Microsilica is a mineral mixture composed of very fine solid glassy areas of silicon dioxide. Most particles are less than 1 micron (0.00004 inch) in diameter. Generally they are 50 to 100 times finer than average cement or fly powder left over after burning particles. Also, microsilica is a main component of the industrial manufacture of ferrosilicon and metallic silicon in high-temperature electric arc furnaces.
Microsilica in concrete gives strength in two ways:
As a pozzolan, it provides a more uniform distribution and a greater volume of watering with water products; as a filler, it decreases the average size of pores in the cement paste.
Used as a mixture, microsilica can improve the properties of both fresh and hardened concrete. Addition of microsilica to a concrete mix changes the cement paste structure. Because the particles are so small they break up and move away among and separate the cement particles. The resulting fine, uniform matrix can give much higher flexural, and bond strength.
Microsilica reduces the rate of carbonation, decreases flow to chloride ions, communicates high electrical resistivity, and has little effect on oxygen transport. Therefore, microsilica concrete can be expected to be strongly defensive of something that strengthens or adds support.
Because of its extreme fineness, it presents handling problems. Some producers mix it with water on a pound-for-pound basis to form a slurry that is transportable in tank trailers designed to handle liquids. The water of the slurry replaces part of that ordinarily added to the mix. When no water reducing agent is used, it calls for more water to maintain a given slump. The gel that forms during the first minutes of mixing microsilica concrete takes up water and stiffens the mixture, necessitating adjustment of the timing of charging and placing.
