Slag Micropowder, Microsilica and Fly Ash

2022-12-01
Slag Micropowder, Microsilica and Fly Ash

Key words
Slag fine powder, Mineral powder, silica fume, fly ash

I. Slag fine powder
Also known as slag powder, granulated blast furnace slag powder. Mineral powder produced in China is mainly used as concrete admixture;
Mineral powder is a high-fineness, high-activity powder obtained by quenching blast furnace slag with water, drying, grinding and other processes. It is a high-quality concrete admixture and cement mixture, and is recognized in the world today for preparing high-performance concrete  important material.  By using granulated blast furnace slag powder, the compressive strength of concrete can be effectively improved and the cost of concrete can be reduced.  At the same time, it has obvious effects on inhibiting the alkali-aggregate reaction, reducing the heat of hydration, reducing the early temperature cracks of the concrete structure, improving the compactness of the concrete, and improving the impermeability and erosion resistance.

According to the national standard GB/T18046-2000, mineral powder is divided into three grades of S105, S95 and S75, and the content of inorganic substances such as active calcium, silicon and aluminum is more than 30%.
Ultra-fine slag powder can not only reduce the cost of concrete, but also make concrete have good mechanical properties, small and delayed hydration temperature peak, and good volume stability.

Performance of granulated blast furnace slag powder:
Save cement consumption and reduce cement and concrete engineering costs.
It has the effect of volcanic ash and increases the compressive, tensile, flexural and shear strength of concrete.
Significantly reduce the heat of concrete hydration, improve the workability of concrete, reduce segregation and bleeding, reduce the temperature difference and internal stress of large-volume concrete, and inhibit the cracks caused by temperature difference.
It can inhibit the alkali-aggregate reaction and significantly improve the ability of concrete to resist alkali-aggregate reaction.
Improve the pore structure in concrete, refine and homogenize the pore size, and improve the impermeability, freeze-thaw resistance and durability of concrete.
Reduce the bleeding of cement concrete and improve the workability of cement concrete.
Improve the corrosion resistance of cement concrete against seawater and fresh water, and resist sulfate corrosion.

II. Silica fume
In the process of industrial electric furnace melting industrial silicon and ferrosilicon at high temperature, the smoke and dust escaping with the exhaust gas are collected and processed by a special collection device.  In the escaped soot, the SiO2 content accounts for about 90% of the total soot, the particle size is very small, and the average particle size is almost at the nanometer level.

During the high-temperature smelting process of industrial silicon and ferrosilicon, the dust that escapes with the exhaust gas is silica fume or silicon powder, and its main component is active SiO2 (the content accounts for about 90%, and the free SiO2 contained in it accounts for about 10%).  The particles are very small, one percent of the particle size of cement, with small capacity and large specific surface area.  Micro silica fume is widely used in construction, chemical industry, metallurgy and other industries.  It is mainly used as an active admixture in concrete to improve the performance of cement or concrete
Due to the physical and chemical characteristics of fine silica particles, large specific area, high purity of active SiO2 and strong pozzolanic activity, adding silica fume as an admixture to concrete improves the performance of concrete in many aspects.
Improve the early strength and final strength of concrete
increase density
Improve concrete segregation and bleeding performance
Improve the impermeability, chemical resistance and specific resistance of concrete
Silica fume concrete can improve the anti-abrasion ability of the project,
Microsilica fume concrete can improve the crack resistance of the project

III. Fly Ash
It refers to the tiny ash particles emitted during the combustion of coal.  Its particle size is generally between 1 and 100 μm.  Also known as fly ash or soot.
Fine solid particles in the flue gas ash produced by fuel combustion.  Such as fine ash collected from flue gases in coal-fired power plants.  Fly ash is formed after the pulverized coal enters the furnace, undergoes heat absorption by the hot surface under the condition of suspended combustion, and then cools down.
Due to the effect of surface tension, most of the fly ash is spherical, with smooth surface and small micropores.  Part of them are cohered due to collision with each other in the molten state, and become honeycomb composite particles with rough surface and many corners.  The chemical composition of fly ash is related to coal composition, coal particle size, boiler type, combustion conditions and collection methods.  Among them, the main phase is glass, accounting for 50-80%; the contained crystal minerals mainly include: mullite, quartzite, calcite, anorthite, calcium silicate, hematite and magnetite, etc., in addition to a small amount  unburned carbon.  Fly ash mainly contains silicon dioxide (SiO2), aluminum oxide (Al2O3) and iron oxide (Fe2O3), and has been widely used in cement and various light building materials.

The role of fly ash in concrete
Fly ash can play an active role in concrete, which can make the gap between cement and fly ash smaller, play a role of filling, and increase the structural density.
The vast majority of fly ash particles are glass spheres, which can reduce the internal friction force when mixed into concrete, thereby reducing the water consumption in concrete, and improving the concrete pore structure.  Concrete mixed with fly ash has good anti-penetration ability.
Fly ash particles contain active SiO2 and Al2O3, which continuously absorb Ca(OH)2 generated by cement hydration to form hydrated calcium silicate and hydrated calcium aluminate, and combine with free lime and high alkalinity hydrated calcium silicate  A secondary reaction occurs to produce low-alkalinity calcium silicate hydrate with higher strength, better stability and more quantity, which improves the composition of hydration gelling substances, reduces or eliminates free lime, and makes the concrete pore solution  The content of effective alkali and chloride ions is greatly reduced, thus effectively inhibiting the alkali-aggregate reaction and reducing the erosion of chloride ions. 
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