The core principle of using silica fume to strengthen high-strength concrete (such as C60 and above) lies in leveraging its ultrafine particle filling effect and pozzolanic activity to optimize concrete density, strength, and durability. Specific application methods are as follows:
I. Determining Optimal Dosage
Silica fume dosage must be tailored to concrete performance requirements (strength, workability, etc.) and cement type, typically ranging from 5% to 10% by cement weight:
- Too low dosage (<5%): Insufficient filling and pozzolanic effects yield limited reinforcement.
- Excessive dosage (＞10%): Significantly increases concrete viscosity, raises water demand, may cause shrinkage cracking, and reduces cost-effectiveness.
II. Selecting the Appropriate Blending Method
- Direct Blending Method: Mix silica fume with cement, aggregates, and other dry materials. Suitable for small-scale production or laboratory mixing. Note that silica fume particles are ultrafine and prone to airborne dispersion. Use enclosed mixing equipment or premix thoroughly with cement before adding.
- **Slurry Method**: Prepare a silica fume slurry (water-to-silica ratio ~1:1) with part of the mixing water, mix thoroughly, then add to the concrete. This prevents agglomeration and ensures uniform dispersion, making it more suitable for large-scale engineering applications.
III. Adjusting Mix Design Parameters
- Water Content and Superplasticizers:
With a large specific surface area (approximately 20,000–25,000 m²/kg), silica fume significantly increases concrete viscosity and water demand. Therefore, the dosage of high-performance water-reducing agent must be increased (typically 20%–30% higher than the reference concrete) to maintain the target slump (e.g., 180–220 mm) while controlling the water-binder ratio (generally ≤0.35 for high-strength concrete) to prevent strength reduction.
- Sand Ratio Adjustment:
The filling effect of silica fume allows for a moderate reduction in sand ratio (typically 1%–2% less than the reference concrete), decreasing friction resistance between aggregates and improving concrete workability.
IV. Controlling Construction Processes
- Mixing Process:
Silica fume must be thoroughly dispersed to prevent agglomeration (“fish eyes”) that compromise concrete uniformity. Recommended mixing sequence: First mix aggregates and cement for 10–15 seconds. Add silica fume (or silica fume slurry) and most of the water, mixing for 30 seconds. Finally, incorporate the water-reducing agent and remaining water. Total mixing time should be extended by 30–60 seconds compared to standard concrete (to ensure complete dispersion).
- Placement and Curing:
Silica fume concrete exhibits rapid early strength development but generates higher hydration heat and significant early shrinkage (particularly autogenous shrinkage). Requirements:
◦ Perform timely compaction after placement (avoid over-vibration to prevent segregation), level the surface, and cover to retain moisture.
◦ Implement accelerated early-stage curing: Spray water or cover with plastic sheeting within 12 hours of placement to maintain surface moisture. Curing period must exceed 14 days (extend in high-temperature environments) to minimize shrinkage cracking.
V. Core Functional Benefits
- Strength Enhancement: Silica fume reacts with Ca(OH)₂ from cement hydration to form additional C-S-H gel, filling pores and increasing concrete density. This boosts 28-day compressive strength by 15%–30%, with continued strength gain over time.
- Durability Optimization: Enhanced density significantly improves concrete’s water resistance, freeze-thaw resistance, and chemical resistance (e.g., against sulfates and chloride ions), making it suitable for high-rise buildings, bridges, and other heavy-load or harsh-environment projects.
Precautions
- Store silica fume in dry, sealed containers to prevent moisture absorption and caking (caked material must be crushed and screened before use).
- For first-time use, determine optimal dosage and water-reducing agent quantities through trial mixes to avoid poor workability or cracking due to improper parameters.
By following these methods, the reinforcing effects of silica fume can be fully leveraged to produce high-performance, high-strength concrete.
