Microsilica properties

A comparative study of microsilicas from 18 sources showed considerable variation in composition and properties, one of those examined containing as little as 23% of SiOj and having a specific surface area of only 7.5 m g (A21). The same study showed that in most of the samples the diffuse XRD peak from the glass accounted for 98-99.5% of the total diffracted intensity and that it peaked at the value of 0.405 nm characteristic of vitreous silica. The commonest crystalline impurities detected were KCl, quartz, metallic iron and iron silicide, and pozzolanic reactivity was found to depend more on the chemical composition and nature of impurities than on the fineness or SiOj content. A surface layer of carbon, if present, greatly decreased reactivity. 

H53.H62). The properties were attributed to a combination of effects. The particles of microsilica, being much finer than those of the cement, partially fill the spaces between the cement grains, and this, together with the superplasticizer, allows the latter to pack more uniformly. They also provide nucleation sites for hydration products, undergo pozzolanic reaction and probably improve the paste aggregate bond. 

Pozzolanic materials or pozzolanas do not exhibit cementing properties if mixed with plain water. However, they possess the capacity to react at ambient temperatures with calcium hydroxide, in the presence of water, to yield strength-developing calcium silicate/aluminate hydrates. They include a variety of materials of natural and artificial origin, such as fly ash, microsilica, burnt clays, and diatomaceous earths. 

It has been reported that the strength properties of concrete made with microsilica may be improved even further by a surface treatment of microsilica with sulfuric acid prior to mixing with other concrete mix components (Li and Chung, 1998). 

Besides microsilica, other ultrafine materials may be used as constiments of DSP systems, as long as their particles are small enough to fill the existing spaces left between the particles of cement. Aldridge et al. (1992) produced a DSP material using ultrafine mtile (TiOj) instead of microsilica as filler. The resulting material had properties comparable to those of a DSP product made with microsilica, in spite of the absence of a pozzolanic reaction in this combination of starting materials. 

The main advantages of using foam concrete is that the density of the material can be varied between 300 and 1600 kg/m, and the foam provides good insulation properties and it is easy to handle by pumping. However, several properties still need to be modified in order to make foam concrete an even more competitive material. There is a need to decrease the shrinkage and the water uptake, and to increase the strength and the insulation ability. At present, the foam structures can be enhanced by the use of various additives, such as microsilica or superplasticizers which reduce the cement/water ratio and improve the stability. Polyester fibre additives enhance the hardening and also improve the tensile properties. 

Low-valued silica sand and quartzite may be converted into specialty silica by various processes. The resulting specialty sihcas are generally classified based on their properties and methods of production and include fumed silica, precipitated silica, silica gels and sols and microsilica. The value of global industrial production of specialty silica is close to 2 billion per anmun, and the capacity of silica production is aroimd 1 million tons per annum, out of which the capacity of precipitated sihca (which is discussed here) is about 85%. These quantities clearly illustrate the importance of the industries and processes dealing with precipitated silica. 

A, Benturand S, Diamond, Effects of direct incorporation of microsilica into GFRC composites on retention of mechanical properties after aging , in S, Diamond (ed,) Proc, Durability of Glass Fiber Reinforced Concrete Syrnp, Prestressed Concrete Institute, Chicago, IL, 1985, pp, 337-351,... 

Myhre B, Hundere AM. Substitution of reactive alumina with microsilica in low cement and ultra-low cement castables. Part I properties related to installation and demoulding. Proc. UNITECR 97, New Orleans, 1997 53 62. 

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