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By: Ashley December 12, 2019 Last Updated :December 12, 2019

What Are the Uses of Blast Furnace Slag (BFS)?

Blast furnace slag (BFS) has been widely studied and used, which can be processed into a variety of valuable materials. It is highly regarded as an environmentally friendly material that can protect the environment by limiting the exploitation of natural resources and reducing energy consumption in the exploitation of natural resources.

What is blast furnace slag?

Blast furnace slag is a by-product of the iron and steel industry. When pig iron is smelted in a blast furnace, the flux is added to the raw materials from the blast furnace in addition to iron ore and fuel (coke). When the furnace temperature reaches 1400-1600 ℃, the lime in the flux reacts with the aluminate, silicate and coke ash in the ore at high temperature and chemically combines to form a nonmetal product called blast furnace slag.

BFS is typically a grey vesicular material that is lighter than most basalt. Because its specific gravity is smaller than that of pig iron, the molten slag rises above the pig iron during heating, making it easy to be separated and recovered. It is pozzolanic with some self-cementing properties. It is estimated that for every ton of pig iron produced, the amount of slag produced is about 300 kg.

Chemical Composition of BFS:

Chemical Composition CaO SiO2 Al2O3 MgO MnO Fe2O3
Content 20-31% 12-32% 13-17% 7-9% 0.3-1.2% 0.2-1.9%

The output of blast furnace slag can not be ignored. If it can be used reasonably, it will save a lot of energy and resources.

Annual Output of Blast Furnace Slag

BFS ouput of several countries

How to process blast furnace slag?

Of course, with the latest trend of recycling society, the effective utilization of blast furnace slag has already attracted people’s attention. During the molten cooling and hardening of blast furnace slag, different cooling methods can be adopted to form various types of blast furnace slag products. It can be broadly categorized into air-cooled slag and granulated slag.

Air-cooled slag

Under natural conditions, the liquid slag is slowly cooled by the surrounding air. A crystal structure will form, resulting in hard, lump slag.

Typical mechanical properties of air-cooled blast furnace slag:

Property Value
Los Angles Abrasion 35-45%
Sodium Sulfate Soundness Loss 12%
Angle of Internal Friction 40-45%
Moh’s Hardness 5-6
Califonia Bearing Ratio (CBR) up to 250%
Water Absorption 2.8%
air-cooled slag

air-cooled slag

Put the slag into the roller crusher and the slag will be broken into a good cube shape with relatively few elongated fragments. The roller crusher is suitable equipment for crushing blast furnace slag, which mainly depends on the extrusion between two rollers to realize the crushing of materials. The fineness of discharge can be controlled to 2-10mm with a handling capacity of 40 tons per hour.

Granulated slag

The molten slag can also be quickly cooled by a column of pressurized water to form a glassy granular slag. Little or no crystallization occurs when the slag is cooled and rapidly quenched to a glassy state. This process produces sand-sized (or fleet-like) fragments, often with some fragile clinker. At this point, the particle size of slag is mostly <5mm. With the use of the ball mill, the granular slag can be processed into ultrafine slag powder. The focus is to enhance the grinding function of the ball mill.

Firstly, adjust the position of the separating board of the ball mill. The length of the first bin can be shortened to 2.6m and the second increased to 3.9m to enhance the grinding capacity of the ball mill. Secondly, transform the partition board to 2mm, so that the slag can be well broken in the first bin and the slag with particle size <2mm enters the second bin for grinding. The residence time of the material in the ball mill can be prolonged, which is conducive to the product fineness reaching the standard. What’s more, increase the filling rate of the second bin to make its forging surface higher than that of the first bin, so as to slow down the flow rate of materials in the ball mill and greatly increase the specific surface area of slag powder.

ground granulated blast furnace slag

ground granulated blast furnace slag

Quality situation of ground granulated blast furnace slag (GGBFS) produced:

Quality Standard Density/ (g/cm2) Specific Surface Area
/(m2/kg)
Mobility Ratio/% Water Content/% SO2 Content/% Ignition Loss/%
GB/T 8064-2000 (S95) ≥2.8 ≥350 ≥90 ≤1.0 ≤4.0 ≤3.0
Actual Product 2.83 402 108 0.46 2.3 0.82

The value of the specific surface area of the slag powder produced by our factory can be control from 420 to 450m2/kg and the average specific surface area 402m2/kg. The output of the ball mill is 6.5 t/h, reaching the average hourly output with the comprehensive power consumption per unit 71 kW.h /t.

production of BFS and GGBFS

production of BFS and GGBFS

What is the application of blast furnace slag?

