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Microsilica

What is microsilica?

Micro Silica, also called Silica Fume or Microsilica, is an ultrafine, amorphous form of silicon dioxide (SiO₂) produced as a by-product during the manufacture of ferrosilicon and silicon metal in electric arc furnaces. Composed mainly of spherical particles about 0.1–0.3 microns in size—roughly 100 times finer than cement—it typically contains 85–98% SiO₂ and appears as a very fine gray powder. Owing to its extremely high surface area and pozzolanic reactivity, micro silica is widely used as a supplementary cementitious material to enhance the strength, durability, and impermeability of concrete, and it also finds applications in refractories, fiber-cement products, polymer composites, and oil-well cementing.

Key Characteristics

  • Ultra-Fine Particle Size: Microsilica particles are extremely small—about 0.1–0.3 microns—which is nearly 100 times finer than cement.
  • Very High Silica Content: Containing typically 85–98% amorphous SiO₂, microsilica provides a rich source of reactive silica.
  • Highly Pozzolanic: Microsilica rapidly reacts with calcium hydroxide during cement hydration to form extra C-S-H gel.
  • Reduces Permeability: Microsilica fills pores and refines the concrete microstructure, reducing the entry of water and harmful chemicals.
  • Increases Strength: Microsilica increases both compressive and flexural strength by densifying the concrete matrix.
  • Improves Cohesiveness & Reduces Segregation: The fine particles improve the viscosity and uniformity of fresh concrete, reducing bleeding and segregation.

Applications

  • Microsilica is a super fine (mean size of 45μ), amorphous, high silica-content, non-crystalline, pozzolanic powder supplied in the densified form. Microsilica is used in concrete mixes to replace part of the cement content and enhance the resultant concrete's water impermeability and/or increase compressive strength. Typical application fields besides others include:
  • Pre-cast & cast-in-situ concrete
  • High performance concretes
  • Marine structures such as bridges, causeways, piers, jetties, ship building yards, ports and quays
  • Oil, gas & petrochemical processing structures
  • Below grade structures exposed to aggressive ground waters such as foundations, rafts, basement walls & floors and underground tunnels
  • Structures exposed to high abrasion such as concrete roads, bridge decks & factory floors
Sr. no Test Parameter Requirement as per ASTM:C1240/IS:15388/IS:1727 Result
1 Physical Test
a Color Acceptable Grey Powder
b Appearance Acceptable Micro Granulated
c Specific Gravity Acceptable 2.22
d Bulk Density kg/m3 600-700 620 Kg/m3
e % Retention in 45 micron IS Sieve 10.0% max 3.25%
f % Retention in 45 micron IS Sieve Average 5.0% max. 1.50%
g Specific Surface M2/g 15 min. 21.5
h Accelerated Pozzolonic strength activity with Portland cement at 7 Days, % of control 105 min. 110
2 Chemical test
a Alkalies as Na2O 1.50% 0.70%
b SiO2 % by mass 85.0% min. 89.50%
c Loss in ignition % by mass 4.0% max. 3.90%
d Moisture 3.0% max. 1.63%