CHARACTERISTICS OF SILICA FUME
1. Physical Properties
- Diameter is about 0.1 micron to 0.2 micron
- Surface area about 30,000 m²/kg
- Density varies from 150 to 700 kg/m³
- When its density is about 550 kg/m³ it is the best suited as concrete additive
2. Chemical Composition
- Contains more than 90 percent silicon dioxide
- Other constituents are carbon, sulphur and oxides of aluminium, iron, calcium, magnesium, sodium and potassium
PROPERTIES OF SILICA FUME CONCRETE
- Too 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
- Silica fume concrete, due to larger surface area of fine particle requires higher water content for same workability than ordinary concrete
A Compressive Strength
- There is a marked increase in compressive strength, with the use of silica fume.
Silica-Fume Concrete: Typical Strengths
High-Strength Silica-Fume Concrete
- Silica fume concrete has a gain in flexural strength from 5th to 20th days
- Upon filling the voids, silica fume densifies the mix, this increases the tensile strength of concrete and hence there is a increase in flexural strength
Flexural Strength of Silica Fume Concrete
For 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
Stress Strain Curves for Silica Fume Concrete
DURABILITY OF SILICA FUME CONCRETE
Resistance to Chemical Attack
Silica fume checks sulphate attack by:
a) Being very fine, it reduces permeability and the entry of sulphate ions.
b) By consuming the calcium hydroxide in course of pozzolanic action, it checks conversion of mono-sulphoaluminate into ettringite.
Resistance against Acidic Environment
a) Silica fume reacts with lime present in paste matrix. Lime is considered as a dangerous compound, as it reacts with various chemicals causing expansion.
b) Silica fume mortar has a better pore structure which vastly reduces permeability.
c) 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.
- Silica fume improves the properties of fresh and hardened concrete
- Fresh concrete made with silica fume is more cohesive
- Silica fume reduces segregation and bleeding
- Silica fume improves the durability of concrete
- Lack of bleeding allows a more efficient finishing process
PROBLEMS WITH THE USE OF SILICA FUME
- Handling problem
- Silica Fume is now widely used for high strength structures
1. Key Bank Tower, Cleveland.
High-strength (83 MPa), high-modulus (47 GPa) concrete columns were specified at the corners of this structure to stiffen against wind sway.
The concrete for this structure contained a combination of Portland cement, blast furnace slag, and silica fume. The water-cementitious materials ratio was 0.24. A combination of chemical admixtures was used to produce a minimum slump of 250 mm.
2. Capital South Parking Structure, Columbus
Designers of parking structures have specified low-permeability silica-fume concrete to slow the entry of chloride ions from deicing salts. A project using silica-fume concrete in a parking structure is described in the projects portion of this post.
3. Kinzua Dam, Western Pennsylvania
Kinzua Dam is located on the Allegheny River in western Pennsylvania. This structure is owned by the Corps of Engineers.
4. Nuclear Waste Storage Facility, Hanford
On this project silica fume was added primarily to enhance constructability. The exterior concrete walls in this structure are 4.5 ft (1.4 m) thick. To prevent thermally induced cracking, the designer specified very tight control over the maximum temperature rise allowed in the concrete. To meet the requirements for concrete temperature, the contractor elected to use a Class F fly ash in the concrete. While the fly ash did help to reduce temperature development, the early strength gain characteristics of the concrete were such that form removal was delayed. Addition of silica fume to the concrete provided additional early strength to facilitate form removal on a satisfactory schedule.
• The 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 concrete
• Durability is improved through significant improvement in resistance to chemical attack. The chief reasons for this are reduced permeability and reduced levels of calcium hydroxide
• Reinforcement corrosion is vastly reduced due to improved permeability, increased resistivity and improved tensile strength.