Precautions for adding silica fume to concrete

Precautions for adding silica fume to concrete

1. Silica fume features fine particles, large specific surface area and high water demand. Therefore, when mixing silica fume into concrete, it must be used together with high-efficiency water reducer to achieve satisfactory effects.
2. There are two mixing methods for silica fume in concrete: internal mixing method and external mixing method.

   The internal mixing method reduces cement dosage, which is generally applied to medium and low strength grade concrete;

   The external mixing method does not reduce cement dosage, which is commonly used for high strength grade concrete.
3. The dosage of silica fume in concrete should neither be too high nor too low, with the typical dosage of 5%～10%.

   Within this range, the beneficial effects of silica fume can be exerted optimally: it can best replace cement, give full play to various excellent performances, and avoid adverse impacts.

   For instance, the frost resistance of concrete will decrease when the dosage exceeds 15%.
4. Silica fume concrete has high consistency. Hence, its designed slump should be 2～3 cm larger than that of ordinary concrete.
5. The mixing time of silica fume concrete shall be prolonged by 0.5～1 minute compared with ordinary concrete, so as to ensure uniform mixing and prevent quality hazards caused by agglomeration of silica fume.
6. After silica fume is added to concrete, early curing must be strengthened to prevent plastic shrinkage cracks of concrete.