Silica fume is available commercially in several forms in both North America and Europe:

• As-produced silica fume is silica fume collected in dedusting systems known as bag houses. In this form, the material is very fine and has a bulk density of about 200 to 300 kg/m3, compared with 1500 kg/m3 for Portland cement (Malhotra et al., 1987).

As-produced silica fume is available in bags or in bulk. Because of its extreme fineness, this form poses handling problems; in spite of this, the material can be and has been transported and handled like Portland cement.

• Compacted silica fume has a bulk density ranging from 500 to 700 kg/m3 and is considerably easier to handle than as-produced silica fume. To produce the compacted form, the as-produced silica fume is placed in a silo, and compressed air is blown in from the bottom of the silo.

This causes the particles to tumble, and in doing so they agglomerate. The heavier agglomerates fall to the bottom of the silo and are removed at intervals.

The air compaction of the asproduced silica fume is designed so the agglomerates produced are rather weak and quickly break down during concrete mixing. Mechanical means have also been used to produce compacted silica fume.

• Water-based silica fume slurry overcomes the handling and transporting problems associated with as-produced silica fume; the slurry contains about 40 to 60% solid particles. Typically, these slurries have a density of about 1300 kg/m3.

Some slurries may contain chemical admixtures such as superplasticizers, water reducers, and retarders. One such product (known as Force 10,000®) has been successfully marketed in North America.

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