Dear Sungwoo, silica gel is clean product, Silica fume is so called because of its nature.

Fumes are waste products so may have inside pollutions (other oxides , even iron oxide and carbon ) that affect colour .

There are several types of these wastes so chemical composition may vary and consequently colour. There are from very white to whitish, from light grey to black .

Every microsilica is very fine and may be densed to permit easy packaging and use.

Cementitiuos Portland technology uses this kind of raw materials to increase viecosity and final compacting of mortars and concretes. There are incredible benefits using these.

Also in geopolymer technology is possible to use these raw materials but not as pozzolana material like before but to partecipate at geopolymerization process.

For these technology colour is a parameter only if you are searching aesthetic effects.

Best regards

Alex Reggiani

Essentially because of the carbon content. The lower the carbon content, the lighter will be the color. You can prove this by doing a secondary combustion of silica fume and you will observe it becoming progressively lighter in color.

Besides carbon, also silicon carbide is often present as impurity. Furthermore, traces of elemental Si can occur.

due to the presence of Iron oxides and silicon content. Also the color observed to be much darker in case of LOI greater then 2%.