Silica fume
An extremely fine pozzolan captured from silicon metal production. Small doses produce dramatic gains in strength, density, and resistance to chloride penetration.
What does silica fume mean?
Silica fume particles are roughly a hundred times finer than cement, so they fill the tiny spaces between cement grains and react aggressively with hydration byproducts. The result is very high strength and very low permeability, which is why it appears in parking structures, bridge decks, and high-rise columns. It makes concrete sticky and prone to plastic shrinkage cracking, so it demands immediate curing and skilled finishing.
In practice
Silica fume flatwork must be protected from drying the moment it is placed, or it will crack before the crew finishes floating it.