Effect of Different Silica Fume Dosage Rates on Mortar and Concrete
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.
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. |
| 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.