Some properties of concrete using microsilica

What is microsilica or silica fume?

Microsilica (silica fume) results from the reduction of quartz (SiO2) by carbon in the electric arc furnace. Part of the only partially reduced quartz evaporates as SiO and is re-oxidized to SiO2, when it comes in contact with oxygen in a cooler part of the furnace. This SiO2 condenses in tiny microscopic spherical particles as amorphous silicon dioxide with average grain size of 0.1 micron. Microsilica particles are thus about 100 times finer than a grain of cement.

Properties of concrete using microsilica or silica fume?

1. Highest strength

By the reaction cement-superplasticizer-microsilica, strengths hitherto never achieved are possible.
Extremely strong concrete using microsilica ca almost reach the strength of steel.

2. Weight saving

Possibility for tall building have been improved, not only because of the high strength of microsilica concrete but also because of weight saving.

3. Permeability to water
An additional of 10% microsilica and superplasticizer reduces the permeability to one tenth.

4. Resistance to frost and road salt
Microsilica improves the chemical resistance to sulphates, chlorides, (road salt), alkali-reative aggregates, acids, carbon dioxide (carbonation), sulphur dioxide and nitrous gases. Resistance to freeze-thaw with road salts are greatly improved.

5. Corrosion resistance
For steel to rust, for important conditions must be fulfilled. There must be: an electrolytic cell in which there is an anode where ions go into solution and electrons are released; an electrical conductor, in this case the reinforcing steel, which transports the electrons from the anode to cathode; the cathode where the electrons are used up in the presence of acid and dampness; and an electrolyte, the dampness of the concrete which allows ions to move between cathode and anode. It is the reactions at the cathode which control the speed of rusting. Thus when neither acid nor water can diffuse to the steel, rusting cannot occur. By reduction of the permeability and of the electrical conductivity through the combination of calcium hydroxide, microsilica improves the rust resistance of steel in concrete.

6. Chloride penetration and carbonation
The high alkalinity of cement protects the reinforcing steel naturally against rust. The pH value if cement paste is about 12.5 and thus passiveness the steel so that even in the presence of acid and damp it cannot rust. The action of carbon dioxide from the air reduces the alkalinity continuously by neutralizing the lime. If the carbonated region reaches the steel it begins to rust. With chlorides, whether from road salt or seawater, rusting begin at high PH value and even at a high rate. The consequence are known. But if carbonation is stopped and the entry of chlorides is made difficult, the reinforcement corrosion can be prevented.

7. Bond with reinforcement
With increasing microsilica content the bond strength between concrete and steel improves.

8. Abrasion resistance
High abrasion resistance is needed for many applications. The property can be greatly improve by the use of microsilica.

9. Temperature resistance
Combination of the free substances, cement, microsilica and superplasticizer can produce materials of very high strength. Surprisingly, the compressive strength is maintained even above 400 Celsius degrees.