What happens to the amount of air entrained in concrete when silica fume is added?

When silica fume is added to concrete, the amount of air entrained typically decreases significantly. Silica fume is a fine, highly pozzolanic material that can enhance the properties of concrete, including its strength and durability. However, it also affects the air void system within the concrete mixture.

The presence of silica fume can lead to a denser microstructure, reducing the volume of air voids that can be entrained. This occurs because silica fume has a high surface area and can fill in voids that might otherwise be occupied by air, thus compressing the air content within the mix. Additionally, silica fume can create more binding material for water and cement particles, resulting in a less porous mixture.

These properties make the addition of silica fume beneficial for certain applications, especially in improving the mechanical and durability aspects of concrete, but they do lead to the reduction in air entrainment. In summary, the addition of silica fume typically reduces the amount of air that can be entrained in the concrete, leading to a denser final product.