Silica fume (micro-silica) is produced in the arc fusion process when silica is separated from Zirconium Silicate.  The resulting fume is very fine amorphous material with purity above 87%, a BET surface area of 12-20 m2/g, and pH range of 5-8.  There are hundreds of industrial uses for fumed silica in sealant formulations, concrete additives, rheology control, and emulsification agents.