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Silica fume is a substance that is used to improve the strength of concrete. It is produced during the synthesis of silicon metal or ferrosilicon alloys, when the reduction of high-purity quartz at high temperature in an electric furnace gives off silicon oxide gas. The gas reacts with oxygen and condenses into silica fume, a fine white powder substance composed mostly of silicon dioxide. This substance is not to be confused with fumed silica, also known as pyrogenic silica, which has a different composition and is used to thicken milkshakes and paints.
Like other pozzolanic materials, silica fume reacts with calcium hydroxide to create strong bonds within the cement mixture. The addition of silica fume to concrete decreases the water content of the concrete and makes it less susceptible to corrosion caused by marine salts and chloride ions. This makes it especially useful in structures exposed to water, such as dams or bridges.
It has been shown that the addition of silica fume to a concrete mixture reduces the level of expansion and cracking resulting from alkali-silica reactions (ASR). ASR typically occurs when hydroxide ions react with silica in a cement mixture to form a calcium and alkali silicate gel, which flows into porous spaces in the concrete. The gel causes expansion and cracking, leading to eventual structural failure.
The extremely fine particulate texture of silica fume gives it advantageous mechanical properties. Silica concrete is less permeable than non-reinforced concrete and can be used to support heavy loads. Skyscrapers and other large buildings that put large amounts of vertical pressure on their structural elements usually use high-strength concrete.
Silica concrete is mixed by adding a specified proportion of fume to cement in either wet or dry form. The proportion of silica fume in the final concrete mixture is determined by the level of material strength required. Stronger mixtures of silica concrete, which might contain as much as 15 percent of silica fume by weight, are more brittle than the more common mixtures, which contain 7-10 percent.
Until about the mid-1970s, the silica fume that was produced in quartz reactions was not conserved for use but was instead released into the atmosphere. This practice created environmental concerns that led to landfilling of the substance. Guidelines have since been developed to standardize physical quality, packaging and other qualities for industrial use.