5 Things to Know Before Buying Silica Fume for Refractories

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There are many kinds of raw materials for refractory bricks, which are mainly divided into six categories: soil, stone, sand, ore, powder and others.

1. soil: aluminum soil, clay, kaolin, diatomite;

2. stone: fluorite, kyanite, andalusite, forsterite, vermiculite, mullite, pyrophyllite, dolomite, chlorite, sillimanite, spinel, silica;

3. sand: ceramic sand, zircon sand, quartz sand, magnesia
4. ore: chromium ore

5. powder: aluminum powder, silica fume, metal powder;
6. others: asphalt, graphite, phenolic resin, perlite, cenosphere, sialon, corundum, silicon carbide, sulfuric acid, water glass, silica sol, boron carbide, calcium aluminate cement, nitride materials, shale, alumina, zirconia alumina sol, etc.

In the Refractories world thirty-five years ago, no one was working with silica fume and few knew what it was. Within a few years, it was being used as an additive to brick. When added to high Alumina brick, mullite was formed in the matrix of the brick on firing, giving the brick good volume stability, strength and chemical resistance. When used in basic brick, high hot strengths resulted, at least at 2700oF, which was about the limit of what could be tested. At the time it was only logical that silica fume would be used in brick not castables. Brick were used for all critical applications, no one would have considered using castables. Today’s refractory castables have gone beyond having
‘brick-like properties’ to actually out performing brick in many applications. Silica fume has played a major role in this transformation.

Silica fume, also known as microsilica, is an amorphous polymorph of silicon dioxide. It is an ultrafine powder collected as a by-product of the silicon and ferrosilicon alloy production. It is a very fine pozzolanic material which can be used in a variety of cementitious products to improve their performances. (such as concrete, grouts, mortars as well as elastomer, polymer, refractory, ceramic and rubber applications)

Silica fume specifications:

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Because of construction materials, micro silica fume (silica fume) can fill the pores between cement and concrete particles, and form gel with hydration products, and react with basic material magnesium oxide to form gel. It can greatly improve the strength and density of concrete, and is widely used in refractory, concrete, industrial floor, dry powder mortar (or dry mixed mortar), grouting material, cementing wellhead, external wall insulation and other industries.

Use of silica fume:
1. the content of micro silica fume is generally 5% to 10% of cementitious material.

The mixing method of microsilica is divided into internal mixing and external mixing.

(1) Internal admixture: under the premise of constant water consumption, 1 phr silica fume can replace 3-5 phr cement (weight) and keep the compressive strength of concrete unchanged, so as to improve other properties of concrete.

(2) External mixing: the strength and other properties of concrete can be significantly improved by adding silica fume. When concrete is mixed with silica fume, there is a certain slump loss. Silica fume must be combined with water reducing agent. It is suggested to mix fly ash and ground slag to improve its constructability.

Microsilica fume addition method:

2. micro silica fume concrete and castables should be constructed by laboratory. Strict accordance with the ratio of construction. During silica fume concrete mixing, silica fume should be added to the blender immediately after aggregate feeding.

There are two ways to add silica fume:
(1) Input aggregate, then put silica fume and cement dry mix, then add water and other additives.
(2) Put coarse aggregate + 75% water + silica fume + 50% fine aggregate, stir for 15-30 seconds, then add cement + admixture + 50% fine aggregate + 25% water, stir until uniform. The stirring time extend 20-25% or 50-60 seconds than ordinary concrete.

Avoid adding silica fume to mixed concrete.

Silica fume construction method:

There is no significant difference between silica fume concrete and ordinary concrete in construction method, but it is necessary to organize and vibrate compactly in construction. The early strength of silica fume concrete will make the final setting time advance, so attention should be paid during the plastering surface. At the same time, adding silica fume will improve the stickiness of concrete and greatly reduce bleeding, making the plastering surface slightly difficult.

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