Microsilica

Compositions of high-alumina cement containing quartz or glass, calcium carbonate, microsilica, carbon black, iron oxide red mud or screened fly ash, and styrene-butadiene latex have been described [141,1803,1804]. 

Microscopy technologies, polymer analysis using, 79 567-568 Micro-Sect formulation, 7 564t Microsilica, world demand for, 22 497 Microspectrometers, 76 484-485 Microspectroscopy, infrared, 76 486... 

Verwej et al. [175] have described a procedure for the determination of PH3-containing insecticides in surface water. In this procedure the insecticide is hydrolysed to methylphosphonic acid, and the acid is concentrated by anion exchange and converted to the dimethyl ester. After clean-up on a microsilica gel column the ester is analysed by gas chromatography using a thermionic phosphorus-specific detector. Detection limit is lnmol L 1. 

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. 

Viewed as a mineral addition in concrete, microsilica is characterized by its small particle size and high pozzolanic activity. In the absence of a water-... 

Table 9.8 Chemical compositions of microsilica from the production of elementary silicon and 75% fcrrosilicon alloy (H53)...

Traetieberg (T47) showed that microsilica used as an addition with cement has considerable pozzolanic activity, mainly in the period 7-14 days after mixing, and that the reaction product formed with CH probably had a Ca/Si ratio of about 1.1. Several subsequent studies have shown that the pozzolanic reaction is detectable within hours and also that the early reaction of the alite is accelerated (H37,H54,H55). Huang and Feldman (H54,H55) studied the hydration reactions in some detail. In pastes with 10% or 30% replacement and w/s ratios of 0.25 or 0.45, the CH content passed through maxima usually within the first day before beginning to decrease in those with 30% replacement, it had reached zero by 14 days. Table 9.9 gives some of the results obtained for CH content and non-evaporable water in these pastes. As with pfa cements, and for the same reason, the non-evaporable water contents of mature pastes are considerably lower than those of comparable pastes of pure Portland cements. 

Table 9.9 Contents of calcium hydroxide and non-evaporable water in some pastes of Portland cement with and without microsilica (percentages on the ignited weight) (H53)... 

Rice husk ash has characteristics broadly similar to those of microsilica but can be even more strongly pozzolanic. A study of pastes with C3S (K48) showed the CH content to pass through a maximum of 3% at 7 days, referred to the ignited weight by 28 days, it had fallen to 1%. The hydration product was C-S-H with a Ca/Si ratio estimated at about 1.3 by analytical electron microscopy, or 0.9-1.2 from the contents of CH and unreacted starting materials. There were indications that an initial product with a Ca/ Si ratio of 0.1-0.2 was formed. 

The effects of the limestone are partly physical and partly chemical. As with many other finely divided admixtures, including pfa, the hydration of the alite and aluminate phases is accelerated. Because of its fineness the material also acts as a filler between the grains of clinker, though it is unlikely to be as effective in this respect as microsilica. Chemically, it reacts with the aluminate phase, producing C ACHjj, thus competing with the gypsum. 

Calculations based on reaction stoichiometry and densities of phases support the conclusions from experimental observations that mature pastes of composite cements are more porous than comparable pastes of Portland cements. This is indicated by the results in Table 7.3, 9.4 and 9.6. Similar calculations for 180-day-old pastes of w/s 0.45 indicate free water porosities of about 24% for a typical Portland cement, 35% for a cement with 40% slag, 35% for one with 40% pfa and 32% for one with 30% microsilica. The calculated values are in all cases somewhat higher than observed mercury porosities (F34,F41). 

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. 

H53 Hjorth, L. Microsilica in Concrete (Nordic Concrete Res., No. 1, Paper 9),... 

Many factors influence the ability of reinforced concrete to resist carbonation induced corrosion. As the carbonation rate is a function of thickness, good cover is essential to resist carbonation. As the process is one of neutralizing the alkalinity of the concrete, good reserves of alkali are needed, that is, a high cement content. The diffusion process is made easier if the concrete has an open pore structure. On the macroscopic scale this means that there should be good compaction. On a microscopic scale well cured concrete has small pores and lower connectivity of pores to the CO2 has a harder job moving through the concrete. Microsilica and other additives can block pores or reduce pores sizes. 

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