The bulk density of silica fume depends on the degree of densification in the silo and varies from 130 (undensified) to 600kg/m3 (densified). The specific gravity of silica fume is generally in the range of 2.2 to 2.3.
Condensed silica fume is essentially silicon dioxide in noncrystalline (amorphous) form. Since it is an airborne material like fly ash, it has a spherical shape and is extremely fine with particles less than 1 micron in diameter and with an average diameter of about 0.1 micron, about 100 times smaller than average cement particles.
Condensed silica fume surface area is about > 20,000 m2/kg (nitrogen adsorption method). In comparison, Type I and Type III cements have surface areas of about 300 to 400 m2/kg and 500 to 600 m2/kg, respectively. The relative density of silica fume is generally in the range of 2.20 to 2.5. Portland cement has a relative density of about 3.15.
