Silica fume is a white powdery substance produced during the synthesis of silicon metal or ferrosilicon alloys. It improves concrete strength, reduces water content, and makes it less susceptible to corrosion. It also reduces expansion and cracking caused by alkali-silica reactions. Silica concrete is less permeable and can support heavy loads. The proportion of silica fume in the mix determines the strength level. Until the mid-1970s, it was released into the atmosphere, but environmental concerns have led to landfill disposal and standardization guidelines.
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 a high temperature in an electric furnace emits silicon oxide gas. The gas reacts with oxygen and condenses into silica fume, a fine white powdery substance composed primarily of silicon dioxide. This substance is not to be confused with fumed silica, also known as fumed 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 mix. Adding this substance to concrete decreases the water content of the concrete and makes it less susceptible to corrosion caused by sea salts and chloride ions. This makes it particularly useful in structures exposed to water, such as dams or bridges.
The addition of silica fume to a concrete mix has been shown to reduce the level of expansion and cracking resulting from alkali-silica reactions (ASR). ASR typically occurs when hydroxide ions react with silica in a concrete mix to form an alkali-calcium silicate gel, which flows into pore spaces in the concrete. The gel causes expansion and cracking, leading to eventual structural failure.
The extremely fine particle structure 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 exert severe vertical pressure on their structural members usually use high-strength concrete.
Silica concrete is mixed by adding a certain proportion of fumes to the concrete in wet or dry form. The proportion of silica fume in the final concrete mix is determined by the strength level of the material required. Stronger mixes of silica concrete, which might contain up to 15 percent silica fume by weight, are more brittle than more common mixes, which contain 7 to 10 percent.
Until about the mid-1970s, the substance produced in quartz reactions was not stored for use but was instead released into the atmosphere. This practice has created environmental concerns which have led to landfill disposal of the substance. Guidelines have since been developed to standardize physical grade, packaging, and other qualities for industrial use.
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