Introduction.
Silica fume also called micro silica, is a light to dark grey cementitious material composed of at least 85 per cent ultra fine, amorphous non-crystalline (glassy) spherical silicon dioxide (SiO2) particles.
It is produced as a by-product during the manufacture of silicon metal or ferrosilicon alloys by reduction of high purity quartz in a submerged-arc electric furnace heated to 2000°C with coal coke and wood chips as fuel.
The individual particles are extremely fine, approximately 1/50th the size of an average Portland cement particle (0.1 to 0.3 µm). The efficiency of silica fume depends upon its mineralogy and particle size distribution.
The extremely fine particle size, large surface area and high content of highly reactive amorphous silicon dioxide give silica fume the super pozzolanic properties.
The effect of silica fume can be explained through two mechanisms—the pozzolanic reaction and the micro filler effect. Like other pozzolanas, silica fume does not have any binding property, but it reacts with Ca(OH2) liberated on hydration of cement. When water is added to cement, hydration occurs forming two primary products.
The first product is calcium-silicatehydrate (C-S-H) gel, that is cementitious and binds the aggregate together in concrete and the other product is calcium hydroxide Ca(OH2) which comprises up to 25 per cent of volume of hydration products. Silica fume reacts with calcium hydroxide to produce more aggregate binding C-S-H gel, simultaneously reducing calcium hydroxide.
The effect of silica fume can be explained through two mechanisms—the pozzolanic reaction and the micro filler effect. Like other pozzolanas, silica fume does not have any binding property, but it reacts with Ca(OH2) liberated on hydration of cement. When water is added to cement, hydration occurs forming two primary products.
The first product is calcium-silicatehydrate (C-S-H) gel, that is cementitious and binds the aggregate together in concrete and the other product is calcium hydroxide Ca(OH2) which comprises up to 25 per cent of volume of hydration products. Silica fume reacts with calcium hydroxide to produce more aggregate binding C-S-H gel, simultaneously reducing calcium hydroxide.
The net result is an increase in strength and durability. The second mechanism is through the micro filler effect. The extreme fineness of silica fume allows it to fill or pack the microscopic voids between cement particles and especially in the voids at the surface of the aggregate particles where the cement particles cannot fully cover the surface of the aggregate and fill all the available space.
This so called interface zone influences the properties of the concrete.
This so called interface zone influences the properties of the concrete.
The effect is credited with greatly reduced permeability and improved paste to aggregate bond, and ultimate strength of concrete.
Advantages.
There are various advantages in using silica fume such as reduction in bleeding and segregation of fresh concrete, and improvements in the strength and durability characteristics of hardened concrete. The combination of high reactivity and extreme fineness results in the possibility of producing more dense concrete with a very low porosity, the pores being small and discontinuous, and therefore, with a high strength and a low permeability.
Physical and chemical properties.
The specific gravity of silica fume is 2.20. The silica fume is available as—produced in untensified from with bulk density of 200-300 kg/m3; densified from with bulk density of 500-600 kg/m3; in the form of micro pellets with bulk density of 600-800 kg/m3; or in a slurry form with the desired concentration (generally with density of 1400 kg/m3). The other basic characteristics are: specific surface area of 1500-2000 cm2/g, standard grade slurry pH value of 4.7, specific gravity of 1.3-1.4 and dry content of micro silica of 48-52 per cent.
Workability.
With the addition of silica fume the slump loss with time is directly proportional to the increase of silica fume content. This is simply due to the introduction of large surface area in the concrete mix by its addition. Although the slump decreases but the mix is highly cohesive. However, with the adjustment in the aggregate grading and by the use of super plasticizer the demand for additional water can be minimized.
Segregation and bleeding.
Silica fume reduces bleeding significantly. This effect is caused mainly by the high surface area of the silica fume to be wetted, thereby reducing the free water left in the mixture for bleeding. Moreover, the silica fume reduces bleeding by physically blocking the pores in the fresh concrete. In the absence of bleeding and due to slow movement of water from interior to the surface, it is essential to finish the concrete as soon as possible after it has been placed and compacted to protect the surface from drying out. It therefore, requires early curing; membrane curing is most effective.
Setting time.
The addition of silica fume in small amounts to ordinary concrete mixtures (250-300 kg/m3) has no significant effect on setting times. However, set retarding occurs with the increased silica fume content. It can also be used in combination with fly ash or blastfurnace-slag to develop strength at early ages.
Applications.
The attributes of silica fume have found their use in shotcrete applications, pumped concrete, mining and chemical industries. The high strength concrete made with silica fume provides high abrasion/erosion resistance. Silica fume influences the rheological properties of the fresh concrete, the strength, porosity and durability of hardened mass. Silica fume concrete with low water content is highly resistant to penetration by chloride ions.
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