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Microsilica, also known as silica fume, is a fine powder with the following characteristics: 
CompositionMicrosilica is mostly made of amorphous silicon dioxide, and usually contains at least 85% silicon dioxide. It may also contain other oxides, such as iron, aluminum, calcium, magnesium, sodium, and potassium. 
Particle shapeMicrosilica particles are spherical and very fine, with an average diameter of about 0.15 microns. 
UsesMicrosilica is used in concrete, construction chemicals, and refractory industries. It's used to increase the strength and durability of concrete, and to improve its performance in aggressive, chemical, and marine environments. 
OriginMicrosilica is a by-product of the smelting industry, produced in electric arc furnaces during the production of elemental silicon or ferro-silicon alloys. 
HistoryBefore the 1970s, silica fumes were vented into the atmosphere, but environmental laws led to the development of new applications for the material. Research in Norway showed that concrete containing silica fume had high strength and low porosity

• Metakaolin grades of calcined clays are reactive allumino silicate pozzolan formed by calcining very pure hydrous china clay. Chemically metakaoline combines with calcium hydroxide to form calcium silicate and calcium alluminate hydrates. Unlike other natural pozzolan metakaolin is water processed to remove unreactive impurities producing an almost 100 percent reactive material. The particle size of metakaolin is significantly smaller than cement particles. Is 456:2000 recommends use of metakaolin as mineral admixture.
• Metakaolin is a pozzolan, probably the most effective pozzolanic material for use in concrete. It is a product that is manufactured for use rather than a by-product and is formed when china clay, the mineral kaolin, is heated to a temperature between 600 and 800ºC. Its quality is controlled during manufacture, resulting in a much less variable material than industrial pozzolans that are by-products. First used in the 1960s for the construction of a number of large dams in Brazil, metakaolin was successfully incorporated into the concrete with the original intention of suppressing any damage due to alkali-silica reaction.
• When used to replace cement at levels of 5 to 10% by weight, the concrete produced is generally more cohesive and less likely to bleed. As a result pumping and finishing processes require less effort. The compressive strength of hardened concrete is also increased at this level of replacement.
• Slightly higher replacement levels (up to 20%) produce a cement matrix that has low porosity and permeability. This results in improvements to resistance of the hardened concrete to attack by sulfates, chloride ions and other aggressive substances, such as mineral and organic acids. Freeze/ thaw resistance is improved and the risk of damage resulting from the effects of impact or abrasion is reduced for metakaolin concrete that has been finished and cured properly.