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The Properties of Silica Fine Powder and Its Application in Calcium Aluminate Cement and Unshaped Refractory

wallpapers Industry 2020-07-14

Silica fine powder is an indefinite type fine powder, which is formed by the aggregation of fine spherical particles, so it has a relatively large specific surface area and high activity. Silica fine powder is widely used in refractory and ceramic industries because of its unique shape and excellent performance.

1. The nature of silica powder

Silicon oxide micro powder, also known as silica fume, is usually a by-product of smelting metallic silicon or ferrosilicon alloys. When quartz is reduced to silicon by the carbon in the electric arc furnace at high temperature, the accompanying SiO gas will be oxidized and cold-polymerized when it contacts cold air to form highly active and enjoyable SiO2 particles, which can be collected and processed by a dust collector.

The specific gravity of the silica powder is 2.2-2.3g/cm3, the colour is greyish white and grey or even black, and it is a standard round sphere. The bulk density is between 150-700kg/m3, and the melting point is between 1550-1570℃. In general, the particles of silicon oxide fine powder are excellent, with an average particle size of about 0.15 μm, so the specific surface area is relatively large, about 15-30m2/g, so it has very high reactivity. Silica powder is non-flammable, but it is breathable dust (d<5μm), which is less harmful to the human body.

The higher the content of SiO2 in the silicon oxide fine powder, the larger the surface area and the higher the activity. Usually, the content of SiO2 is between 87-98%. Generally, the SiO2 content in the silicon oxide fine powder obtained by smelting the ferrosilicon alloy is lower than that obtained by the metal silicon. Since domestically produced silica fine powder is an industrial by-product, it generally has a very different chemical composition.

As a by-product, the fine silica powder can also be collected when zirconite is used to produce desiliconized zirconium. Compared with metal silicon by-products, the average particle size of the silicon oxide powder obtained by this method is smaller, and the content of alkaline oxide is lower, so the pH value is more economical so that the castable can achieve excellent fluidity and more prolonged use. Because silicon oxide micro-powder is an industrial by-product, it is not limited by cost factors and mechanical equipment. It has a unique shape, so it is the most significant amount in refractory castables.

2. Effect of silica powder on the hydration process of calcium aluminate cement

At present, there is no literature to systematically analyze the impact of silica powder on calcium aluminate cement hydration process. However, from the hydration process of calcium aluminate cement described above, it can be speculated that the chemical composition, specific surface area, particle size distribution, and pH value of fine silica powder may affect the cement hydration reaction.

There is literature showing that the content of Na2O has a significant effect on the hydration reaction of hydrate phases CA and CA2 in calcium aluminate cement. The Na2O content in different silica powders is very different, so it can be speculated that the influence of varying silica powders on the hydration reaction of calcium aluminate cement is very different.

After adding fine silica powder, calcium aluminate cement hydration products can not only nucleate on the surface of cement particles but also nucleate on the surface of silica fine powder particles, so it can increase the nucleation sites of hydration products, which is beneficial to hydration products Crystallization nucleation.

Silica powder and calcium aluminate cement particles will wrap each other when they are in contact, so the particle size distribution of silica powder will affect the accumulation state of the particles. Water must penetrate the accumulation layer to contact the cement particles so that it may change the cement Hydration process.

The pH value of fine silica powder may affect the "hydroxylation" rate in the cement hydration process, thereby affecting the cement hydration rate.

3. Application of silica fine powder in unshaped refractory materials

In the traditional formulation of aluminate cement combined with castable, the water consumption is generally between 9%-13%, and the cement dosage is usually between 12%-30%. Although high cement dosage can make the castable obtain higher normal-temperature strength, the medium-temperature strength of the castable will decrease due to the crystal form transformation of calcium aluminate during the heating process. At the same time, a certain amount of CaO and The oxide reaction in the castable ingredients produces low-melting compounds such as travertine or feldspar, which results in the corrosion resistance and high-temperature strength of the castable being reduced at the same time, and the pores of the castable will also increase significantly due to the high water consumption. Finally, the strength of the castable is reduced.

The development of micro powder technology can solve the above problems. The silicon oxide powder is the more commonly used powder in castables. In the low cement castable series, almost all the gaps between the aggregate particles will be filled with the added silica fine powder, which can reduce the amount of water and cement, thereby reducing the porosity of the castable, the denser structure, and higher strength, It is more resistant to wear and corrosion.

The role of silica powder in refractory castables mainly includes two aspects.

(1) Filling effect

Silica powder is generally in a standard spherical state, and its particle size is small, belonging to the nanometer level, so it has a "ball effect", which can replace water with filling the pores between the particles in the castable. It is precise because of this principle that the fine powder reduces the water consumption of the castable, thereby increasing its high density. After the cast material added with silicon oxide fine powder is calcined at high temperature, the porosity will be significantly reduced so that the middle-temperature strength of the traditional cast material will gradually increase.

(2) Improve the rheology of castable

Silica powder generally has a smaller particle size, so it has higher surface energy. Therefore, a small amount of free water squeezed out by the silicon oxide fine powder can be adsorbed, thereby reducing the open water in the castable and promoting the setting and hardening of the castable. Moreover, the addition of silica fine powder to the casting material will release the free water in the flocculation structure in a short time, thereby reducing the water consumption of the casting material and significantly improving the fluidity of the casting material.

As a by-product of industry, silica micro powder has various physical and chemical properties that are quite different. At present, there is no literature to systematically study the effect of silica powder of different qualities on cement hydration rate. This work is to establish the relationship between the impact of silica fine dust on the cement hydration rate and the influence on the flowability and cast strength of the castable.

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Tag: Silica Fine Powder