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Blast Furnace Slag

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Generated from the Hyphae knowledge graph. Drafted by claude-sonnet-4-6 · Reviewed by claude-opus-4-7

A calcium-magnesium-alumino-silicate co-product of pig iron production in a blast furnace. Formed when calcium oxide (derived from limestone flux calcination) reacts with silica and alumina gangue from the iron ore and ash from the coke, producing a molten oxide melt that floats on the denser liquid iron in the hearth. Blast furnace slag is tapped separately from pig iron, then either air-cooled (crystalline aggregate) or rapidly water-quenched (glassy granulate). The quenched form — ground granulated blast-furnace slag (GGBS/GGBFS) — is a latent hydraulic binder widely used as a supplementary cementitious material in Portland cement concrete, improving durability and reducing heat of hydration. Approximately 180–350 kg of slag is produced per tonne of pig iron in a modern blast furnace using iron-rich ores.

Common forms

  • Air-cooled blast furnace slag (ACBFS): slag poured into pits and solidified slowly; forms a crystalline, dense aggregate used for road base, railway ballast, and fill material.
  • Granulated blast furnace slag (GBFS): molten slag quenched by high-pressure water jets or steam, producing a glassy, sand-like granulate. Precursor to GGBS.
  • Ground granulated blast-furnace slag (GGBS or GGBFS): granulated slag dried and ground to Portland cement fineness; a supplementary cementitious material (SCM) classified as a latent hydraulic binder. Used as 30–85% replacement for Portland cement clinker in CEM III cements (European standard EN 197) and as a concrete addition per EN 206:2013. [CIT-SLG-02]
  • Pelletized slag: partially water-cooled then projected by a rotating drum to produce lightweight aggregate.
  • Expanded (foamed) slag: controlled water injection into molten slag produces a vesicular, porous material for lightweight aggregate and insulation applications.

Common sources

  • Blast furnace ironmaking — sole industrial source of blast furnace slag specifically. Co-produced at every blast furnace hearth tap. 180–350 kg per tonne of pig iron in modern operations with iron-rich ores; extreme values ~100 kg/t (charcoal furnace + very rich ore) to ~1300 kg/t (poor ores). [CIT-SLG-01]

Composition

Major oxides (typical range for hematite pig iron slag, wt%): SiO₂ 34–36%, Al₂O₃ 10–12%, CaO 38–41%, MgO 7–10%, FeO/Fe₂O₃ 0.16–0.2%, P₂O₃ ~1%, TiO₂ 1–1.5%, S (minor). [CIT-SLG-01]

The GGBS/GGBFS literature quotes a slightly wider compositional range reflecting variation in raw materials across global operations: CaO 30–50%, SiO₂ 28–38%, Al₂O₃ 8–24%, MgO 1–18%. [CIT-SLG-02]

Slags also incorporate MnO (<1%), alkalis (Na₂O, K₂O), and sulfur (as CaS/oldhamite). Composition is deliberately managed by adjusting limestone and dolomite additions to the burden to achieve the target basicity and viscosity. [CIT-SLG-01]

Hazards

  • Molten slag at ~1400°C — contact with water causes steam explosions; all equipment entering contact with liquid slag must be dry. Severe burns from radiant heat and splash. [Common high-temperature metallurgy hazard knowledge; consistent with CIT-BF-01.]
  • Crystalline slag dust — air-cooled slag may contain trace quantities of crystalline silica; dust generated during crushing may pose silicosis risk. GGBS grinding dust is amorphous silicate (lower silicosis risk than crystalline silica, but respiratory protection is good practice). [Occupational health general knowledge.]
  • Alkaline leachate — fresh slag in contact with water generates highly alkaline leachate (estimated pH >12) due to CaO, Ca(OH)₂, and CaS species; requires containment at storage and road-base sites. [General civil engineering practice knowledge — not directly verified from a fetched source; flagged in needs_verification.]

Properties

  • basicity_ratio: Basicity index i = CaO/SiO₂ (by weight). For ironmaking: typical target i ≈ 1.0–1.2 (basic slag) to maintain fluidity and desulfurization capacity. Basic slag (i > 1) is fluid and favors sulfur removal; acidic slag (i < 1) is more viscous and favors alkali removal. A practical compromise is normally required. [CIT-SLG-01]
  • density_g_per_cm3: ~2.4 g/cm³ (molten slag). Significantly less dense than liquid iron (~7.0 g/cm³), causing slag to float on the iron pool and enabling physical separation at the hearth. [CIT-SLG-01]
  • hydraulic_activity: Rapidly quenched (granulated) slag is predominantly glassy (90–100% glass content). The glassy structure confers latent hydraulic activity: in contact with water and an activator (Portland cement clinker or alkali), it forms calcium silicate hydrates (C-S-H), contributing to concrete strength and durability. Slow-cooled slag is crystalline and largely non-reactive. [CIT-SLG-02]
  • melting_temperature_C: Approximately 1350–1550°C (composition-dependent). [CIT-BF-01]
  • enthalpy_of_fusion_MJ_per_tonne: ~1800 MJ/tonne slag; accounts for ~3.5% of blast furnace energy balance. [CIT-SLG-01]

Claims

Connections

Outgoing

Incoming

  • Produces ← Blast Furnace Ironmaking — Blast furnace slag is co-produced with pig iron; tapped separately from the hearth. Yield: approximately 180–350 kg slag per tonne of pig iron in modern operations with iron-rich ores (Wikipedia Slag, sha256:261ddb4882d5bed5fbc08a9ba185ea27c9e1bed346bab69fb1e5188ed0fc3587). Slag is a secondary output — pig iron is the primary product. Molten slag floats on liquid iron due to density difference (~2.4 vs. ~7.0 g/cm³) and is tapped through a separate notch. Downstream: granulated and ground to GGBS for Portland cement use, or air-cooled for aggregate.

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