Microsilica range from light to dark gray. Because SiO2 is colorless, the color is determined by the nonsilica components, which typically include carbon and iron oxide.In general, the higher the carbon content, the darker the microsilica. The carbon content of microsilica is affected by many factors relating to the manufacturing process, such as: use of wood chips versus coal, wood chip composition, furnace temperature, furnace exhaust temperature, and the type of product (metal alloy) being produced.
The specific gravity of microsilica is approximately 2.2, as compared with about 3.1 for portland cement. Table 1 lists the specific gravity of Microsilica from several sources.Variations in specific gravity are attributed to the nonsilica components of the various Microsilica.
Table 1—Microsilica specific gravity versus alloy type
3.1 As-produced silica fume—The bulk density of as-produced silica fume collected from silicon metal and ferrosilicon alloy production usually ranges from 8 to 27 lb/ft3(130 to 430 kg/m3), although it is most common to see values near the middle of this range.
3.2 Slurried silica fume—Slurried silica fume will typically have a bulk density of approximately 11 to 12 lb/gal. or 83 to 90 lb/ft3 (1.3 to 1.4 Mg/m3). The nominal silica fume content of most slurries is approximately 50% by mass. The actual silica fume content may vary, depending on the particular
source and whether chemical admixtures have been added to the slurry.
3.3 Densified (compacted) silica fume—Densification from an initial bulk density of 13 lb/ft3 (200 kg/m3) to a densified value of 31 lb/ft3 (500 kg/m3) has been reported .The bulk density of commercially available densified silica fume ranges from approximately 25 to 45 lb/ft3 (400 to 720 kg/m3).* At higher bulk densities, it may become increasingly difficult to disperse densified silica fume particles within concrete. Concrete made with a silica fume with a bulk density of 50 lb/ft3 (810 kg/m3) showed decreased performance in both strength and frost resistance (Fidjestøl 1992).
4—Fineness, particle shape, and oversize material
Microsilica consists primarily of very fine smooth spherical glassy particles with a surface area of approximately 98,000 ft2/lb (20,000 m2/kg) when measured by the nitrogen-adsorption method. The extreme fineness of microsilica is illustrated by the following comparison with other fine materials (note that the values derived from the different measuring techniques are not directly comparable):
• Microsilica: 63,000 to 150,000 ft2/lb (13,000 to 30,000 m2/kg), nitrogen adsorption
• Fly ash: 1400 to 3400 ft2/lb (280 to 700 m2/kg), Blaine air permeability
• Ground-granulated blast-furnace slag:1700 to 2900 ft2/lb (350 to 600 m2/kg), Blaine air permeability
• Portland cement: 1500 to 2000 ft2/lb (300 to 400 m2/kg),Blaine air permeability
The nitrogen-adsorption method is currently the most frequently used test to determine the surface area of silica fume. The Blaine air permeability apparatus is not appropriate for measuring the surface area of silica fume because of difficulties in obtaining the necessary 0.50 porosity to conduct
the test. Nitrogen-adsorption surface area results for various microsilica range from 63,000 to 150,000 ft2/lb (13,000 to 30,000 m2/kg) (Malhotra et al. 1987b). One study of Si and FeSi-75% microsilica reported results between 88,000 and 110,000 ft2/lb (18,000 and 22,000 m2/kg).
Another study (Nebesar and Carette 1986) reported average surface area values of 98,000 and 84,000 ft2/lb (20,000 and 17,200 m2/kg) for Si and FeSi-75% microsilica, respectively.Individual microsilica particles have a diameter of less than 1 μm (4 × 10–5 in.), which is approximately 1/100 of the size of an average cement particle. The individual particles,however, are usually found in agglomerations that may range in size from 1 to 100 μm (4 to 400 × 10–5 in.) (Dingsoyr et al.1992). The degree and extent of agglomeration will vary depending on the fume type and the furnace gas exhaust temperature.
Any attempt to show particle-size distribution of silica fume must take the agglomeration tendency into account. Figure 1 shows several particle-size distributions from the same as-produced microsilica subjected to differing degrees of dispersion. ACI Committee 234 is not aware of data showing that different degrees of agglomeration of asproduced microsilica affect its performance in concrete.
