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Steel Slag is a by-product generated during the steel manufacturing process by extracting molten metal from iron ores through the sand, which is commonly used as an abrasive material.
Basic Oxygen Furnace Slag (BOF Slag)
In large quantities, steel is produced from molten pig iron mixed with iron scraps by adding oxygen. The resultant slag has a density of 2-3 g/cm3 and a particle size ranging from 0.1 mm to 10 mm. It is used in cement production, road construction, and as aggregates in concrete.
EAF Slag (Electric Arc Furnace Slag)
This is derived from the Electric Arc Furnace operations, where steel scrap is melted using electric arcs. It has a density of about 2.9 g/cm3 and a similar particle size to BOF slag. EAF contains higher quantities of metallic iron and minor elements, making it suitable for use as aggregate materials in construction.
Ladle Slag
It is produced during ladle refining. It is less voluminous compared to BOF and EAF, but it has a density of about 2.7 g/cm3 and a comparable particle size. Due to its lower density, it is used in lightweight concrete, asphalt pavements, and as a soil amendment.
Induction Furnace Slag
This is produced in Induction Furnaces, where scrap metal and ferroalloys are melted. It is comparable to EAF in composition and production process. It provides a high content of calcium silicate, making it ideal for cement and building material manufacturing.
Cupola Furnace Slag
It is produced in cupola furnaces, which are vertical cylinders used to recycle foundry sand. It has a density of about 2.5-3.5 g/cm3 and a particle size ranging from 0.2 mm to 5 mm. It can be utilized in niche applications like block manufacturing and sand casting operations.
The manufacturing process of steel slag involves several critical steps that ensure the efficient extraction of molten steel from iron ores. Common techniques include:
Blast Furnace:
It is where iron ore is mixed with coke (a form of coal) and limestone, then placed into a blast furnace with hot air. This helps the oxygen to escape from the ore, allowing the carbon to bind to it. Consequently, molten iron sinks to the bottom and forms the bulk while the slag floats on top.
Smelting:
It is the general process of melting ore at very high temperatures to separate metal from impurities. In this case, ore is usually mixed with coke to help drive the chemical reactions. Lime or other fluxing materials are also added to assist in impurity removal.
Reduction:
This process uses chemical agents like carbon or hydrogen to convert iron oxide to metal. Example techniques include the use of blast furnaces, which incorporate carbon and coke for iron reduction, or hydrometallurgical methods, which utilize water and chemical solutions for extraction.
Recycling:
Waste materials such as iron scraps and steel slag, sourced from industrial activities, are collected for recycling. Induction furnaces or electric arc furnaces are utilized to melt down these scraps, with added materials to purify the metal for reuse. This continuously helps in reducing environmental impact and enhances resource efficiency.
After iron extraction, the slag is then allowed to cool naturally or through water quenching, which rapidly cools the slag by immersing it in water. It is subsequently crushed to achieve the required particle size for the intended application, ranging from road construction to cement production.
There are several reasons that make steel slag ideal for construction purposes. Some of them include:
Durability and Strength:
Steel slag is very strong and durable, hence BOF and EAF steel slag are suitable for heavy-load and high-stress applications like road bases and railway ballast. Additionally, it is resistant to wear and weathering compared to other aggregates, thereby ensuring long lifespan of the structures.
Environmental Benefits:
As discussed earlier, steel slag is derived from steel production. Therefore, using it for construction creates a positive impact on the environment by reducing the landfills where it is taken to and thus reducing the environmental dangers that come with it. In addition, it conserves the natural aggregate resources and reduces carbon emissions associated with the production of new materials.
High Load-Bearing Capacity:
Slag aggregate possesses high load-bearing capacity, hence making it suitable for use in concrete mixes in different construction applications such as pavements, roads, bridges, and structures that carry heavy loads. It helps in distributing weight and resisting deformation in the long run.
