Steel making slag

                          Steel making slag

Steel making slag is defined as the solid material resulting from the interaction of flux and impurities in the smelting and refining of steels. It is a by product of steel making. It is produced during the separation of the molten steel from impurities in steel making furnaces. The slag occurs during steel making in liquid state. It is a complex solution of silicates and oxides that solidifies during cooling.

Steel making slag is mainly calcium alumino silicate. This compound contains elements like sulfur, selenium, carbon, cadmium, lead, copper, and mercury. Many of the residuals are encased within a glassy matrix. The matrix is soluble and releases calcium and manganese oxides, which can increase the pH of the dissolving fluid to 10 or 11. As slag is a coarse glass, it maintains high permeability (~4.5 x 10-2 cm/sec) regardless of how much water has passed through it. The permeability of the slag is reduced when it is compacted or grounded into smaller particles.

Unlike lime, steel making slag does not absorb CO2 from the air and convert back to relatively insoluble limestone. This is an important property. Because of this property steel making slag, when left outside exposed to the atmosphere for years, achieves high levels of alkalinity upon dissolution.

The neutralization potential (NP) of steel making slag is in the range of 45 % to 78 %. Most of the residuals are in the form of alumino silicates and iron oxides

Steel making slag aggregates usually exhibit a tendency to expand. This is due to the presence of free lime and magnesium oxides which have not reacted with the silicate structures and which can hydrate and expand in humid environments. This potentially expansive nature (volume changes of up to 10 % or more attributable to the hydration of calcium and magnesium oxides) may cause difficulties with products containing steel slag. This is one of the reasons why steel making slag aggregates are not suitable for use in Portland cement concrete or as compacted fill beneath concrete slabs.

Steel making slag, which is to be used as an aggregate, is stored outdoors for several months to expose the material to moisture from natural precipitation and/or application of water by spraying. The reason for such storage (aging) is to allow potentially destructive hydration and its associated expansion to take place prior to use of the material in aggregate applications. A pile of steel making slag is at Fig. 1.

 sms slag                      

Fig 1 A pile of steel making slag

Physical properties

Steel making slag aggregates are highly angular in shape and have rough surface texture. They have high bulk specific gravity and moderate water absorption (less than 3 percent). Typical physical properties of steel making slag are as follows:

  • Specific gravity –  > 3.2
  • Bulk density – 1.6 – 1.9 tons per cubic meter
  • Absorption – Up to 3 %

Chemical properties

The chemical composition of steel making slag is usually expressed in terms of simple oxides calculated from elemental analysis determined by x-ray fluorescence. The following is the range of compounds present in steel making slag from a typical base oxygen furnace. Virtually all SMS slags fall within these chemical ranges but not all steel making slags are suitable as aggregates. Of more importance is the mineralogical form of the slag, which is highly dependent on the rate of slag cooling in the steel making process.

  • CaO – 46 % to 52 %
  • SiO2 – 12 % to19%
  • MgO – 9 % to 11%
  • Al2O3 – 1 % to 3%
  • MnO – 3 % – 6 %
  • FeO 15 % to 25 %
  • P2O5 – 0.5 % to 1.0 %
  • S – < 0.1 %
  • Metallic Fe – 0.5 %to 6 %

The cooling rate of steel making slag is sufficiently low so that crystalline compounds are generally formed. The predominant compounds are dicalcium silicate, tricalcium silicate, dicalcium ferrite, merwinite, calcium aluminate, calcium-magnesium iron oxide, and some free lime and free magnesia (MgO). The relative proportions of these compounds depend on the steel making practice and the steel slag cooling rate.

Free calcium and magnesium oxides are not completely consumed in the steel making slag, and the hydration of unslaked lime and magnesia in contact with moisture is largely responsible for the expansive nature of most steel making slags. The free lime hydrates rapidly and can cause large volume changes over a relatively short period of time, while magnesia hydrates much more slowly and contributes to long term expansion that may take long time to develop.

Steel making slag is mildly alkaline, with a solution pH generally in the range of 8 to 10. However, the pH of leachate from steel making slag can exceed 11, a level that can be corrosive to aluminum or galvanized steel pipes placed in direct contact with the slag.

Tufalike precipitates, resulting from the exposure of steel making slag aggregates to both water and the atmosphere, have been reported. Tufa is a white, powdery precipitate that consists primarily of calcium carbonate (CaCO3). It occurs in nature and is usually found in water bodies. The tufa precipitates associated with steel slags are attributed to the leachate combining with atmospheric carbon dioxide. The free lime in steel slags can combine with water to produce calcium hydroxide (Ca(OH)2) solution. Upon exposure to atmospheric carbon dioxide, calcite (CaCO3) is precipitated in the form of surficial tufa and powdery sediment in surface water. Tufa precipitates have been reported to clog drainage paths in pavement systems.

Mechanical properties

Processed steel making slag has favorable mechanical properties for aggregate use, including good abrasion resistance, good soundness characteristics, and high bearing strength. Some typical mechanical properties of steel making slag are as follows:

  • Los Angeles abrasion (ASTM C131) – 20 %to 25 %
  • Sodium sulfate soundness loss (ASTM C88) – < 12 %
  • Angle of Internal Friction – 40º to 50º
  • Hardness -6 to 7 Moh’s scale of mineral hardness
  • California Bearing Ratio (CBR)  – Up to 300 for maximum size of 19 mm

Thermal properties

Due to the high heat capacity, steel making slag aggregates have been observed to retain heat considerably longer than conventional natural aggregates. The heat retention characteristics of steel making slag aggregates can be advantageous in hot mix asphalt repair work.

Uses of steel making slag

Steel making slag is recycled in sinter plant as sinter feed since it has high percentage of CaO and MgO. It is used as railroad ballast and as a road base. Steel making slag is also used for soil treatment, hot mix asphalt and as a land fill.