Blast Furnace Slag


Blast Furnace Slag

 Blast furnace (BF) slag is a nonmetallic by product produced during the process of iron making in a blast furnace. It consists primarily of silicates, aluminosilicates, and calcium-alumina-silicates. The molten slag usually absorbs most of the sulfur from the blast furnace charge.

Blast furnace slag is mildly alkaline and exhibits a pH in solution in the range of 8 to 10. Although blast furnace slag contains a small component of elemental sulfur (1 % to 2 %), the leachate tends to be slightly alkaline and does not present a corrosion risk to steels in pilings or to the reinforcement steels embedded in concrete structures made with blast furnace slag cement or aggregates.

In certain situations, the leachate from blast furnace slag may be discolored (characteristic yellow/green color) and have a sulfurous odor. These properties appear to be associated with the presence of stagnant or slow moving water that has come in contact with the slag. The stagnant water generally exhibits high concentrations of calcium and sulfide, with a pH as high as 12.5. When this yellow leachate is exposed to oxygen, the sulfides present react with oxygen to precipitate white/yellow elemental sulfur and produce calcium thiosulfate, which is a clear solution. Aging of blast furnace slag can delay the formation of yellow leachate in poor drainage conditions but does not appear to be a preventative measure, since the discolored leachate can still form if stagnant water is left in contact with the slag for an extended period.

BF slag is normally produced in two forms (Shown in Fig. 1). These two forms are described below.

Granulated and air cooled BF slag

 Fig 1 Granulated and air cooled BF slag

1.   Granulated blast furnace slag – When liquid slag is cooled and solidified by rapid water quenching to a glassy state, little or no crystallization occurs. This process results in the formation of sand size (or frit-like) fragments, usually with some friable clinker like material. The physical structure and gradation of granulated slag depend on the chemical composition of the slag, its temperature at the time of water quenching, and the method of production.

Granulated blast furnace slag is a glassy granular material that varies, depending on the chemical composition and method of production, from coarse popcorn like friable structure greater than 4.75 mm (No. 4 sieve) in diameter to dense sand size grains passing a 4.75 mm sieve. When crushed or milled to very fine cement sized particles, ground granulated blast furnace slag (GGBFS) has cementitious properties.  This property of slag makes it a suitable partial replacement for or additive to Portland cement.

When ground to the proper fineness, the chemical composition and glassy (noncrystalline) nature of vitrified slags are such that in combination with water, these vitrified slags react to form cementitious hydration products. The magnitude of these cementitious reactions depends on the chemical composition, glass content, and fineness of the slag. The chemical reaction between GGBFS and water is slow, but it is greatly enhanced by the presence of calcium hydroxide, alkalis and gypsum (CaSO4).

Because of these cementitious properties, GGBFS can be used as a supplementary cementitious material either by premixing the slag with Portland cement or hydrated lime to produce a blended cement (during the cement production process) or by adding the slag to Portland cement concrete as a mineral admixture.

 2.  Air cooled blast furnace slag (ACBFS) – If the liquid slag is      poured into beds and slowly cooled under ambient conditions, a crystalline      structure is formed to produce a hard lump slag which can subsequently be      crushed and screened. Crushed ACBFS is angular, roughly cubical, and has      textures ranging from rough, vesicular (porous) surfaces to glassy      (smooth) surfaces with conchoidal fractures. There can be considerable      variability in the physical properties of blast furnace slag depending on      the iron production process. For example, some samples of ACBFS have reported      to have a compacted unit weight as high as 1940 Kg / Cum. Higher unit      weights, if they are reported, are generally due to increased metals and      iron content in the slag. The water absorption of ACBFS can be as high as      6 %. Although ACBFS can exhibit these high absorption values, ACBFS can be      readily dried since little water actually enters the pores of the slag and      most of it is held in the shallow pits on the surface.

Chemical properties

Chemical composition of BF slag can vary in the following range under Indian condition.

  • Calcium Oxide (CaO) –  31 % – 40 %
  • Silicon Dioxide (SiO2) – 29 % – 38 %
  • Aluminium Oxide (Al2O3) – 14 %- 22 %
  • Magnesium Oxide (MgO) – 7 % – 11 %
  • Ferrous Oxide (Fe0) – 0.1 % – 1.9 %
  • Manganese Oxide (MnO) – 0.01 % – 1.2 %
  • Sulphur – 1 % – 1.9 %
  • Basicity (CaO/SiO2) – 0.9 % – 1.3 %

Mechanical properties

Air cooled blast furnace slag is most commonly used as an aggregate material. Processed ACBFS exhibits favorable mechanical properties for aggregate use including good abrasion resistance, good soundness characteristics, and high bearing strength. The typical mechanical properties of air cooled blast furnace slag are given below:

i)            Los Angeles Abrasion (ASTM C131) – 35 % – 45 %

ii)           Sodium Sulphate soundness loss (ASTM C88) – 12 %

iii)         Angle of Internal Friction – 40-45 deg

iv)          Hardness – 5-6 Moh’s scale of mineral hardness

v)           California Bearing ratio (CBR) – Up to 250 % for maximum size of 19 mm

Other propertie

Because of their more porous structure blast furnace slag aggregates have lower thermal conductivities than conventional aggregates. Their insulating value is of particular advantage in some applications.