Physical properties of silica fume

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Silica fume is an ultrafine material with spherical particles less than 1 μm in diameter, the average being about 0.15 μm. This makes it approximately 100 times smaller than the average cement particle. The bulk density of silica fume depends on the degree of densification in the silo and varies from 130 (undensified) to 700 kg/m3 (densified). The specific gravity of silica fume is generally in the range of 2.2 to 2.3 m2/g. The specific surface area of silica fume can be measured with the BET method or nitrogen adsorption method. It typically ranges from 15,000 to 30,000 m2/kg.

Chemical composition of silica fume:

Property Silica
SiO2 content (%) 85–97
Al2O3 content (%) 1.0±0.2%
Fe2O3 content (%) 0.9±0.3%
CaO content (%) 0.3±0.1%
MgO content (%) 0.7±0.1%
SO3 content (%)
Specific surfaceb (m2/kg) 15,000–30,000
Specific gravity 2.22
General use in concrete Property enhancer