8 Advantages of Using Silica Fume In Concrete

The concrete conventionally used for construction uses cement with a mix of sand and limestone. However, the common concrete is brittle, rigid, and prone to forming cracks.

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While sand is already in short supply in Gujarat, and cases of illegal sand mining abound, the rising number of vehicles, and consequent rubber tyre waste, also constitutes a major concern with few ways to recycle. Prajapti's method of mixing fine rubber waste with silica fume, instead of sand, gives more flexible and durable concrete, with higher earthquake resistance, he claims.

Silica fume is a mineral admixture composed of very small solid glassy spheres of silicon di-oxide. Most silica fume particles are less than one micron india meter, which is generally 50 to 100 times finer than average cement or flyash particles.

Microsilica in concrete has a set of advantages.

1. By adding micro silica in the concrete, the overall weight of the concrete increases up to 15 percent, where else the normal proposition should be 7 to 10 percent.

By adding micro silica, the potential of the cement becomes high in quality as the cement becomes more strong and brittle. It also increases the water demand in concrete mix; however, dosage rates of less than 5 percent will not particularly require a water reducer. High replacement rates will require the use of high range of water reducer.

Function of adding silica fume is that it increases the durability of the concrete. It also reduces sponginess in the cement, making it robust. Along with this, it also improves the resistance to corrosion giving the concrete resilient life.

2. when micro silica is used in concrete, it acts as a plaster in the cementitious material. The fine particles of micro silica fill spaces between cement particles and also between the cement previous milieus and collective elements. Micro silica also combines with calcium hydroxide to form additional calcium hydrate through the pozzolanic reaction. Both of these actions result in a thicker, solider, denser and less permeable material. The concretes ability to protect the embed steel against corrosion depends on the alkalinity of the pore water, resulting in destroying the passivity of the aggressive ions resulting in corrosion. Fortunately, silica fume with all its pore filling capabilities reduces the rate of carbonation,decreases permeability to chloride ions, imparts high electrical resistivity,and has little effect on the oxygen transport. Therefore silica fume concrete can be expected to be strongly protective and strengthening.

3. Silica fume is a highly reactive pozzolan and gives concrete a denser microstructure, lower permeability & higher electrical resistivity leading to improved durability.

The durability of a reinforced concrete structure can be defined as the capability of the structure to maintain its original functional and structural characteristics during the expected service life in the exposure conditions it was intended for.

Combined with correct design and workmanship, silica fume in concrete offers characteristics that will ensure long term durability and service life.

4. Silica fume adding into concrete helps to increase time to initiation of corrosion – by reducing the permeability of concrete, microsilica significantly reduces the rate of penetration of chloride ions into the concrete.

In addition to protection provided by reduced permeabilty, concrete with silica fume has higher electrical resistivity. Increased electrical resistivity helps reduce the rate of corrosion. Increased electrical resistivity of silica fume in concrete is due to the reduced alkali ion concentration in the pore solution, and a more discontinuous capillary pore structure.

5. Silica fume is used to make the high strength concrete that is often essential in major projects, such as record breaking tall buildings.

These engineering applications requiring high strength concrete (>80 MPa) use microsilica as a cost effective means to increase compressive strength. Ultra-high strength concrete (UHSC), typically defined as >150MPa can be achieved using higher dosages of silica fume. Increased strength results from microsilica's particle packing & pozzolanic reaction. In microsilica concrete, the thickness of the transition zone (interface) between the cementitious paste and aggregate is reduced and the CH crystals are smaller and more randomly oriented, resulting in decreased porosity & increased interface bonding strength.

6. Silica fume is used to help give concrete integral protection from damage by chemicals, including alkali-silica reaction (ASR) and sulfate attack.

Chemical attack on concrete takes place either by leaching of calcium hydroxide or by ingress of harmful substances, such as sulfates or nitrates. Since microsilica reduces the amount of soluble calcium hydroxide, leaching is reduced, and less is available for the harmful expansive reactions caused by sulfates and nitrates.

The addition of microsilica reduces the permeability of concrete. This retards the leaching and ingress of materials from, or to, the interior of the concrete. Consequently, high performance concrete designed with silica fume can possess superior resistance to chemical attack.

7. Silica fume plays an important role in improving the rheology of concrete, mortar and grout mixtures.

Microsilica helps prevent both bleeding and segregation. This is due to the strong internal forces caused by the large surface of the microsilica particles, and improved particle packing in the mixture.

A small dose of micro silica fume helps improve pumpability of concrete and makes self-compacting concrete (SCC) more robust.

8. Silica fume is extensively used in sprayed concrete applications. Benefits are so many, both in the dry and wet processes and include:

– Reduced rebound;

– Increased cohesiveness and stickiness;

– Less dust generation;

– Increased layer thickness;

– Reduced need for accelerator;

– Improved wash-out resistance;

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– Improved bond to substrate and reinforcement;

– Improved properties of hardened concrete, including strength and durability;

The unshaped refractories, commonly known as Monolithic refractories are manufactured by suitably blending graded refractory aggregates, binders, fillers & (/or) special additives used for modification of ultimate properties. The refractory aggregates chosen for the formulation of refractory castables have a major contribution in determining their ultimate product quality. Along with this and many other input parameters, especially the high temperature properties of all type of Castables (Aluminous / Basic) depend a lot on the additive’s level and type used. All ingredients, binders, and additives of different chemical compositions and grading are chosen, blended to provide the proper characteristics for various applications of monolithics. 

Effect of Water Addition vs. Ultrafine Additives (Silica Fume) in Castable Refractories

For castable refractories the mobility of various particles is essential for proper placement of the castable. One method to achieve mobility is by adding more water, but this increases porosity & thus affects the performance both at normal & elevated temp. Therefore, addition of water should be minimized. The desired fluidity can be achieved by maintaining the coarser particles separated from one another by suspension of fines and ultrafine additives combined with the state of flocculation within the suspension. The role of the grains of these ‘Microfines or Ultrafine Additives’ can be compared with those of the balls in a ball-bearing.

What are the commonly used Ultrafine Additives in Refractory Castables?

Read: Benefits of using Steel Fibers and Organic Fibers in  Refractory Castables and other Monolithic refractories

The commonly used microfines or ultrafine additives are Silica Fume also known as Microsilica or Fumed Silica, microfine Alumina (Al2O3), ultrafine green Chrome Oxide (Cr2O3) etc. Silica fume is a by-product of producing silicon metal or ferrosilicon alloys. Silica fume consists primarily of amorphous (non-crystalline) silicon dioxide (SiO2). Because of its super fine particles, large surface area, and the high SiO2 content, silica fume is a very reactive.

The addition of Microsilica or Silica Fume has other advantages also, since these micro fine silica particles easily react with alumina present in the material to form Mullite which, in turn, helps in enhancing refractory properties of the product.

The addition of certain percentage of superfine green Chrome Oxide (Cr2O3) in Alumina castables increases the slag corrosion resistance & HMOR of the product significantly because of the formation of Alumina-Chrome (Corundum) solid-solution.

There are several manufacturers of fumed silica. Some well-known global brands of fumed silica are of Elkem Materials, AEROSIL of Evonik Industries, Norchem Concrete Products. 

Read: Advantages of using Gel Bond and Colloidal Silica in Monolithic Refractories

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