2 INTRODUCTIONSilica fume is also referred to as micro silica or condensed silica fume but the term silica fume has been generally acceptedThe addition of ultra fine particles in concrete improves strength of concreteThe optimum proportion of silica fume is found in between 20% and 25% by weight of concreteSilica Fume when react with calcium hydroxide on hydration of cement produces calcium silicate hydrate which has good binding properties.
3 CHARACTERISTICS OF SILICA FUME 1 Physical PropertiesDiameter is about 0.1 μ to 0.2 μSurface area about 30,000 m²/kgDensity varies from 150 to 700 kg/m³When its density is about 550 kg/m³ it is the best suited as concrete additive
4 2 Chemical CompositionContains more than 90% silicon dioxideOther constituents are carbon, sulphur and oxides of aluminium, iron, calcium, magnesium, sodium and potassium
5 PROPERTIES OF SILICA FUME CONCRETE 1 Properties of Fresh ConcreteToo much silica fumes cause the concrete to become sticky and thus reduces the workability.Silica fume addition up to 15% by weight of cement does not result in any loss of workability.
6 2 Properties of Hardened Concrete A Compressive StrengthThere is a marked increase in compressive strength, with the use of silica fume.
8 B Flexural StrengthSilica fume concrete has a gain in flexural strength from 5th to 20th daysUpon filling the voids, silica fume densifies the mix, this increases the tensile strength of concrete and hence there is a increase in flexural strength
10 C. Elastic ModulusFor silica fume concrete the strain at failure was more or less equal to that of ordinary concrete. This means that the modulus of elasticity is substantially higher than that of normal concrete
12 DURABILITY OF SILICA FUME CONCRETE Resistance to Chemical AttackSilica fume checks sulphate attack byBeing very fine, it reduces permeability and the entry of sulphate ions.By consuming the calcium hydroxide in course of pozzolanic action, it checks conversion of mono-sulphoaluminate into ettringite.
13 Resistance Against Acidic Environment Silica fume reacts with lime present in paste matrix. Lime is considered as a dangerous compound, as it reacts with various chemicals causing expansion. Silica fume mortar has a better pore structure which vastly reduces permeability.Addition of silica fume as a partial replacement of cement reduces C3A content of the paste. C3A is seemed to react with acids causing expensive products.
14 ADVANTAGESSilica fume improves the properties of fresh and hardened concreteFresh concrete made with silica fume is more cohesiveSilica fume reduces segregation and bleedingSilica fume improves the durability of concreteLack of bleeding allows a more efficient finishing process
15 PROBLEMS WITH THE USE OF SILICA FUME AvailabilityHandling problemCost
16 APPLICATIONSSilica Fume is now widely used for high strength structures
24 Concrete in marine structures is also susceptible to chloride-induced corrosion.
25 Deicing salt damage to a bridge curb and parapet. conc057
26 CONCLUSIONThe use of silica fume give rise to an increase in the fresh as well as hardened properties of concrete, with less cement and a concrete having better resistance for freezing and thawing than ordinary concreteDurability is improved through significant improvement in resistance to chemical attack. The chief reasons for this are reduced permeability and reduced levels of calcium hydroxideReinforcement corrosion is vastly reduced due to improved permeability, increased resistivity and improved tensile strength.
27 REFERENCES1. Silica Fume Association <http//:www.silicafume.org>2.3.