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;
&#; Improved bond to substrate and reinforcement;
&#; Improved properties of hardened concrete, including strength and durability;
Silica fume, or microsilica, presents an important by-product produced during the manufacturing process of ferrosilicon and silicon metal alloys. The compound finds wide uses in concrete and provides important benefits for the marine, building construction, and oil and gas industries around the world.
At HSA, the leading provider of silica fume in China, we know all the ins and outs of silica fume applications. Today, we are going to take a close look at the advantages and disadvantages of silica fume when added to concrete.
What is Silica Fume?
Before diving into the advantages and disadvantages of silica fume in concrete, let&#;s have a brief review of what silica fume is.
Silica fume or microsilica is a pozzolanic material that resembles fly ashes or Portland cement in appearance. Silica fume itself consists of non-crystalline or amorphous Silicon Dioxide (SiO2), with particles that are 100 times smaller than a standard cement particle.This means that silica fume particles measure less than 1 micron in diameter and make up a superfine powder with a density of 2.2 g/cm3.
Silica fume can be categorized into densified and undensified silica fume:
·&#; Undensified silica fume posessesbulk density of 200-350 kg/m3.
·&#; Densified silica fume has a bulk density of 380-750 kg/m3.
When one needs to modify or enhance the natural properties of concrete, silica fume can be added to the solution during batching, enhancing the strength and durability of the material.
Why is Silica Fume Important?
The fine silica fume powder is one of the key materials used in the manufacture of refractories and concrete. When added to concrete, silica fume is able to augment its mechanical properties and achieve better corrosive strength, increased sulphate resistance, and superior abrasion resistance. In addition, using silica fume in concrete helps to reduce the permeability of the mixture and improve its durability.
Silica fume presents a truly versatile material that is used to produce virtually all grades of concrete. It can also be used to improve the strength and high-temperature performance of refractories and finds applications in the rubber and water glass industries.
How is Silica Fume Used in Concrete?
In order to better understand the advantages and disadvantages of silica fume, it is important to know what chemical reactions occur once silica fume is added to concrete.
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When added to concrete, silica fume initially remains inert. Then, once the water starts reacting with portland cement in the mixture (hydrating), the primary chemical reaction produces two important chemical compounds:
·.Calcium Silicate Hydrate (CSH), the crystalline compound responsible for the material&#;s strength
·.Calcium Hydroxide (CH), a by-product that acts as a filler and lines pores within the concrete
Next, a pozzolanic reaction occurs between Calcium Hydroxide and silica fume, producing additional Calcium Silicate Hydrate compounds that fill up the voids around the hydrated particles of cement. These additional CSH compounds provide concrete with a denser matrix in areas that would have otherwise remained as small voids subject to potential ingress. As such, concrete develops improved flexural, compressive, and bond-strength properties.
Important Advantages of Silica Fume
Due to its properties, silica fume is widely used around the world where increased strength and durability of concrete are required. Silica fume is available from suppliers like Henan Superior Abrasives Import & Export Co., Ltd and can be easily added during the concrete production process to improve the characteristics of both fresh and hard concrete.
Here are some important advantages of silica fume that are responsible for its wide use in the construction and concrete industry:
1. Very Stable Chemical and Physical Properties
Silica fume serves as a neutral filler with incredibly stable properties, both chemical and physical. It does not contain crystalline water, does not take part in the curing reaction, and does not impact the reaction process in any way.
2. Increased Thermal Conductivity
Not only can silica fume increase thermal conductivity, but it can also improve flame retardancy and change the adhesive velocity.
3. Good Adsorption Performance
Silica fume has a good adsorption performance, as well as good infiltration for various types of resin. In addition, it is easy to mix with no agglomeration phenomenon.
4. Reduced Precipitation and Stratification
Due to its reasonable distribution and fine grain size, silica fume can effectively eliminate stratification and reduce precipitation.
5. Improved Strength
The reasonable size distribution of silica fume ensures good hardness, strong densification, and great wear resistance. Adding silica fume to concrete can improve the wear resistance, compressive strength, and tensile strength of cured products. In addition, the abrasion resistance can be increased by 0.5-2.5 times.
6. Improved Insulation
Silica fume presents pure silicon powder with a low amount of impurities. As such, it possesses stable chemical and physical properties that ensure good arc resistance and insulation of the curing material.
7. Reduced Overall Costs
The bulk density of silica fume ranges between 0.2 and 0.8 or 1.0 and 2.2. When used as polymer filling material, microsilica can reduce the total cost of the product by saving the amount of polymer and minimizing the amount of loading.
8. Improved Corrosion Resistance
Silica fume is composed of silica, or SiO2, which is an inert material. This means that silica fume doesn&#;t react with most alkaloids or acids, and the silicon powder is evenly distributed over the surface of objects. As such, silica fume contributes to improved cavitation and corrosion resistance by 3 to 16 times.
9. Shortened Induction Period
Adding silica fume to concrete can shorten the induction period and provide early strength.
10. Crack Prevention
Silica fume can reduce the peak temperature achieved during the curing reaction of epoxy resin. In addition, it reduces the shrinkage rate and linear expansion coefficient in solidified concrete, thus reducing the internal stress and preventing cracking.
11. Improved Frost Resistance
The relative elastic modulus of microsilica is only 10 to 20 percent after 300-500 freeze-thaw cycles. At the same time, the average elastic modulus of usual concrete is 30 to 73 percent after 25-50 cycles. As such, adding microsilica to concrete can significantly improve its frost resistance.
Disadvantages of Silica Fume in Concrete
While microsilica, undoubtedly, offers a broad range of advantages when added to concrete, it does come with certain drawbacks:
1. Difficulty of Construction
The workability of concrete is an important parameter to consider when designing the proportions for a specific concrete mixture. For example, adding excessive amounts of silica fume powder will reduce the material constructability and cause the concrete to become more compacted. It would be more difficult to plaster concrete or make it close-grained, negatively affecting the concrete quality, its smoothness, and uniformity of the surface.
2. Dry Shrinkage
As compared to regular concrete, the shrinkage rate of silica fume concrete is notably higher, especially the early dry shrinkage. This can be explained by the fact that the filling effect of silica fume leads to decreased moisture content within the concrete, which, in turn, leads to dry shrinkage cracks. This affects the overall strength of the product and limits the potential applications.
In addition, when the external temperature is high, the surface water evaporation increases beyond the bleeding rate, causing dry shrinkage cracks to appear on the surface. Luckily, the issue can be alleviated by focusing on concrete maintenance after construction, such as using strengthening water and sprinkler procedures.
3. Temperature Cracks
Microsilica concrete is also more likely to produce temperature cracks. While the early strength develops quickly with the addition of silica fume powder, the corresponding heat from concrete hydration dissipates quickly, leading to an increase in concrete temperature. This, in turn, places high-temperature stress on the material, which concentrates on the top of the dry shrinkage cracks. This could make these cracks extend deeper, leading to the formation of transfixion cracks.
4. High Price
The price of microsilica is relatively high, when compared to fly ash or cement. While the use of silica fume does increase upfront expenses, the better performance of silica fume cement will offset the costs in the long-term.
Shop for High-Quality Silica Fume at HSA
While the use of silica fume comes with both advantages and disadvantages, the powder presents an important component that finds wide applications in the construction, marine, and oil and gas fields all around the globe.
To take advantage of the improved concrete qualities associated with the use of microsilica, while also minimizing the potential challenges, it is important to choose a trusted silica fume supplier, such as Henan Superior Abrasives Import & Export Co., Ltd. Shop our high-quality silica fume products today.
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