Micro Silica (Silica Fume): Production & Application
Source: | Author:Arfa Falak | Published time :2024-09-02 | 113 times view: | Share:
Micro silica, also known as silica fume, is an ultrafine powder produced as a by-product during the manufacturing of silicon and ferrosilicon alloys. It is primarily used as a pozzolanic material in high-performance concrete, enhancing its durability and strength. When added to concrete, micro silica fills gaps in the wet mixture and chemically reacts with Portland cement, resulting in a hardened, compact concrete with improved strength and durability. The production of micro silica involves heating high-purity quartz and coal to over 2000°C, consuming significant electrical power. Micro silica's applications in concrete include increasing corrosion resistance, sulphate resistance, and abrasion resistance, as well as reducing heat generation and permeability. Despite its higher cost and potential for increased shrinkage and reduced workability, micro silica contributes to sustainable building development by improving concrete properties and reducing environmental impact.
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Properties of Micro Silica

  • The diameter of a silica fume particle ranges from 100 to 200nm.

  • Its particles are spherical in shape.

  • The density of condensed silica fume required to be used as an additive in concrete must be about 500 to 700 kg/m3.

  • It contains 85 to 97% of silicon dioxide.

  • The other constituents present in micro silica are carbon, sulphur along with the oxides of aluminium, iron, calcium, magnesium, sodium and potassium.

  • Replacing the cement with about 10% silica fume subsequently results in the increase of compressive strength of the concrete.

  • Reinforcement corrosion is greatly reduced due to increased resistivity, high tensile strength and less permeability.

Effects of Silica Fume on Concrete

01. Pozzolanic Effect

According to ‘Verma Ajay et al.’ (Published in Research Journal of Engineering Sciences), when water is added to ordinary Portland cement, hydration of concrete takes place resulting in the formation of the following products:

OPC + H2O  →  CSH + Ca (OH)2

In the presence of silica fume, the silicon dioxide reacts with the calcium hydroxide to form more aggregate binding CHS (Calcium silicate hydrate) which is as follows:

Ca (OH)2 + SiO2 + H2O  →  CSH

The reaction diminishes the amount of calcium hydroxide in the concrete.  This is helpful as a weaker calcium hydroxide does not impart any strength to the concrete. When it is combined with carbon dioxide, it forms a soluble salt which will leach through the concrete causing efflorescence, a familiar architectural problem. When high amount of calcium hydroxide is added to the concrete, the concrete becomes more vulnerable to sulphate attack, chemical attack as well as adverse alkali-aggregate reactions.

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02. Micro Filler Effect

According to ‘Md Athar Kazmi et al.’ (Published in International Journal of Science and Research (IJSR)), micro silica is a finer material, approximately 100 times finer than cement in terms of average diameter. By adding nearly 8% of cement by weight, approximately 100,000 particles for each particle of cement will fill up the water spaces in fresh concrete. This eliminates bleeding and weak transition zone between aggregate and concrete paste. This micro filler effect greatly reduces permeability and improves paste-to aggregate bond in silica fume concrete compared to conventional concrete. The reaction of silica fumes is rapid, providing high strength and durability at an early stage. In terms of efficiency of silica fume, it is 3-5 times more than that of OPC and simultaneously performance of concrete can also be drastically improved.

03. Effects on Air Entrainment of Fresh Concrete

The dosage of admixtures such as the air- entraining agents required to maintain the desirable air content while using micro silica is slightly higher than that required for conventional concrete. This is because of the high surface area in addition to the presence of carbon. Hence, the dosage of air entraining agents is increased with increasing amounts of micro silica content in concrete.

04. Effects on Water Requirements of Fresh Concrete 

On adding silica fume to concrete, there is an automatic increase in water demand. This often results in requiring one additional pound of water for every pound of silica fume added. This issue can easily be solved by using High range water reducers (HRWR).

05. Effects on Consistency and Bleeding of Fresh Concrete 

Adding more than 10% of silica fume in concrete induce stickiness in concrete. Therefore, in order to enhance the workability of concrete, the initial slump should be increased. It has been observed that condensed silica fume reduces the bleeding of concrete because of its effect on the rheological properties.

Applications of Micro Silica

Micro silica concrete is generally used to achieve following properties,

01. Corrosion Resistance

According to ‘Umesh Sharma et al.’ (Published in International Journal of Civil Engineering Research), the reduced permeability of micro-silica provides protection against intrusion of chloride ions thereby increasing the time taken for the chloride ions to reach the steel bar and initiate corrosion. In addition, micro-silica concrete has much higher electrical resistivity compared to OPC concrete thus slowing down the corrosion rate. This combined effect increases the life of a structure by 5 – 10 times.

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