Micro Silica (Silica Fume): Production & Applications

Micro silica is an amorphous polymorph of silica (SiO2 / silicon di-oxide). It is also known as silica fume or condensed silica fume. It is an ultrafine powder collected as a by-product during the production of the silicon and ferrosilicon alloys.  It consists of spherical particles having an average diameter of 150nm. The main field application is as pozzolanic material for high performance concrete.
It is uniquely pure and efficient.  When used as an additive for the concrete, micro silica dramatically improves the durability and shelf life of concrete, thus saving the maintenance costs.

When added to the concrete mix, a small dose of micro silica introduces millions of tiny silicon dioxide particles into the concrete. It fills the gaps in wet concrete mixture and reacts chemically with the Portland cement. The result is a hardened compact concrete with greatly improved strength and durability.

Production of Micro Silica

According to ‘A.M. Neville’ (Author of the Properties of Concrete), micro silica is a by-product obtained during the manufacturing of silicon and ferrosilicon alloys in a submerged-arc electric furnace. These alloys are produced from high-purity quartz and coal. Silica fume is also collected as a by-product during the manufacture of other silicon alloys such as ferrochromium, ferromanganese, Ferro magnesium, and calcium silicon.
Silicon is an element that is found in large quantities in most rocks but never in a pure state. Approximately 26% of the earth’s crust is made up of silicon. Quartz is one of the rocks that contain high amounts of silicon, but it is strongly combined with oxygen. The raw materials that normally goes to the smelter are metallurgical grade quartz, coke, wood chips and coal. The mixtures are transported to the furnace silo and then directly downed into the furnace via rotating charging tubes that distribute the mixture around the electrodes in the furnace.
The mixture is heated to more than 2000°C by three large electrodes. At this temperature, the oxygen in the quartz is released in a reduction process and it recombines with the carbon. It oxidizes and condenses in the low temperature zone to tiny particles, consisting of non-crystalline Silica. The production of silica fume requires a huge electrical power. 11,000 kilowatt hours are required in order to produce one ton of pure silicon.

Production of Micro Silica

Refining of the silicon is performed by adding oxygen to the melt through the bottom of the refining label. When the refining process is completed, the silicon is cast in large iron chills. Casting is an important part of the production because it is vital that the slag produced by the refining process remains in the refining label. The presence of even a small amount of slag may spoil the quality of the final product.
When the ingots have cooled, they are crushed to the desired size, filled in containers and transported to the customer. Efficient filtering removes almost all the dusts from gases emitted from the furnaces. This dust consisting of tiny particles of silica is the micro silica and is proved to be very useful for various purposes, for example, as an additive to concrete.

Micro Silica

Mixing and Placing Considerations

Handling Micro Silica

Owing to its extreme fineness, micro silica presents handling problems. A cement tanker that could ordinarily haul 35 metric tons of cement accommodates only 7 to 9 tons of dry micro silica and requires 20 to 50% more time for discharging.

Water Requirement of the Mix

When no water reducing agents are used, the addition of micro silica to a concrete mix requires more water to maintain a given slump. In order to avoid using excess water, a water reducer or super plasticizer is used along with the micro silica.

Placing, Finishing, and Curing

The gel that forms during the first few minutes of mixing micro silica stiffens the mixture. It is necessary to finish the concrete and apply a curing compound or cover instantly. Various effects have been reported differently with lean concrete mixes or mixes having fly ash replacement for cement.