The primary sources of silica fume (microsilica) are concentrated in the ferroalloy and silicon metal industries.
1.Ferroalloy Industry
Silicon iron alloy is one of the main sources of silica fume (microsilica) . In the production of silicon iron alloy, varying amounts of silica fume (microsilica) are produced depending on the silicon content and application. For instance, high-silicon silicon iron alloy production typically generates more silica fume (microsilica) than low-silicon silicon iron alloy. - During the smelting of ferroalloys (such as silicon iron alloy, industrial silicon, etc.), silicon and oxygen in the raw materials react at high temperatures in a submerged arc furnace to form silicon dioxide. As the reaction progresses, some of the silicon dioxide escapes as gas. These gaseous silicon dioxides are collected in specific dust collection equipment, and after cooling and condensation treatments, they form silica fume (microsilica) .
2.Silicon Metal Industry
Silica fume (microsilica) is one of the by-products in the production process of metallic silicon. The production of industrial silicon (metallurgical-grade silicon) also generates a significant amount of Silica fume (microsilica). Industrial silicon is mainly used in the manufacturing of semiconductors, solar cells, and other high-tech fields, and the Silica fume (microsilica) produced in this process has a high purity and activity. The production of metallurgical silicon typically uses the carbothermal reduction method, where silicon ore (mainly composed of silicon dioxide) and carbonaceous reductants (such as coal, coke, etc.) are reacted at high temperatures. In this process, a large amount of gaseous silicon dioxide is produced. These gaseous silicon dioxides are collected through a dust collection system to form Silica fume (microsilica). The quality of Silica fume (microsilica) in the production process of metallurgical silicon is usually related to factors such as the quality of silicon ore, production processes, and the efficiency of dust collection equipment.
II. Performance Indicators of Silica fume ( microsilica) and Its Impact on Concrete Properties.
1.Physical Performance Indicators
(1) Appearance:Silica fume (microsilica) is typically a grayish-white powder with extremely fine particles, resembling the appearance of cement. Due to its fine particles, Silica fume (microsilica) has a large specific surface area, generally between 15,000 to 25,000 square meters per kilogram.
The color of Silica fume (microsilica) may vary depending on the source and production process, but it is generally close to grayish-white.
(2) Particle Size: The particle size of Silica fume (microsilica) is very small, with an average particle diameter generally between 0.1 to 0.3 micrometers. This allows Silica fume (microsilica) to fill the gaps between cement particles, increasing the density of concrete.
The particle size distribution of Silica fume (microsilica) is usually narrow, with most particles concentrated in a smaller diameter range. This helps to enhance the activity and filling effect of Silica fume (microsilica).
(3)Specific Surface Area: As mentioned earlier, Silica fume (microsilica) has an extremely high specific surface area, which is one of its important physical properties. The high specific surface area allows Silica fume (microsilica) to fully contact cement and other binding materials, undergo chemical reactions, and improve the strength and durability of concrete.
The size of the specific surface area is usually measured using the nitrogen adsorption method or the BET method.
(4)Water Demand Ratio:
The water demand ratio of silica fume (microsilica) refers to the ratio of the water used in silica fume (microsilica) concrete to the water used in the reference concrete under the same flowability. The lower the water demand ratio, the less impact Silica fume (microsilica)has on the fluidity of concrete. Silica fume (microsilica) has a relatively high water demand, generally between 120% and 130%. This is because silica fume (microsilica) particles are fine, with a large specific surface area, requiring more water to wet their surfaces. When using Silica fume (microsilica), water-reducing agents and other admixtures can be added to reduce the water demand of concrete.
(5)Loss on Ignition (LOI):
The LOI of Silica fume (microsilica) refers to the content of volatile substances such as organic matter and carbonates in silica fume (microsilica) at a certain temperature. The lower the LOI, the higher the purity of Silica fume (microsilica). Generally, the LOI of high-quality Silica fume (microsilica)should be less than 5%.
