01-06-2012, 01:19 PM
MICROSILICA– CHARACTERIZATION OF AN UNIQUE ADDITIVE
MICROSILICA.doc (Size: 1.25 MB / Downloads: 40)
ABSTRACT
Microsilica, or silica fume is an amorphous type of silica dust mostly collected in baghouse filters as by-product of the silicon and ferro-silicon production. The paper summarizes important physical and chemical properties of microsilica and uses those results for an evaluation of microsilica from a Health Safety and Environment (HSE) standpoint.
Microsilica consists of spherical particles with an average particle size of 150 nm and a specific surface area of typically 20 m2/g. The chemical and physical properties of this inorganic product are different as compared to other amorphous and crystalline silica polymorphs. More than 500.000 MT of microsilica are sold to the building industry world-wide and are used in fibre cement, concrete, oil-well drilling, refractories, and even in polymers.
Microsilica contains trace amounts of heavy metal oxides and organic deposits, which originate from natural raw materials. Since the concentration of these impurities is very low, microsilica complies with company policies and international regulations. Traces of crystalline silica in microsilica do not seem to represent a health risk, neither for silicosis nor for lung cancer, due to the low levels and the large particle size.
According to the International Agency for Research on Cancer (IARC), there is insufficient evidence for the carcinogenicity of silica fume, in contrast to crystalline silica.
INTRODUCTION
In order to tailor or enhance concrete properties for specific applications, a multitude of concrete additives is available on the market. Among these are also pozzolans. According to ASTM C618, pozzolans are defined as
“siliceous or siliceous and aluminous material which, in itself, possesses little or no cementitious value but which will, in finely divided form in the presence of moisture, react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties”. Typical pozzolans are for example volcanic ashes, mineral slag, fly ash, and silica fume.
CHEMICAL AND STRUCTURAL CHARACTERIZATION OF MICROSILICA
Characteristics of microsilica
The chemical composition of microsilica depends strongly on the choice of raw materials and the production process parameters. According to international standards (e.g. ASTM 1240-01, prEN 13263-1), the silica content of silica fume for the use in concrete must be at least 85 %. Typical impurities are carbon, silicon, phosphorus, sulfur, silicon carbide, chloride, and a variety of metal oxides. The specific surface area is between 15 and 30 m2/g, which is relatively high, compared to crystalline silica flour (e.g. 4 m2/g). The color of microsilica varies from dark black to almost white, depending on the carbon content. Undensified microsilica is a fluffy powder with low bulk density around 100-300 g/l.
As depicted in Figure 3, microsilica is amorphous to X-rays and thus, lacks long range order. This means that the primary building units of amorphous silica, SiO4 tetrahedra, are randomly connected to each other and do not exhibit a regular pattern, in contrast to the crystalline silica polymorphs quartz, cristobalite or tridymite.