Pozzolanic reactivity of mesoporous amorphous rice husk ash in portlandite solution

•The higher the SSA of RHA, the more the pozzolanic reactivity.•The pozzolanic reactivity of SF is higher than that of RHA.•An appropriate criterion suitable for comparing their pozzolanic reactivity is proposed. Pozzolanic reactivity of rice husk ashes (RHA) and silica fume (SF) was assessed by measuring electrical conductivity, concentration of silicon, calcium ions and pozzolanic reaction product in saturated portlandite solution at 40°C. The differences in surface structure, alkali content and possible difference in nature of dissolved Si between RHA and SF are important parameters influencing their Ca ion adsorption, dissolution–precipitation process, and hence electrical conductivity of portlandite-pozzolan suspensions. The variation in electrical conductivity after 2min or 210min of hydration of portlandite-pozzolan suspensions is not appropriate to compare the pozzolanic reactivity of RHA and SF which possess different Ca ion adsorption capacity. The electrical conductivity reduction rate of portlandite-pozzolan suspensions between 30 and 210min is a suitable parameter for comparing the pozzolanic reactivity of RHA and SF. SF possesses a higher pozzolanic reactivity than RHA. The higher the specific surface area of RHA, the more the pozzolanic reactivity.