The higher the content of SiO2, the higher performance of micro silica. Generally, the silica fume SiO2 is required to be more than 85%, and the carbon in the silica fume will absorb some air-entraining agent in the concrete, so the loss on ignition is required to be less than 7% (control the carbon and water content); The fineness of micro silica should be controlled below 45um, and the residue of particles with >45um should be less than 10%.
The amount of micro silica added in concrete should not be too high or too low, generally is 5% – 10%, and the quality of concrete in this range is good. For example, the cold resistance of commercial concrete will be reduced after the dosage is more than 15%.
Add 2%, 4%, 6%, 8% and 10% silica fume (the percentage of micro silica fume in the total amount of cement and silica fume) into the concrete, and measure the water absorption and compressive strength of the concrete as follows:
| Cement (/%) | Silica Fume (/%) | W/C | Water-reducing agent (/%) | Foaming agent (/%) | `Calcium chloride (/%) | Water absorption (/%) | 7d compressive strength (/MPa) | 28d compressive strength (/MPa) |
| 100 | 0 | 0.5 | 0.2 | 1 | 3 | 65 | 1.4 | 1.7 |
| 98 | 2 | 0.5 | 0.2 | 1 | 3 | 65 | 1.5 | 1.9 |
| 96 | 4 | 0.5 | 0.2 | 1 | 3 | 60 | 1.7 | 2.2 |
| 94 | 6 | 0.5 | 0.2 | 1 | 3 | 63 | 2.3 | 2.7 |
| 92 | 8 | 0.5 | 0.2 | 1 | 3 | 69 | 2.1 | 2.5 |
| 90 | 10 | 0.5 | 0.2 | 1 | 3 | 71 | 1.9 | 2.3 |
With the increase of silica fume content, the water absorption rate of foam concrete increases continuously, and the 7d compressive strength fluctuates. When the silica fume content is 6%, the compressive strength reaches 2.3 MPa, is higher than that of the sample without silica fume. The 28d compressive strength is 2.7 MPa, and the water absorption rate decreases by 2%. The overall effect is obvious.
