Obtained as a by-product of the manufacture of ferrosilicon and silicon metal alloys, silica fume provides important benefits for the marine, building construction, as well as oil and gas industries around the world.
The grey powder of silica fume, also referred to as “volatilized silica” or “microsilica,” presents valuable pozzolanic material that can be categorized into multiple grades and concentrations. Of course, proper classification requires thorough and careful testing of chemical and physical properties alike. In this article, we are going to take a look at silica fume testing standards all around the globe.
Measuring Bulk Density, Surface Area, and Grain Size of Silica Fume
Among the most common applications of silica fume is its addition to cement in order to improve its durability and strength. In fact, silica fume works to reduce the cement permeability and refine its pore structure, which can help to increase its mechanical properties to a great extent.
In order to be able to achieve the maximum benefits, silica fume manufacturers need to be able to measure and understand various physical properties of the silica fume particles themselves, including their bulk density, surface area, and grain size.
Bulk density, also referred to as “volumetric density” or “apparent density,” is a property of granules and powders that describes the mass of the divided solid that occupies a unit of volume, including the volume of the voids in between the solid particles.
Bulk density is not intrinsic to a specific material. Instead, it can change based on the way the material is handled. For example, when a powder is placed inside a cylinder, it will have a specific bulk density. Once the cylinder starts rotating, the powder particles will move and end up being closer together, leading to higher bulk density.
The bulk density of granules, powders, and other divided solids is commonly measured with a bulk density apparatus. Remember that this bulk density tester does not measure an intrinsic property of a material; as such, the bulk density of powders is generally reported both as freely settled and tapped density.
Bulk Density of Silica Fume – When it comes to silica fume, the standard bulk density will depend on the type of material in question:
The Elkem 920 undensified micro silica fume usually has 200 and 350 kg/m3 bulk density, the densified micro silica fume is 500 to 700 kg/m3.
Surface area is a physical property describing the total area of any given surface. When talking about powders like silica fume, the surface area presents the total area of all of the particles contained in one gram of the material.
The most common test used to measure the surface area of silica fume is the nitrogen adsorption method. Other methods, such as the Blaine apparatus, may not be appropriate for measuring the surface area of this particular material for a variety of reasons. In the case of the Blaine apparatus, the issue lies in the difficulties associated with obtaining the 0.50 porosity level required to conduct the test.
It is important to note that the nitrogen adsorption measurement result is considerably affected by the carbon content of the silica fume. As such, when reporting results, the carbon content should be reported along with the surface area. Many times, the Loss on Ignition (LOI) is reported instead of the carbon content.
Silica fume consists mostly of fine spherical particles with a smooth, glassy surface. The surface area of each particle is estimated to be approximately 20,000 m2/kg, when measured by the nitrogen adsorption method.
To better illustrate the extreme fineness of micro silica, let us compare it to other fine materials:
Keep in mind that the values produced with different measuring techniques shouldn’t be compared directly.
Grain or particle size presents the diameter of individual grains or particles within a granular material. When talking about silica fume, grain size refers to the diameter measurement of each silica fume particle.
One of the most common tests used to measure the grain size of silica fume is the residue (oversize) on the 45 um (No. 325) sieve. During the test, a sample of silica fume material is washed through a 45 um sieve, and the composition and size of the oversize particles are reported. These oversize particles typically contain quartz, wood, coal, rust, carbon, and relatively large silica fume agglomerates. During the test, the amount of oversize material will be considerably influenced by the silica-fume collection system, with the amount of oversize material varying from one system to another.
Because many non-silica components of silica fume tend to have larger particles, some silica fume suppliers choose to remove these oversize particles from their silica fume materials. Some of these beneficiating or oversize removal processes use various types of classifiers or cyclones to work with the dry fume, while others involve running slurried silica fume through screens after the silica fume has been passed through a dry beneficiating process.
While the particle-size distribution of silica fume may vary depending on the furnace gas exhaust temperature and the fume type, most silica fume particles are smaller than 1um, with an average diameter of approximately 0.1 um. For comparison, this presents approximately 1/100 of the average size of a cement particle.
A number of countries, including China, Canada, Brazil, Japan, and more, have developed their own standards for the local use of silica fume powder. These standards outline the mandatory physical and chemical requirements for silica fume to be used as an addition to concrete or grouts, mortars, and other mixtures.
The various standards for different countries across the globe are summarized below:
China’s Chemical Content Testing Standards for Silica Fume
Within China, there are also different standards for testing silica fume’s chemical content:
ASTM Chemical Content Testing Standards for Silica Fume
Choose High-Quality Silica Fume Products Available from Henan Superior Abrasives Import & Export Co., Ltd.
Without a doubt, silica fume presents a valuable material for a broad range of industries and applications. As such, it is of utmost importance to find high-quality silica fume materials with the desired physical and chemical properties. The good news is that at Henan Superior Abrasives Import & Export Co., Ltd., we carry silica fume of various concentrations and grades to meet all of your demands. Find your ideal micro silica product now.
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