Description of the use of silica fume

Description:

Silica fume(micro silica) reacts chemically with the calcium hydroxide in the cement paste which yields a calcium silicate hydrate gel that significantly enhances strength and durability. The superfine silica fume fills the voids between cement particles creating a very dense, less permeable concrete.

Directions for Use:

Due to the fineness of the Silica fume particles, the admixture needs to be thoroughly mixed and dispensed in and around the cement particles. The proper dispersion is accomplished by adding the silica fume first in the truck mixing process. A typical mix sequence would be:

1. Silica fume.

2. 75% coarse aggregate, plus sand and 75% water.

3. Air entraining admixture (if required)

4. Cement.

5. High range water reducer.

6. Coarse aggregate 25% and 25% water.

Water demand will be increased when using silica fume. Most mixes will require the use of a high range water reducer to maintain workability, low water content and a low water/cement ratio.

Dosage:

The use of silica fume is normally used at the rate of 5 to 10% by weight of cement. Contact The NewWorld Company for guidance where higher dosages of up to 15% by weight of cement are needed.

Placement:

Concrete treated with silica fume may be placed in the same fashion as conventional concrete.

Finishing:

Concrete containing silica fume will bleed much less than conventional concrete; at higher dosage rates bleeding will be essentially eliminated. Plastic shrinkage cracks occur due to rapid moisture loss from the surface of the concrete. Because concrete containing silica fume will have a reduced amount of bleed water to replenish what has evaporated, it will be more susceptible to plastic shrinkage cracking.

Curing:

Proper curing of concrete containing silica fume is absolutely critical in order to achieve the designed high strength and high durability. Proper curing requires the maintenance of proper moisture and proper temperature conditions in the concrete.

All curing of concrete containing silica fume should begin immediately after the finishing procedure is completed. Acceptable curing methods are wet burlap, polyethylene and the use of a high solids liquid membrane-forming curing compound.

Precautions / Limitations

• Test batches/mix designs/sample slabs may be required due to variations in local cement and aggregates.

• Keep concrete from freezing until a minimum strength of 1,000 psi (7 MPa) is reached.

• In all cases, consult the Safety Data Sheet before use.