Silica Fume is a very reactive pozzolana because of its chemical and physical properties. Pozzolona reacts with the Calcium Hydroxide which is by product of the hydration reaction to form compounds having cementitious properties. Hence when silica fume is not used, the calcium hydroxide (Formed after hydration reaction) remains as it is which does not have any cementitious properties. By adding Silica fume, calcium hydorxide is converted to cementitious products. Solution Silica Fume is a very reactive pozzolana because of its chemical and physical properties. Pozzolona reacts with the Calcium Hydroxide which is by product of the hydration reaction to form compounds having cementitious properties. Hence when silica fume is not used, the calcium hydroxide (Formed after hydration reaction) remains as it is which does not have any cementitious properties. By adding Silica fume, calcium hydorxide is converted to cementitious products..