Microsilica properties

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1. Overview

Micro silica (scientific name Microsilica or Silica Fume), in the department of smelting ferrosilicon and industrial silicon, through the flue exhaust steam oxidation, specially designed silica dust collector to collect the amorphous powder SiO2. The average particle size of the micro silicon powder is 0.15-0.20μm, Specific surface area is 15000-20000 m2/kg, which has a very strong surface activity. A large number of micro silica is mainly used in cement or concrete admixture, in order to improve the performance of cement or concrete, preparation with high strength (more than C80), wear resistance, erosion resistance, corrosion resistance, anti permeability and frost resistance, early strength special concrete, used for large dams, reservoirs, hydropower, wharf, railway bridges, highway the airport, runway, tunnel and high-rise building engineering.

2. Physical properties of micro silicon fume

(1) color: micro silica is the by-product reduction generated by high pure quartz in the furnace, coke and sawdust in ferrosilicon and silicon metal production process, the main ingredient is SiO2, general micro silica color is between light gray and dark gray, SiO2 itself is colorless, its color mainly depend on carbon and iron oxide content, the higher the carbon content, color is dark; in addition the encryption silica fume color is dark than natural silica powder.

(2) size: micro silicon powder average particle size is 0.15-0.20μm, is 1 / 100 of cement particle diameter, so silicon fume can highly disperse on the concrete, fill in cement particles and improve the density, while micro silica has high activity, can faster and fully occur oxygen and hydrogen reaction with cement.

(3) specific surface area: 15000-20000 m2/kg.

(4) the proportion of density:
Silica fume has two main supply form, namely the original state of micro silica and densification silicon powder.

original state of micro silica

A, the original state of micro silica, directly collected through the dust collector, loose volume is about 150-300 kg/m3, the original state general use bagged, because the density is small, the long-distance transportation efficiency is low, when use, direct manual broken bags, and pour micro silica into the concrete mixer, great dust of work environment, low working efficiency.

densified silica fume

B. densification microsilica powder, in order to solve the problem that the original micro silica powder is not suitable for long distance transportation and low efficiency, improve the microsilica loose density, develop microsilicon increasing technology. Improve the loose volume of silica fume to 350 – 700 kg/m3, convenient use, densified silica fume particles cohesion weak, so it is easy to disperse in the concrete mixing process, so after feeding aggregate, pour micro silica particles into the mixer, in order to ensure the densification micro silicon particles disperse and good dispersion.

The physical properties of micro silicon powder and cement, slag and fly ash:

Item Silica Fume Cement Slag Fly ash
Specific gravity (N/m3) 21000 31500 29000 21000
Density (Kg/m3) 200-300 1200-140 1000-1200 900-1000
Specific surface area (m2/kg) 20000 200-500 200-600

It can be seen from the table that the proportion of silicon powder is about 2/3 of cement, but the density is only about 1/6 of cement.

(5) particle morphology and mineral structure: during the formation process of silica fume, the amorphous spherical particles is formed by the action of surface tension due to the formation of phase change, and the surface is smooth. Some are aggregates with multiple spherical particles adhered together. It is large specific surface area and highly active volcanic ash. With silica fume, tiny spheroids can be lubricated.

Mechanism of silica fume’s pozzolanic reaction in a cementitous system:

Silica fume in contact with water goes into solution within an hour. The silica in solution forms an amorphous silica rich, Ca poor, gel on the surface of the silica fume particles and agglomerates. After time the silica rich, Ca poor, coating dissolves and the agglomerates of silica fume reacts with free lime(CaOH2) to form calcium silicate hydrates(CSH). This reaction is called the pozzolanic reaction.

Storage of microsilica fume:

Silica fume should be kept dry, out of weather and the elements.

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