2. Chemical Performance Indicators
(1) Main Chemical Components:
-The main chemical component of Silica fume (microsilica) is silicon dioxide (SiO₂), with a content usually between 85% and 95%. In addition, Silica fume (microsilica) also contains small amounts of impurities such as aluminum oxide (Al₂O₃), iron oxide (Fe₂O₃), calcium oxide (CaO), etc.
- The high content of silicon dioxide endows Silica fume (microsilica) with good pozzolanic activity, which can react with calcium hydroxide produced by cement hydration to form calcium silicate gel, improving the strength and durability of concrete.
(2) Activity Index:
- The activity index of Silica fume (microsilica) is an important indicator to measure its pozzolanic activity. The activity index is usually determined by comparative tests with cement, that is, mixing a certain proportion of Silica fume (microsilica) and cement, then measuring the compressive strength at a specific age and comparing it with the compressive strength of pure cement.
- The higher the activity index, the stronger the pozzolanic activity of Silica fume (microsilica), and the better the strengthening effect on concrete. Generally, the activity index of high-quality Silica fume (microsilica) is above 100%.
(3) pH Value:
- The pH value of Silica fume (microsilica) is generally between 7 and 11, showing weak alkalinity. This is due to the presence of small amounts of alkaline oxides in Silica fume (microsilica), such as calcium oxide.
- The size of the pH value affects the compatibility of Silica fume (microsilica) with cement and other binding materials, as well as the alkali-aggregate reaction and other properties of concrete. Therefore, when using Silica fume (microsilica), it is necessary to consider its impact on the pH value of concrete properties.
3.Impact on Concrete Properties
(1) Impact on Mixing Properties
-Fluidity: The addition of Silica fume (microsilica) generally reduces the fluidity of concrete mixing. This is because silica fume (microsilica) particles are extremely fine, with a large specific surface area and high water demand. Under the same water content, silica fume (microsilica) adsorbs a large amount of water, reducing the free water in concrete, thereby reducing fluidity. For example, in slump tests, the slump of concrete with Silica fume (microsilica) may be smaller than that of concrete without Silica fume (microsilica). However, the fluidity of concrete can be improved by adjusting the mix ratio, such as increasing the amount of water-reducing agents.
-Cohesion: The addition of Silica fume (microsilica) can improve the cohesion of concrete mixing. Due to the fine particles of Silica fume (microsilica) can fill the gaps between cement particles, making the concrete structure more compact. This helps to improve the cohesion of concrete mixing, reducing the occurrence of segregation and bleeding phenomena. In actual construction, it can be observed that concrete with Silica fume (microsilica)is more stable during transportation and pouring, and is not prone to layering.
-Water Retention: Silica fume (microsilica) also has a positive impact on the water retention of concrete mixing. The micro-aggregate effect of Silica fume (microsilica) can improve the pore structure of concrete, and reduce the number of large pores, thereby improving the water retention of concrete. Concrete mixing with good water retention is not prone to bleeding during construction, which can ensure the quality and appearance of concrete.
(2) Impact on Strength
-Early Strength:Silica fume (microsilica) can significantly improve the early strength of concrete. Silica fume (microsilica) has high pozzolanic activity, which can quickly react with calcium hydroxide produced by cement hydration to form calcium silicate gel. These gels fill the gaps between cement particles, making the concrete structure more compact, thereby improving the early strength of concrete. For example, in some projects that require rapid formwork removal or early use, the addition of silica fume (microsilica) can greatly shorten the curing time of concrete.
-Later Strength: Silica fume (microsilica) also has a continuous strengthening effect on the later strength of concrete. Over time, the hydration reaction between Silica fume (microsilica)and cement continues, generating more calcium silicate gel, further improving the density and strength of concrete. In the long term, the strength increase of concrete with Silica fume (microsilica) is usually greater than that of concrete without Silica fume (microsilica).
