Nowadays Silica Fume is widely used for producing high durable concrete for structures particularly exposed to aggressive environments, such as marine structures. It is combined with Ordinary Portland Cement OPC (or portland cement type I according ASTM C150) as a substitution for sulfate resistance cement (cement type V according ASTM C150) in producing sulfate resistance concrete.
Silica Fume is a by product of the silicone and ferro silicone industry.
It results from the reduction of quartize to silicone using high quality charcoal in an electric arc furnace.
Furnaces rotate slowly round carbon electrodes. These create electric arcs deep in the furnace and hence the high temperature needed to produce molten silicon.
The off gas and air drawn in over the furnace is exhausted by large fans to the baghouse. The Silica Fume drop into filter bin and are conveyed to silos.
Silica fume particles average size is 0.1 micron. These particles are thus 100 times finer than a grain of cement.
By blowing air through the Silica Fume in the silo, the particles will touch other particles and adhere to them by surface forces. This is principle of densification for producing Densified Silica Fume, much more practical and easy to use in concrete.
SPECIFICATIONS FOR USING IN HIGH DURABLE CONCRETE
The requirements for used in concrete are generally given in terms of:
SiO2 content: minimum specified of 85%
SiO3 content: maximum of 1%
Loss on Ignition: maximum of 6%
Moisture Content: maximum of 3%
The SiO2 minimum content will limit the quantity of impurities and contaminants in the Silica Fume. The SiO3 is restricted to limit the expansion due to the formation of ettringite. Moisture content of Silica Fume living the furnace is generally negligible. The 3% limit is generous and is likely to be exceed with poor storage. The standard Specification for Use in Hydraulic Cement Concrete is ASTM C1240.
MAIN ADVANTAGES OF USE FOR CONSTRUCTION
The material has been used all over the world for many years in the area where high strength and durable concrete were required. A certain know-how is needed for making good concrete. Simply state, the fresh concrete must have a consistency such that it can be transported, placed and compacted by available site equipment without segregation.
The use in combination with superplasticiser (for improve the dispersion of cement and thus greatly reduce the water demand) has been specified worldwide for producing high quality concrete. This help to improve the characteristics of both fresh and hard concrete.