The particles of silica fume are extremely small. On average, the particles are about 0.1 micrometers (100 nanometers) in diameter. To put this into perspective, if a typical silica fume particle was the size of a football, then a single grain of cement would be about the size of a car.

The small particle size is a key reason why silica fume is so effective as a pozzolan in concrete. It fills in the gaps between larger particles, reducing the porosity and increasing the strength and durability of the finished product.