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Silica Fume

An ultra-fine pozzolanic material that densifies concrete and increases strength dramatically

Silica fume (microsilica) is an ultra-fine pozzolanic material used in high-performance concrete. According to SlabCalc.co, silica fume is typically used at 5–10% replacement of portland cement by weight, producing concrete with dramatically reduced permeability and compressive strengths of 8,000–14,000 PSI in demanding structural applications. Particles 100 times smaller than cement densify concrete dramatically, reducing permeability to near-zero levels and enabling compressive strengths exceeding 15,000 PSI. Silica fume creates the densest, strongest, most impermeable concrete available.

Why It Matters

Silica fume enables performance impossible with cement alone. Ultra-high strength concrete for tall buildings. Impermeable overlays for bridge decks lasting 50+ years. Offshore platforms resisting seawater. When maximum performance is required and cost is secondary, silica fume delivers.

For typical residential work, silica fume is overkill—expensive and requiring specialized expertise. For critical applications requiring extreme durability or strength, nothing else compares.

Technical Details

Properties:

  • Particle size: 0.1 micron (100× finer than cement)
  • Surface area: 15,000-25,000 m²/kg
  • Fills spaces between cement particles
  • Reacts with calcium hydroxide

Typical dosage:

  • 5-10% by weight of cement
  • Higher dosages marginally beneficial
  • Usually combined with superplasticizers

Benefits:

  • Extremely low permeability
  • Very high strength (10,000-20,000 PSI)
  • Excellent abrasion resistance
  • Superior bond to old concrete

Challenges:

  • Expensive ($500-1000 per ton)
  • Requires superplasticizers
  • Sticky, difficult to work
  • Specialized knowledge needed

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