Fig. 1—Particle-size distribution of silica fume subjected to varying amounts of dispersion. The set of curves shows the importance of the amount of dispersion on the measured distribution. Varying the intensity and duration of ultrasonic dispersion gives very different results for the same microsilica.The values shown are percentages of full stroke in a Microtrac apparatus; higher percentages indicate increased dispersion effort.
One of the most common tests conducted on microsilica is the residue (oversize) on the 45 μm (No. 325) sieve. In this test, a sample of microsilica is washed through the sieve, and the mass and composition (wood, quartz, carbon, coal, rust, and relatively large microsilica agglomerates) of the oversize particles are reported.
The amount of oversize material is strongly influenced by the microsilica collection system, and the amount of oversize material may vary considerably from one system to another. Various values have been reported for the amount of oversize: 0.3 to 3.5% (Elkem 1980), 3.7 to 5.6% (Pistilli et al.1984a), and 1.8 and 5.4% for Si and FeSi-75%, respectively (Nebesar and Carette 1986).
Because many nonsilica components of microsilica are associated with the larger particles, some microsilica suppliers routinely remove oversize particles from the microsilica. Some oversize removal (beneficiating) processes work with the dry fume using various kinds of cyclones or classifiers. Other systems run slurried microsilica through sieves, usually after the microsilica has been passed through one or more of the dry beneficiating processes.
Table 2 gives the chemical composition of typical microsilica from silicon furnaces in Norway and North America.
Table 2—Variations in chemical composition of microsilica from several sources
The microsilica generally contain more than 90% silicon dioxide (SiO2). The chemical composition of the silica fumes varies with the type of alloy that is being produced .
The acid-soluble chloride content of as-produced and densified silica fume is typically below 0.1%, but individual plants have reported 0.4% as an upper limit.* In cases where chloride limits in concrete are critical, chlorides contributed by the microsilica should be included in the calculation of
total chloride content.
The pH of microsilica and water slurries may be determined.This test may be performed on a sample prepared by adding 0.7 oz (20 g) of microsilica to 2.8 oz (80 g) of deionized water. Typical pH values at one silicon metal source were between 6.0 and 7.0.
Trace amounts of silicon metal may be found in some microsilica. ACI Committee 234 is not aware of data describing effects of variations in any other nonsilicon dioxide components on concrete performance.
Testing by x-ray diffraction has shown microsilica to be essentially amorphous (Nebesar and Carette 1986; Aïtcin et al. 1984). Silicon carbide (SiC), an intermediate compound occurring during the production of silicon and ferrosilicon alloys, has been observed (Popovic et al. 1984). Reported x-ray diffraction patterns exhibit a broad hump centered around the area where cristobalite would normally be found. The absence of a distinct peak at this location suggests that cristobalite is not present in significant quantities.
Although microsilica source-to-source variations and within-source variations have been monitored, only a limited amount of this information has been published. Available results of within-source micro-silica variability studies for chemical composition and physical properties are presented
in Tables 2.2 and 2.3. These results indicate that uniformity of microsilica from a single source is reasonably similar to that associated with ground-granulated blast-furnace slags (GGBFS), and the variations are smaller than those associated with fly ashes (Malhotra et al. 1987b). This observation is not surprising considering that the metallurgical processes used in the production of silicon and alloys containing silicon are well controlled.
Within-source variations occur in microsilica from a single furnace due to the age or condition of the furnace. Typical furnace linings are rebuilt biennially or as refractory deterioration occurs. Changes in the materials used to produce silicon or silicon alloys will cause variations in the microsilica collected from the furnace. If the silicon-alloy type is changed in a furnace, then the microsilica recovered from this furnace will also change.
An approach toward minimizing within-source variations has been to blend silica fume from several furnaces or from many days of production or both. One microsilica supplier produces densified and slurried silica fume products by blending silica fume from four furnaces producing the same alloy.
Because there are few published data available to relate particular physical or chemical properties of microsilica to its performance in concrete, quality-control measures should aim at ensuring uniformity of properties of a particular microsilica to minimize variations in the performance of the
concrete. Changes in the microsilica or in the silicon alloy should be reported by the microsilica supplier. Laboratory testing to verify performance in concrete is recommended if a change occurs.