General Info

Amorphous Silica: Creating durable Structures. 

A very fine pozzolanic material composed mostly of non-crystalline silica. Note: The materials may contain some crystalline silica and individual suppliers should be contacted for detailed information if required.

Silica Fume

A very fine pozzolanic material composed mostly of amorphous silica produced by electric arc furnaces as a co-product of the production of elemental silicon or ferrosilicon alloys.

Physical Forms of Amorphous Silica

Silica Fume

Undensified Amorphous Silica

The term ‘undensified’ describes condensed silica fume sourced directly from a silicon or ferro silica furnace; the bulk density at source can be as low as 150kg/m³ and has the consistency of a thick smoke. The bulk density of undensified silica fume is <400 kg/m³. Naturally occurring amorphous silica can have bulk densities in the same range as undensified silica fume.

Densified Amorphous Silica

The term ‘densified’ was originally adopted to define the densification process for condensed silica fume processing. In densification, air is blown through the base of a silo to cause the particles to move around. As the particles come into contact they amalgamate together (due to Van der Vaal’s forces). Over a period of time, the material becomes granular. The bulk density of Densified silica fume is >400 kg/m³ and <700 kg/ m³.

Slurried Amorphous Silica

Amorphous silica is mixed with water to form slurry. A conditioner such as a pH neutraliser may be added to stop setting. Other chemical admixtures for use with concrete such as high range water reducers (HRWR) may be incorporated into the slurry. The specific gravity of slurried amorphous silica increases up to 1.7, depending on the solid content of the slurry. The nominal solid content of most slurries is approximately 50 to 70 per cent by weight.

Naturally Occurring Amorphous Silica

A raw, calcined or refined pozzolanic material, composed mostly of amorphous silica, derived from material occurring in a natural state, such as:


Free flowing fine material


Naturally occurring amorphous silica mixed with water to produce a slurry. Can be mixed with or without the use of conditioners or other admixtures such as high range water reducers or retarders. Slurries will typically contain 50-70% amorphous silica by weight but they are not necessarily limited to this range.

Pre-wetted compressed filter cake

Moist, naturally occurring amorphous silica. Used with or without conditioners or admixtures, in cake form. Typically filter cakes will contain 65-85% amorphous silica by weight but they are not necessarily limited to this range.

Characteristics of Amorphous Silica

Typical Uses of Amorphous Silica

Bulk Density
Description Typical Uses
< 400 Undensified: Raw material. Difficult to collect and package in this form. Fine granular in appearance with a grain size less than 0.5mm. (Minimal densification improves handling whilst agglomerations are loose). Slurries, Blended Cement; Grouts, concretes, mortars, and refractories.
> 400 to < 700 Densified: Grain size greater than 0.5mm. Normal cement-handling techniques can be used on fully Densified amorphous silica. Agglomerates can require high shear mixing action to break up for good dispersion. Concrete and Refractories
1300 - 1700 Slurries: Raw materials mixed with water. The nominal solids content of most slurries is approximately 50 to 70 percent by weight. Slurries, Grouts, concretes, mortars, Concrete and Refractories.