Cement ultrafine

Finely ground cements often called ultrafine cements, having particles less than 10 p.m and an average size of 4 pm are used to grout soils with fine pore spaces, such as fine sand with a permeabiUty of lO " cm/s. These cements can be made with a wide combination of Pordand cement, slag, or siUca fume (72). 

F. De Larrard. Ultrafine particles for the making of very high strength concretes. Cement Concrete Res, 19(2) 161-172, March 1989. 

The addition of silica fume to dry-mix shotcrete in proportions of 10-15% by mass of Portland cement substantially improves the adhesive and cohesive properties of the freshly applied shotcrete. Silica fume creates a very dense and sticky mix with an almost complete lack of bleeding. These characteristics of the material are attributed to the extreme densification achieved by packing of the ultrafine silica fume particles between the cement particles in the plastic (fresh) shotcrete. Practical ramifications of these characteristics include the observations which follow [107, 117-119]. 

Cemented Carbides, Fig. 6 Nano, ultrafine, and sub-micrrai grades (Source http //www.allabontcemen-tedcarbide.cran702 01.html [Date of access 16.5.2013])... 

Nehdi, M., Mindess, S., and Aitcin, P.C. (1998) Rheology of high performance concrete effect of ultrafine particles. Cement and Concrete Research 28,687-697. 

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. 

Nehdi et al. (1998) indicated the importance of ultrafine particles, in most cases made by grinding various kinds of waste materials. These particles of dimensions down to 0.3 pm replace part of the cement grains, decrease bleeding and densify the structures the use of waste materials is also beneficial. 

Deflocculated Castables Deflocculating castables contain at least one deflocculating agent in addition to a cement. They should also contain a minimum of 2% by weight of ultrafine particles. These are particles less than one micron in size. 

Examples of ultrafine particles in the deflocculated castables are microsilica and reactive alumina. These are further divided according to CaO content CaO is used as the cement. The divisions are shown in Table 20.1. 

Recent developments in low- and ultralow-cement castables/pumpables, the effects of the ultrahne particles are of great significance. The water requirements are low since the ultrafine silica fume particles (mostly used in these compositions) occupy part of the space of water. Although the silica fiune helps in reducing water requirements in the castable, it affects high-temperature properties due to the formation of anorthite and gehlinite phases at temperatures around 1250°C- 1400°C. The effects are somewhat minimized in ultralow cement castables due to the use of lesser quantity of calcium aluminate cement. 

Ultrafine particles are, for example, pyrogenic microsilica or reactive aluminas. The family of the deflocculated castables is subdivided according to the content of CaO—and not ( ) to the content of cement—as shown in Table 5. 

Pearson, D., and A. J. Allen (1985). A Study of Ultrafine Porosity in Hydrated Cements Using Small Angle Neutron Scattering , journal of Materials Science 20 (1) 303-315. 

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