(3) Impact on Durability
-Freeze-Thaw Resistance: Silica fume (microsilica) can improve the freeze-thaw resistance of concrete. This is because the fine particles of Silica fume (microsilica) can fill the pores of concrete, increasing the density of concrete, reducing water penetration and freeze expansion. In addition, the reaction between Silica fume (microsilica) and calcium hydroxide produced by cement hydration, forming calcium silicate gel, can enhance the strength and durability of concrete, improving its freeze-thaw resistance.
-Permeability Resistance: Silica fume (microsilica) can significantly improve the permeability resistance of concrete. Due to the fine particles of Silica fume (microsilica), which can fill the pores of concrete, forming a compact structure, preventing the penetration of water and harmful substances.At the same time, the reaction between Silica fume (microsilica) and calcium hydroxide produced by cement hydration, forming calcium silicate gel, can block the capillary pores in concrete, further improving the permeability resistance of concrete.
-Chemical Resistance: Silica fume (microsilica) can improve the chemical resistance of concrete. The calcium silicate gel formed by the reaction between Silica fume (microsilica) and cement hydration products has high chemical stability and can resist the erosion of acids, alkalis, salts, and other chemical substances. In some chemically corrosive environments, such as wastewater treatment plants, chemical plants, etc., concrete with Silica fume (microsilica) can better maintain its structural integrity and performance stability.
III. Testing Methods for Silica fume (microsilica)
1.Chemical Analysis Methods
(1) Determination of Silicon Dioxide Content:
- The main component of Silica fume (microsilica) is silicon dioxide, and its content is usually determined by the gravimetric method or the potassium fluosilicate volumetric method.
- The gravimetric method involvesSilica fume (microsilica) samples with fluxes such as sodium carbonate, fusing at high temperatures to convert silicon dioxide into soluble silicates. Then, the fused material is dissolved in hydrochloric acid, filtered, washed, and ignited to determine the weight and calculate the content of silicon dioxide.
- The potassium fluosilicate volumetric method utilizes the reaction of silicon dioxide in silica fume (microsilica) with hydrofluoric acid to form silicic acid, which is then converted into potassium fluosilicate precipitate by adding potassium salts. After filtration, washing, and neutralization, the content of silicon dioxide is calculated by titration with a standard sodium hydroxide solution.
(2) Determination of Loss on Ignition (LOI):
- LOI refers to the percentage of mass lost by Silica fume (microsilica) after calcination at a certain temperature compared to the original sample mass. The determination of LOI can be done by the calcination method.
- Silica fume (microsilica)samples are calcined in a high-temperature furnace until a constant weight is reached, and the LOI is calculated based on the mass difference before and after calcination. LOI mainly reflects the content of volatile substances such as organic matter and carbonates in silica fume ( microsilica ).
(3) Determination of Impurities such as Iron Oxide and Aluminum Oxide:
- The content of impurities in Silica fume (microsilica) can be determined by chemical analysis methods, such as spectrophotometry and atomic absorption spectroscopy.
- These methods can accurately determine the content of impurities such as iron(III) oxide, aluminum oxide, calcium oxide, and magnesium oxide in Silica fume (microsilica), thereby assessing the quality of Silica fume (microsilica).
2. Physical Property Testing Methods
(1) Determination of Specific Surface Area:
The specific surface area of Silica fume (microsilica) is usually determined by the nitrogen adsorption method (BET method). Silica fume (microsilica) samples are degassed at a certain temperature and then adsorb nitrogen at liquid nitrogen temperature. By measuring the amount of adsorbed nitrogen, the specific surface area of Silica fume (microsilica) can be calculated. The specific surface area is an important indicator for measuring the activity and filling performance of Silica fume (microsilica).
(2) Particle Size Distribution Determination:
The particle size distribution of Silica fume (microsilica) can be determined using a laser particle size analyzer. The laser particle size analyzer uses the principle of laser scattering to measure the scattering intensity of silica fume (microsilica) particles against the laser, thereby calculating the size and particle size distribution of the particles. Particle size distribution can reflect the uniformity and fineness of silica fume (microsilica) particles, which has a significant impact on the performance of Silica fume (microsilica).
