This article aims to study the pozzolanic activity of silica fume by the reaction with Ca(OH)₂ (CH) at room temperature. The pozzolana includes the natural or artificial type containing amorphous alumina or silica structure. It can be added or substituted with OPC in concrete that can react with CH liberated during the hydration of cement forming calcium silicate hydrate (C-S-H) gel, the main binding agent in the matrix. This hydrated gel is deposited in some of the open pores of concrete then the pores are diminished and improved its durability in sulphate, chloride and seawaters. The pozzolanic activity of silica fume (SF) was investigated by the determination of free lime, free silica, and combined water contents as well as the hydration products were investigated by the aid of DTA, XRD, FT-IR and SEM, techniques. The results of chemical reaction are in good agreement with those determined by the different equipment. It was also concluded that the reaction is complete for all mixes in the first 28 days.