Microsilica ( Grey ) is a mineral admixture composed of very fine solid glassy spheres of silicon d i oxide (SiO2). Each particles are less than 1 micron in diameter, generally 50 to 100 times finer than average cement or fly ash particles. Frequently called condensed silica fume, microsilica is a by- product of the industrial manufacture of ferro silicon and metallic silicon in high-temperature electric arc furnaces. The ferrosilicon or silicon product is draw n off as a liquid from the bottom of the furnace. Vaporizing from the 2000-degree-C furnace bed is oxidized, and as it cools condenses into particles which are trapped in huge cloth bags. Processing the condensed fume to remove impurities and control particle size yields microsilica.