The pozzolanic reaction refers to the chemical process in which pozzolanic materials, such as fly ash, silica fume, and volcanic ash, react with calcium hydroxide—a byproduct of cement hydration—to form additional calcium silicate hydrate (C-S-H), the primary binding agent in concrete that contributes to its strength and durability. This reaction not only enhances the mechanical properties of concrete but also significantly improves its impermeability and resistance to chemical attack, which are crucial in severe environmental conditions. By reducing the concrete’s pore size and filling voids with C-S-H, the pozzolanic reaction decreases the material’s susceptibility to aggressive agents like chlorides and sulfates, thereby extending the lifespan of concrete structures exposed to such conditions.