(3) Determination of Water Demand Ratio:
- The water demand ratio refers to the ratio of the water used in Silica fume (microsilica) concrete to the water used in reference concrete under the same flowability. The determination of the water demand ratio can be done using the cement mortar flow test method.
- Silica fume (microsilica) is mixed with cement in a certain ratio, standard sand and water are added, and the mixture is stirred into mortar. Then, the flowability of the mortar is measured and compared with the flowability of the reference cement mortar to calculate the water demand ratio. The water demand ratio reflects the impact of Silica fume (microsilica) on the fluidity of concrete.
(4) Determination of Activity Index:
- According to a certain mix ratio, Silica fume (microsilica) is mixed with cement, sand, stone, and water, stirred evenly, and concrete specimens are prepared. At the same time, reference cement concrete specimens are prepared for comparison.
- The concrete specimens are cured under standard curing conditions until the specified age, such as 7 days, 28 days, etc. Then, the compressive strength of the concrete specimens is measured using a pressure testing machine.
- Based on the measured compressive strength values, the activity index of Silica fume (microsilica) concrete is calculated. The calculation formula for the activity index is: Activity Index = (Compressive Strength of Silica fume (microsilica) Concrete / Compressive Strength of Reference Cement Concrete) × 100%.
(5) When testing silica fume (microsilica), the following points should be noted:
-Sample Representativeness: When collecting Silica fume (microsilica) samples, it is important to ensure that the samples are representative. Samples can be collected from different parts and batches of Silica fume (microsilica) and mixed evenly.
-Accuracy of Testing Methods: Choose appropriate testing methods and strictly follow standard operating procedures for testing. Use accurate and reliable testing equipment and reagents to ensure the accuracy of the test results.
-Control of Testing Environment: The environmental conditions during the testing process should be controlled, such as temperature and humidity. Avoid the impact of environmental factors on the test results.
-Data Processing and Analysis: The test data should be processed and analyzed correctly, and the calculated results should be accurate. At the same time, the test results should be reasonably evaluated and judged based on the actual situation.
IV. Difference Between Silica fume (microsilica) and Silicon Powder
1.Different Definitions and Sources
(1) Silica fume (microsilica): Silica fume (microsilica), also known as microsilica, is amorphous, powdery silicon dioxide (SiO₂) micro-particles collected by specially designed dust collectors from the oxidation of silicon vapor emitted through the smokestacks during the smelting of silicon iron alloys and industrial silicon. Mainly sourced from the ferroalloy industry, it is a by-product of industrial production. During the smelting process, silicon reacts with oxygen at high temperatures to form silicon dioxide vapor, which is then cooled and collected to form Silica fume (microsilica).
(2) Silicon Powder: Silicon powder typically refers to powdery silicon dioxide made from natural quartz or fused quartz through processing techniques such as crushing and grinding. Mainly derived from natural minerals or obtained by processing quartz. Natural quartz, after mining and ore dressing, is then subjected to crushing, grinding, grading, and other processes to obtain silicon powder of different particle sizes.
2. Different Physical Properties
(1) Appearance:Silica fume (microsilica): Grayish-white or light gray powder, extremely fine particles, resembling the appearance of cement. Silicon Powder: Usually white or colorless and transparent, with particle size slightly coarser than silica fume ( microsilica).
(2) Particle Size:Silica fume (microsilica): Average particle diameter generally between 0.1 to 0.3 micrometers, belonging to nano to sub-micron particles.Silicon Powder: Particle size usually ranges from a few micrometers to several tens of micrometers, relatively larger than Silica fume (microsilica).
(3) Specific Surface Area:Silica fume (microsilica): Extremely large specific surface area, generally between 15,000 to 25,000 square meters per kilogram.Silicon Powder: Relatively smaller specific surface area, generally around a few thousand square meters per kilogram.
