What is the optimum percentage of silica fume?

Effect of Different Silica Fume Dosage Rates on Mortar and Concrete

The construction industry has extensively researched the use of silica fume, which has shown significant improvements in various properties of mortar and concrete. Determining a reasonable dosage rate is essential for optimizing performance and reducing costs. Factors such as conveying volume, mixing time, and on-site maintenance play crucial roles in production control. This article examines the impact of different silica fume dosage rates on the compressive strength and other properties of mortar and concrete, providing feasible mix ratios and techniques for better overall performance.

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Preparation of Materials

• Cement: Ordinary Portland cement PO 42.5
• Silica fume: G95D Microsilica (densified silica fume with SiO2 content of 95%, pozzolanic activity index of 110%, density of 670kg/m3)
• Superplasticizer: Polycarboxylate superplasticizer with a water reduction rate no less than 30%
• Aggregate
• GGBFS (Ground Granulated Blast Furnace Slag)
• Fly ash

Test variable: Silica fume dosage rate; Test specimens: 10cm x 10cm x 10cm.

Compressive Strength Test of Mortar Specimens with Silica Fume

The 28-day compressive strength of mortar specimens with varying silica fume contents, which replace a portion of the cement (while keeping the total amount of cement and silica fume constant), were tested. The results are as follows:

Silica Fume Dosage (%)

28-Day Compressive Strength of Mortar (MPa)

0: 57.4

3: 57.9

5: 58.4

6: 63.4

7: 62.8

8: 55.2

10: 54.6

12: 53.7

According to the results, the optimum content of silica fume is between 5% and 8%. Within this range, the 28-day compressive strength increases, reaching a peak at 6%. Beyond 8%, the strength decreases.

The Proportion of Silica Fume to Replace Cement in Concrete

Every concrete mix ratio has an optimal silica fume proportion to replace cement, determined by testing based on actual raw material conditions. In this test, the water content is constant (165kg). Below are the results of the compressive strength of concrete and mixture conditions with different proportions of silica fume replacing cement.

Cement (kg)

Silica Fume (kg)

28-Day Compressive Strength (MPa)

60-Day Compressive Strength (MPa)

Mixture Performance

490: 0 -> 70.7 -> 72.5 -> Low cohesiveness, low water retention, bleeding in 30 min.

460: 30 (1:1) -> 61.8 -> 68.4 -> Good workability, no bleeding in 30 min.

445: 30 (1.5:1) -> 56.3 -> 67.7 -> Good workability.

430: 30 (2:1) -> 63.4 -> 70.4 -> Good workability, softer mixture.

415: 30 (2.5:1) -> 57.4 -> 63.4 -> Good workability, softer mixture.

The data indicates that the 28-day and 60-day compressive strengths initially decrease with increasing replacement ratios, but improve when the replacement ratio is 2:1 (1 kg of silica fume replaces 2 kg of cement). This ratio yields strengths comparable to an ordinary mix without silica fume, along with better mixture conditions and reduced raw material costs.

For ordinary concrete, bleeding creates capillary pores, weakening the bond strength among the cement paste, aggregates, and steel bars. Adding silica fume, due to its large specific surface area, restricts capillary water inside the concrete, reducing bleeding and enhancing bond strength. The pozzolanic activity of silica fume further alters the concrete's internal structure, significantly boosting strength. Data shows that adding 5%-8% silica fume by mass of cement can increase compressive strength to 80-120 MPa, with a 5%-35% gain in 28-day compressive strength. However, as silica fume content continues to rise, strength gains diminish due to increased water demand. Thus, using silica fume requires combining it with a superplasticizer to achieve both cement reduction and strength enhancement.

Use of Silica Fume in Concrete

Micro silica fume fills the pores between cement and concrete particles, forming a gel with hydration products and reacting with basic material magnesium oxide to form a gel. This greatly improves the strength and density of concrete, making it widely used in refractory, concrete, industrial floor, dry powder mortar, grouting material, cementing wellhead, external wall insulation, and other industries.

Use of Silica Fume:

The content of micro silica fume generally ranges from 5% to 10% of cementitious material.

The mixing method of micro silica is divided into internal mixing and external mixing:

Internal Mixing: Under constant water consumption, 1 part silica fume can replace 3-5 parts cement (by weight) while maintaining compressive strength, thus improving other concrete properties.

External Mixing: Adding silica fume significantly improves concrete strength and other properties. As it may cause some slump loss, it should be used with a water-reducing agent. Adding fly ash and ground slag can enhance constructability.

Microsilica Fume Addition Method:

Concrete and castables containing micro silica fume should follow lab-verified constructions ratios. During mixing, add silica fume to the blender immediately after aggregate feeding.

Adding Silica Fume:

1. Input aggregate, then dry mix silica fume and cement, followed by adding water and other additives.

2. Mix coarse aggregate + 75% water + silica fume + 50% fine aggregate for 15-30 seconds. Then, add cement + admixture + 50% fine aggregate + 25% water and mix until uniform. Extend mixing time by 20-25% or 50-60 seconds compared to ordinary concrete.

Silica Fume Construction Method:

The construction method for silica fume concrete is similar to ordinary concrete, but special attention is needed for vibration and compacting. Due to early strength gain, the final setting time advances, influencing plastering surfaces. Adding silica fume increases concrete stickiness and reduces bleeding, making plastering slightly more challenging.

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