Densified microsilica for high-strength concrete, easier to handle and transport than the undensified form.
Silica fume is roughly a hundred times finer than cement and almost entirely amorphous silica, so it reacts fast and fills the pore structure aggressively. At 5 to 10 percent it takes concrete into the 80 to 120 MPa range and drops chloride permeability to very low levels.
The densified form is the practical choice for ready-mix and precast because it handles and ships without the dust problems of the loose material.