The blast furnace slag can be crushed and processed into blast furnace slag aggregate by roller crusher and ground granulated blast furnace slag by ball mill. These two materials can be widely used in construction, infrastructure and agriculture.

Blast furnace slag aggregate

The processed BFS aggregate does not contain organic impurities, clay, shells or similar materials, and has many valuable properties, which is one of the reasons for chemical reactions with alkaline aggregates. It reduces the impact on the environment, preserves the precious natural resources needed to sustain the ecosystem, and can reduce the energy consumed in mining, crushed rock and other activities. It is called almighty engineering aggregate.

Coarse aggregate for concrete

In general, concrete containing BFS aggregates has similar plastic properties to natural aggregate. Concrete made of BFS aggregates can be pumped, placed and refined successfully. BFS aggregate results in a better cement paste-aggregate interaction due to its vesicle nature. In many cases, the performance of hardened concrete is improved when BFS aggregates are added to properly designed blends.

Road base

Blast furnace slag can be used as paving material after mechanically stabilizing. It takes longer to harden, which can be used to create a thinner pavement than using natural gravel (i.e., mechanically stable gravel). It is easy to process which enables the road can be opened to traffic immediately after the end of the work, and road compaction base can continue to be carried out even on rainy days, which is widely praised. In addition to being used as the base material, it is also used as aggregate for asphalt mixture due to its excellent hardness and wear resistance.

uses of blast furnace slag aggregates

uses of BFS

Ground granulated blast furnace slag

Ground granulated blast furnace slag is widely used with its characteristics of cost-effective, green effect, hydraulic property as raw material for cement, civil engineering works, fertilizer and so on.

Raw material for Cement

The largest demand for blast furnace slag is cement. Ground granulated blast furnace slag has potential hydraulic performance that can be shown under the action of cement clinker, lime, gypsum and other activators, so it is a high-quality cement raw material. About 60% of the blast furnace slag is made into granulated slag as a cement mixing material. Using granulated blast furnace slag can save energy by 20-40%, reduce costs by 10-30%, and reduce CO2 emissions by 44%.

Civil engineering works

Granulated blast furnace slag for civil engineering is used for bank protection, soft soil filling, road subgrade, embankment, etc. It is lighter than natural sand (11-16 kN/m3) and has a large shear angle (above 35℃). In addition, its hydraulic property makes it hydrate and solidify over time, resulting in high resistance to liquefaction during earthquakes. It also has the same or better water permeability as high-quality sand.

Using GGBFS as a coastal protection backfill material can take advantage of its light weight and large shear angle to effectively reduce the earth pressure of the front sheet pile and reduce the sheet pile section. Once fully solidified, it will not liquefy during an earthquake, so no additional liquefaction measures are necessary.

When used as earth covering for correcting soft ground, its light weight reduces the danger of lateral soil flow as well as the amount of consolidation settlement. As a road structure, it has the advantage with its hydraulic property that will not cause strength loss due to seepage under traffic loading. In addition to cost performance and operability, it also offers excellent durability.

Characteristics Backfill Protection Earth Covering
light weight
large shear angle
reduce the earth pressure reduce the danger of lateral soil flow
reduce the amount of consolidation settlement
hydraulic property no additional liquefaction measures no loss of strength

Fertilizer

Because of the fertilizer components CaO, SiO2 and MgO in ground granulated blast furnace slag, it is used as calcium silicate fertilizer in rice cultivation.

By using granulated blast furnace slag, it improves harvest quantities and flavor by improving photosynthesis, reducing blight on lower leaves, preventing rice blast and promoting ripening.

uses of ground granulated blast furnace slag

uses of GGBFS

The following video shows the production of BFS slag and emphasizes the important role it plays in the construction of our world:

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