... is collected and sold as silica fume, rather than being landfilled. Silica fume consists primarily of amorphous (non-crystalline) silicon dioxide (SiO2), its ... fine particles, large surface area, and the high SiO2 content, silica fume is a very reactive pozzolan when used in concrete ... . At the same time, silica fume can still be used in the production of refractory ...