Ultra fine ash at 8000 cm2/g Blaine for high-performance and self-compacting concrete.
At this fineness the ash starts to work like a mild silica fume: it fills the space between cement grains, refines the pore structure and drives chloride permeability down hard. Self-compacting mixes benefit twice over, because the spherical particles also improve flow without extra water.
Used at 8 to 15 percent, usually alongside a conventional Class F replacement rather than instead of it.