Microsilica (BRAND ORISIL), also known as Silica fume, is a byproduct of producing silicon metal or ferrosilicon alloys. Microsilica (ORISIL) consists primarily of amorphous (non-crystalline) silicon dioxide (SiO2). The individual particles are extremely small, approximately 1/100th the size of an average cement particle. Because of its fine particles, large surface area, and the high SiO2 content, silica fume is a very reactive pozzolan when used in concrete.