C. RHA samples contain macro- and mesopores. Grinding partly collapses the porous structure of RHA and reduces SSA. There is an optimum grinding time/fineness of RHA for maximum compressive strength. Despite having a higher SSA, RHA consumes less portlandite but induces similar mortar compressive strength compared to SF. Because of the mesoporous structure, ground RHA can absorb an amount of aqueous phase to decrease the effective water content in mixture. Moreover, calcium ions have access to internal surface of RHA particles, enhancing the pozzolanic reactivity of RHA.