Microsilica is a mineral admixture composed of very fine solid glassy spheres of silicon dioxide. Most microsilica particles are less than 1 micron (0.00004 inch) 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 ferrosilicon and metallic silicon in high-temperature electric arc furnaces.