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Micro silica ,also called silica fume or micro silica, is a byproduct of production silicon metal or ferrosilicon alloys. Silicon metal and alloys are produced in electric furnaces.The raw materials are quartz,coal and wood-chips.The smoke that results from furnace operation is collected and sold as silica fume, silica fume consists primarily of amorphous(non-crystalline) silicon dioxide(SiO2),its average granule diameter is 0.15~0.20um,specific surface area is 15000~20000m2/kg,and it has extremely strong surface active.
How Does Silica Fume in Concrete?
When silica fume is added to concrete, initially it remains inert. once portland cement and water in the mix start reacting with each other (hydrating), primary chemical reactions produce two chemical compounds: Calcium Silicate Hydrate (CSH), which is the strength producing crystallization, and Calcium Hydroxide (CH), a by-product also called free lime which is responsible for nothing much other than lining available pores within concrete as a filler or leaching out of inferior concrete.
Pozzolanic reaction occurs between silica fume and the CH, producing additional CSH in many of the voids around hydrated cement particles. This additional CSH provides the concrete with not only improved compressive, flexural and bond- strength but also a much denser matrix, mostly in areas that would have remained as small voids subject to possible ingress of deleterious materials.
