Micropozzolan spheres, condensed from the vapors of a metallic silicon/ferrosilicon liquid, possess multifunctional applications in oilwell cements. Commonly available and cost-effective, this amorphous substance imparts significant improvements in the physical and mechanical properties of Portland cements. The practical utility of silica fume as an admixture is practical utility of silica fume as an admixture is primarily attributable to its particle size, chemical composition, primarily attributable to its particle size, chemical composition, and reactivity. Composed largely of silicon dioxide (SiO2), silica fume has an average particle diameter of 0.1 pm (100 times finer than cement or fly ash) and yields a surface area on the order of 20 m2/g.
silica fume was added to cements or cement/fly ash mixtures, several beneficial effects were noted:
the water consumptive nature of silica fume allows it to function as an extender and a pozzolan substitute for lightweight cements;
high water adsorption combined with an increased pozzolanic reactivity promotes enhanced compressive strengths;
the purity and solubility of the material makes it suitable for combating strength retrogression in cements at temperatures above 230 deg. F (110 deg. C); (4) permeability and alkali content of the set product we reduced, both desirous properties in a carbon dioxide (CO2) environment. properties in a carbon dioxide (CO2) environment
As a special function admixture for construction concretes, silica fume has been shown to provide such beneficial properties as very high strength, impermeability', high electrical resistivity, superior abrasion endurance, freeze-thaw durability, and sulfate resistance. First used commercially in 1969, silica fume is being used increasingly for oilwell cementing applications.
Silica fume is a waste product generated during the production of silicon/ferrosilicon metals. In the production of silicon/ferrosilicon metals. In the presence of carbon, quartzitic silica is reduced in the presence of carbon, quartzitic silica is reduced in the 2000 deg. C (3600 deg. F temperatures of electric arc furnaces. A portion of the produced silicon reacts with air to form silicon dioxide (SiO2). As the resultant SiO2 fumes cool, they condense into tiny vitreous particles composed primarily of amorphous silica. The condensed silica fume is then processed to remove impurities and control particle size.
Also known in the construction industry as ferrosilicon dust, silica dust, microsilica, amorphous silica, or volatilized silica, the chemical composition of this material varies somewhat depending on its source. A typical analysis shows silica fume to be composed of 91-95% SiO2 0.5-3% Fe2O3, 1-3% mixed alkalis and 12% carton. The surface area of silica fume is generally in the range of 19-21 m2/g. Portland cements and fly ash have specific surface areas ranging from 0.3 to 0.7 m2/g. Even tobacco smoke with a specific surface area of 10 m2/g, has only half the surface area of silica fume.