3. Different Chemical Properties
(1) Chemical Composition:
- Silica fume (microsilica): The main chemical component is silicon dioxide, with a content usually between 85% and 95%, and also contains small amounts of impurities such as aluminum oxide (Al₂O₃), iron oxide (Fe₂O₃), calcium oxide (CaO), etc.
- Silicon Powder: The main component is silicon dioxide, with a higher purity, generally above 98%, and relatively fewer impurities.
(2)Activity:
- Silica fume (microsilica): Has high pozzolanic activity, which can quickly react with calcium hydroxide produced by cement hydration to form calcium silicate gel, improving the strength and durability of concrete.
- Silicon Powder: Relatively low activity, but can also react with calcium hydroxide in cement under certain conditions, which has a certain improvement effect on the performance of concrete.
4.Different Application Fields
(1) Silica fume (microsilica) is mainly used in concrete and refractory materials.
- Concrete field: Widely used in high-performance concrete, can improve the strength, durability, impermeability, freeze-thaw resistance and other properties of concrete. Especially in high-strength concrete, self-compacting concrete, shotcrete and other special concrete, the application effect is significant.
- Refractory materials: As an additive for refractory materials, can improve the strength, thermal shock resistance and corrosion resistance of refractory materials.
- Chemical industry: Used in coatings, rubber, plastics and other industries, can improve the strength, wear resistance and corrosion resistance of products.
(2) Silicon powder is mainly used in electronic materials and ceramics industry.
- Electronic materials: Due to its high purity and good insulating properties, it is widely used in the fields of semiconductors, integrated circuits, electronic packaging materials, etc.
- Ceramic industry: As a ceramic raw material, can improve the performance of ceramics, such as improving strength, hardness, wear resistance, etc.
- Coating industry: Used in high-grade coatings, can increase the hardness, wear resistance and corrosion resistance of coatings.
There is a serious difference between Silica fume (microsilica) and silicon powder, which needs to be carefully identified when used in concrete.
(1) Reduce Volume:Silica fume (microsilica) is a very fine powder with a very low bulk density when not densified, usually around 150-300 kg/m³. This means that the same mass of un densified Silica fume (microsilica)occupies a very large volume. Through densification treatment, the bulk density of silica fume (microsilica) can be increased to 500-800 kg/m³ or even higher. This can greatly reduce the volume of Silica fume (microsilica), facilitate storage and transportation, and reduce transportation costs. For example, in the transportation process, densified Silica fume (microsilica) can carry more goods in the same transportation space, improving transportation efficiency.
(2)Prevent Dust Flight: Un densified Silica fume (microsilica) particles are very fine and easy to fly and form dust. During storage and transportation, if silica fume (microsilica) flies, it will not only cause environmental pollution but also harm the health of operators. After densification, the gaps between Silica fume (microsilica)particles are reduced, and the friction between particles is increased, making it less likely to fly. This can effectively prevent dust pollution of Silica fume (microsilica) during storage and transportation, and improve the safety of the working environment.
2.Improve Use Performance
(1) Improve Fluidity:
After densification, the agglomeration phenomenon between silica fume (microsilica) particles is improved, and the fluidity is enhanced. This makes it easier for silica fume (microsilica) to disperse in materials such as concrete, improving construction efficiency and material performance stability. For example, during the concrete mixing process, densified Silica fume (microsilica) can mix more evenly with cement, aggregate, etc., improving the quality of concrete.
(2) Enhance Stability:
Densified silica fume (microsilica) has stronger resistance to external environmental factors due to closer particle binding. This makes Silica fume (microsilica) more stable during storage and use, less affected by humidity, temperature and other factors.
In summary, due to its high activity, Silica fume (microsilica), as a high-quality auxiliary binding material, can partially replace cement, reduce the amount of cement used while ensuring the strength and performance of concrete. This can not only reduce the cost of concrete but also reduce energy consumption and environmental pollution in the cement production process. However, it should not be mixed in too high a proportion during the application process, which may increase the risk of concrete shrinkage and cracking. Therefore, it should be strictly implemented according to the national standard specification of not more than 10% mixing ratio.
