Silicon is primarily used in the form of silica (silicon dioxide, SiO 2) and silicates in cement and concrete. In these materials, silicon is a fundamental component that contributes to the matrix’s strength and durability. In its reactive form, such as silica fume, silicon participates in pozzolanic reactions with calcium hydroxide to form additional calcium silicate hydrate (C-S-H), which is crucial for the strength and durability of concrete. This reaction helps to fill pores and refine the microstructure of concrete, making it denser and less permeable.