The primary difference between blast furnace slag (BFS) and steel slag (SS) lies in their origin, chemical composition (especially iron content), and subsequent physical properties and applications. While both are by-products of metallurgical processes, they are formed under different conditions, leading to distinct characteristics.
Key Distinctions at a Glance
| Feature | Blast Furnace Slag (BFS) | Steel Slag (SS) |
|---|---|---|
| Origin Process | Ironmaking (production of pig iron in a blast furnace) | Steelmaking (refining pig iron/scrap into steel) |
| Primary Function | Collect impurities during iron production | Collect impurities and remove phosphorus/sulfur during steel production |
| Iron Concentration | Very low (FeO roughly 0.70%) | High (Total iron content ranges from 16% to 25%) |
| Main Components | Calcium oxide (CaO), silica (SiO₂), alumina (Al₂O₃), magnesia (MgO) | Calcium oxide (CaO), iron oxides (FeO, Fe₂O₃), silica (SiO₂), magnesia (MgO) |
| Color | Typically light grey or off-white | Dark grey to black |
| Density | Generally lower than steel slag | Higher density due to higher iron content |
| Pozzolanic Activity | Highly pozzolanic (especially ground granulated BFS) | Limited or no pozzolanic activity |
| Volume Stability | Highly stable after cooling | Can exhibit volume instability due to free CaO/MgO |
| Typical Uses | Cementitious material, aggregate, road construction | Aggregate, base material, soil stabilization, asphalt filler |
Understanding the Production Processes
To fully grasp the differences, it's essential to understand how each slag is formed:
- Blast Furnace Slag (BFS): This slag is a by-product of the ironmaking process where iron ore, coke, and flux (typically limestone) are heated in a blast furnace to produce molten pig iron. The flux combines with non-ferrous impurities (like silica, alumina, magnesia) from the iron ore and coke to form a molten slag layer that floats on top of the heavier molten iron. This slag is then tapped off. Depending on the cooling method, BFS can be granulated, air-cooled, or expanded.
- Resource: Learn more about the blast furnace process.
- Steel Slag (SS): Steel slag is generated during the steelmaking process, which involves refining molten pig iron and/or scrap steel in a basic oxygen furnace (BOF) or electric arc furnace (EAF). During this process, fluxes (like lime and dolomite) are added to remove impurities such as phosphorus, sulfur, and silicon, which then combine to form the steel slag. This slag helps remove undesirable elements and protects the furnace lining.
- Resource: Understand the basics of steelmaking slag.
Chemical Composition: The Primary Differentiator
The most significant chemical distinction lies in their iron concentration. Blast furnace slag has a very low iron oxide (FeO) level, typically around 0.70%. In contrast, steel slag has a substantially higher total iron content, ranging from 16% to 25%. This difference directly impacts their color, density, and reactivity.
Beyond iron, the general chemical compositions are:
- BFS is primarily composed of calcium oxide (CaO), silica (SiO₂), alumina (Al₂O₃), and magnesia (MgO). Its high content of glassy silicates and aluminosilicates makes it a valuable latent hydraulic or pozzolanic material.
- SS also contains high levels of CaO, but significantly more iron oxides (FeO, Fe₂O₃), along with silica (SiO₂), magnesia (MgO), and small amounts of other elements. The presence of free lime (CaO) and periclase (MgO) in steel slag can sometimes lead to volume expansion if not properly managed, which is a key consideration for its use in construction.
Physical Properties
Due to their differing chemical compositions and cooling methods, BFS and SS exhibit distinct physical properties:
- Color and Density: BFS is typically lighter in color (light grey to off-white) and generally has a lower density due to its lower iron content. Steel slag, with its higher iron content, is usually dark grey to black and denser.
- Pozzolanic Activity: Ground granulated blast furnace slag (GGBFS) is a well-known supplementary cementitious material (SCM) because of its excellent pozzolanic and latent hydraulic properties. This means it can react with calcium hydroxide in the presence of water to form compounds with cementitious properties, enhancing concrete strength and durability. Steel slag, on the other hand, typically has limited or no pozzolanic activity.
- Volume Stability: While BFS is generally volume stable, steel slag can sometimes contain unreacted free lime (CaO) and periclase (MgO), which can hydrate and expand over time. This requires careful processing, aging, or specific applications to prevent issues like swelling in pavements or concrete.
Applications and Practical Insights
The unique properties of each slag lead to diverse applications:
Blast Furnace Slag Applications:
- Cementitious Material: Ground granulated blast furnace slag (GGBFS) is widely used as a partial replacement for Portland cement in concrete and mortar.
- Benefits: Improves concrete durability, reduces permeability, enhances resistance to sulfate attack and alkali-silica reaction, and lowers heat of hydration.
- Example: Used in large concrete pours, marine structures, and infrastructure projects requiring high durability.
- Aggregates: Air-cooled blast furnace slag (ACBFS) is crushed and sized for use as aggregates in asphalt concrete, road base, and unbound fill applications.
- Mineral Wool: BFS can be spun into mineral wool for insulation.
Steel Slag Applications:
- Aggregates: Due to its hardness and angular shape, steel slag is an excellent aggregate for:
- Road Construction: Used in hot-mix asphalt, road base, sub-base, and unbound road layers. Its high friction angle improves skid resistance.
- Railroad Ballast: Its durability helps maintain track stability.
- Soil Stabilization: Its alkaline nature can be beneficial for stabilizing acidic soils.
- Agricultural Lime Substitute: In some cases, it can be used to neutralize soil acidity.
- Heavy Fill Material: Used in embankments and general earthworks.
Impact on Concrete Properties
As noted, the chemical composition of slag significantly affects the properties of concrete. The high pozzolanic nature of BFS makes it invaluable for enhancing concrete performance and sustainability by reducing the clinker content in cement. While steel slag's direct use as a cementitious material is limited, its potential as a concrete aggregate is being explored more extensively, provided volume stability issues are addressed through proper processing like aging or stabilization. Research focuses on finding ways to mitigate its expansion potential to unlock its full potential in concrete mixtures.