Cost-Effectiveness:
Steel slag is affordable as it comes from the by-product of steel manufacturing. Since it is abundant and easily accessible, it helps in lowering the overall construction material prices. Thus, it can be used for various infrastructure applications.
Granular Properties:
Slag has angular and rough texture particles that provide excellent interlocking and adhesion properties. This makes it enhance stability in asphalt and concrete mixtures, thus improving the mechanical performances of pavements.
There are some key factors to consider when choosing the suitable steel slag in India for a specific application. They include:
Composition:
Different types of steel slag used in India have different chemical compositions. For instance, Electric Arc Furnace Slag (EAF) has higher traces of manganese and metals, while Basic Oxygen Furnace Slag (BOF) contains lime and iron. Therefore, to choose the ideal steel slag, consider the chemical elements and minerals that will offer the required mechanical strength, hardness, and bind property benefits for the intended use.
Moisture and texture contents:
These are also crucial elements to consider when choosing steel slag because they influence the strength and stability of the structure. When highly moistened, they can reduce concrete strength and lead to rusting of reinforcement bars. Therefore, go for steel slag that contains minimal moisture content and with an angular texture, as it promotes better interlocking and solid anchorage in pavement applications.
Size and gradation:
The steel slag particle size and grading significantly affect the mechanical performances when it is used in concrete. Larger particles are ideal for structural applications because they provide strength and support, while smaller ones can be used for enhanced workability in order to fill voids. Consider choosing slag with a uniform gradation that promotes compaction and reduces the empty spaces in between.
Applications:
Different kinds of steel slag in India are used for different purposes. BOF Slag is used in cement, ballasts, and road constructions, while EAF Slag is utilized for aggregates in structures and concrete. Therefore, choose the type that is widely used for the application intended to utilize the steel slag for.
Steel slag is renowned for its multiple benefits as listed below:
Excellent Durability:
It is highly durable and strong, thus making it long-lasting even under harsh environmental conditions. It can withstand extreme temperatures and wear and tear. Therefore, it is ideal for heavy-duty construction applications like roads, railways, and pavements.
Good Load-Bearing:
Steel slag is able to support substantial weights without deformation because of its high load-bearing capacity. This property makes it suitable for uses in road bases, foundation supports, and structural concrete, where heavy loads are frequently endured.
High-Cement Content:
Steel furnace slag contains high amounts of cementitious materials that allow it to bind with other materials and thus promote concrete strength whenever it is blended with it. It enhances the properties of concrete, such as improving the compressive strength and reducing the empty spaces, hence creating dense concrete.
Eco-Friendly:
Using steel blast furnace slag in construction reduces environmental impact by recycling industrial waste. It reduces the need for virgin aggregates, which aids in conserving natural resources and minimizes carbon emissions as well as the destruction of habitats associated with quarrying.
Good Drainage Properties:
Steel slag’s angular and porous particle structure helps in promoting effective drainage in road and erosion control applications. It aids in water movement through layers without clogging, thereby reducing the risks of waterlogging and associated structural defects.
Better Stabilization:
The angular shape of the steel slag particles offers good interlocking properties, therefore better stabilizing soil and other pavement layers. This leads to enhanced ground stability in construction, which reduces erosion and surface runoff.
Steel slag is used as a partial replacement for Portland cement in concrete due to its cementitious properties. Consequently, when used, it improves the mechanical strength of concrete, enhances workability, and reduces empty spaces, hence creating dense concrete, which increases the durability and sustainability of concrete structures.
While there are lots of benefits of using steel slag, some of the downsides include its variable composition, which may lead to inconsistent concrete properties. Also, it has a high density, which may result in heavier concrete structures, and sometimes it can have unweathered free lime and hábitos emprendedores present, which may cause expansion cracking if not properly treated.
The steel slag particles are non-porous and highly dense; hence, they can hold minimal moisture within them. Even though they have low absorption capacity compared to natural aggregates, they can still retain some amounts of water on the surface, which may affect the concrete mix if not addressed.
