Factors Affecting the Flowability of Silica Fume

Factors Affecting the Flowability of Silica Fume

1. Physical Properties

• Particle size and specific surface area

  Silica fume has an extremely fine particle size (approx. 0.1–0.3 μm) and a large specific surface area (15,000–30,000 m²/kg), which intensifies van der Waals forces between particles, leading to easy agglomeration and reduced flowability.
• Particle morphology

  Particles are mostly spherical, which theoretically favors flowability. However, the ultra-high specific surface area and surface energy counteract this spherical advantage.
• Bulk density

  It has a low loose bulk density (approx. 200–300 kg/m³), with large interparticle voids that easily create air resistance and impair flow.

2. Moisture and Moisture Content

• Hygroscopicity

  Silica fume has high surface adsorptivity. Even a small amount of moisture (>0.5%) forms liquid bridges on particle surfaces, causing severe agglomeration and a sharp decline in flowability.
• Storage environment

  Silica fume tends to cake in humid environments and requires strict moisture protection.

3. Chemical Composition and Surface Condition

• SiO₂ content and impurities

  High-purity silica fume (SiO₂ > 90%) has high surface activity and adsorbs moisture or gas more easily. Impurities such as alkali metal oxides may affect surface charge and agglomeration behavior.
• Surface modification

  Hydrophobic treatment (e.g., with silane coupling agents) or nano-silica coating can reduce agglomeration and improve flowability.

4. Processing and Treatment Methods

• Densification treatment

  Mechanical compaction or fluidized bed processes increase bulk density (up to 500–700 kg/m³), reduce interparticle air gaps, and improve flowability.
• Vibration and aeration

  Applying vibration or slight air fluidization breaks up agglomerates and temporarily improves flowability (e.g., in pneumatic conveying).

5. Environmental and External Conditions

• Temperature and humidity

  High temperature and low humidity help maintain dryness and reduce liquid bridge formation; low temperature and high humidity promote agglomeration.
• Electrostatic effect

  Particles tend to accumulate static electricity in dry conditions, causing adhesive agglomeration and uneven flow.

6. Interaction with Other Materials

• Mixing system

  In cement or mortar, silica fume readily adsorbs water-reducing agents. Improper formulation increases paste viscosity and impairs overall flowability.