Silica Fume is an ultrafine powder. Primary silica fume particles that exhibit a perfectly spherical shape are in the size range of 0.02 - 1.0 Ám (average particle size 0.15 Ám). During collection, cooling and bagging, the primary particles form larger agglomerates (not aggregates), as shown in the Transmission Electron Microscopy (TEM) pictures.
The specific surface area (BET) is typically around 20 m2/g (range 15-30).
Although silica fume consists of sub-micron particles, the generation of respirable dust during handling is fairly low. The results of the dustiness tests (EN 15051) of densified and undensified silica fume products show that, out of the total inhalable silica fume, the proportion of respirable particles is only 1-3 wt%.
Silica fume is amorphous to X-rays, as shown in the Figure