What is the optimum percentage of silica fume?

The construction industry is an area where silica fume has the earliest research and is mostly used and applied. A reasonable dosage rate of silica fume, which must be determined through tests, can improve various properties of mortar and concrete as well as reduce costs. Additionally, in production control, attention should be paid to aspects like conveying volume, mixing time, on-site maintenance, etc. This article will study the effect of different dosage rates of silica fume on the compressive strength and other properties of mortar, concrete, and provide a more feasible mix ratio and technology for obtaining mortar and concrete with 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 water reduction rate no less than 30%;
• Aggregate;
• GGBFS;
• Fly ash.

Test variable: the dosage rate of silica fume; 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 different silica fume contents to replace cement are tested (the total amount of cement and silica fume is unchanged), and the test 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, when the content of silica fume is 0-5%, the 28-day compressive strength of mortar is basically stable, and when the content of silica fume is 5%-8%, the 28-day compressive strength gradually increases. When the content is greater than 8%, the strength value of mortar gradually decreases. It can be seen that the optimum dosage range of silica fume is 5%-8%.

The proportion of silica fume to replace cement in concrete

Each set of concrete mix ratios has an optimal silica fume proportion to replace cement. The final replacement ratio must be determined through tests based on the actual raw material conditions.

The water content in this test is constant (165kg). And the results of the compressive strength of concrete and the conditions of the mixture under different proportions of silica fume to replace cement are shown in the table below.

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 occurs in 30 min.

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460

30 (1:1)

61.8

68.4

Good workability, bleeding doesn’t occur 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.

It can be seen that the 28-day and 60-day compressive strengths of concrete decrease first as the replacement ratio increases, then increase when the replacement ratio is 2:1 (that is, 1 kg of silica fume for replacing 2 kg of cement), which is near the strengths of ordinary mix ratio without silica fume and has much better mixture conditions. With the replacement ratio of 2:1, the amount of cement reduces from 490 kg/m3 to 430 kg/m3, the cost of raw materials also decreases.

For ordinary concrete, due to bleeding, a large number of capillary pores are formed inside the fresh concrete, and a porous structure is formed under the aggregate and steel bars, which reduces the bond strength among the cement paste and the aggregate and the steel bars, resulting in a decrease in the strength of the concrete. By adding silica fume, because of the large specific surface area of the silica fume, capillary water inside the concrete is constrained, which reduces the bleeding inside and increases the bond strength. The pozzolanic activity of silica fume also changes the structure inside the concrete, and the strength of the concrete is greatly improved. According to data, with the addition of 5%-8% silica fume by mass of cement (the water-cement ratio is controlled at 0.2-0.3, and it must be used with superplasticizer), the compressive strength of concrete can reach 80-120 MPa, and the 28-day compressive strength can be increased by 5%-35%. But as the amount of silica fume keeps increasing, the increase in strength will become less obvious, which may be due to the increase in water demand. Therefore, the concrete mixed with silica fume must be used together with the superplasticizer to achieve the dual effect of reducing cement and increasing strength.

Because of construction materials, micro silica fume (silica fume) can fill the pores between cement and concrete particles, and form gel with hydration products, and react with basic material magnesium oxide to form gel. It can greatly improve the strength and density of concrete, and is widely used in refractory, concrete, industrial floor, dry powder mortar (or dry mixed mortar), grouting material, cementing wellhead, external wall insulation and other industries.

Use of silica fume:
1. the content of micro silica fume is generally 5% to 10% of cementitious material.

The mixing method of microsilica is divided into internal mixing and external mixing.

(1) Internal admixture: under the premise of constant water consumption, 1 phr silica fume can replace 3-5 phr cement (weight) and keep the compressive strength of concrete unchanged, so as to improve other properties of concrete.

(2) External mixing: the strength and other properties of concrete can be significantly improved by adding silica fume. When concrete is mixed with silica fume, there is a certain slump loss. Silica fume must be combined with water reducing agent. It is suggested to mix fly ash and ground slag to improve its constructability.

Microsilica fume addition method:

2. micro silica fume concrete and castables should be constructed by laboratory. Strict accordance with the ratio of construction. During silica fume concrete mixing, silica fume should be added to the blender immediately after aggregate feeding.

There are two ways to add silica fume:
(1) Input aggregate, then put silica fume and cement dry mix, then add water and other additives.
(2) Put coarse aggregate + 75% water + silica fume + 50% fine aggregate, stir for 15-30 seconds, then add cement + admixture + 50% fine aggregate + 25% water, stir until uniform. The stirring time extend 20-25% or 50-60 seconds than ordinary concrete.

Avoid adding silica fume to mixed concrete.

Silica fume construction method:

There is no significant difference between silica fume concrete and ordinary concrete in construction method, but it is necessary to organize and vibrate compactly in construction. The early strength of silica fume concrete will make the final setting time advance, so attention should be paid during the plastering surface. At the same time, adding silica fume will improve the stickiness of concrete and greatly reduce bleeding, making the plastering surface slightly difficult.

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