Particle size distribution refers to the proportion of different-sized particles in a material and is critical in determining the properties and performance of both cement and concrete. In the context of cement, finer particles hydrate more quickly and contribute to early strength gains, while coarser particles provide bulk and influence the long-term strength and durability. The distribution of particle sizes affects the packing density, water demand, workability, and hydration rate of the cement paste. The ideal PSD maximizes packing density and minimizes the voids between particles, which in turn reduces the water requirement and enhances the mechanical properties and durability of the hardened concrete.