Hyphae

Blast Furnace Slag

8 min read

Material · draft · confidence 0.93

Generated from the Hyphae knowledge graph.

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

  • Modern blast furnaces using iron-rich ores produce 180–350 kg of slag per tonne of pig iron; extreme values range from ~100 kg/t (charcoal furnace, rich ore) to ~1300 kg/t (poor ores). (confidence 0.97; sources: CIT-SLG-01)
    • Directly and explicitly stated in Wikipedia Slag (sha256 verified). Note: BF Ironmaking node prose states ‘200–350 kg/t’ — minor discrepancy with this source’s ‘180–350 kg/t’; the 180 figure is the lower bound given in the source.
  • Blast furnace slag density is approximately 2.4 g/cm³ (molten), far less dense than liquid iron (~7.0 g/cm³), causing slag to float and enabling tapping separation. (confidence 0.95; sources: CIT-SLG-01)
    • Wikipedia Slag states density 2.4 and cites 7.85 for steel at 20°C. Molten iron density ~7.0 g/cm³ is consistent with general metallurgical knowledge (not independently fetched for this value).
  • Blast furnace slag typical composition (hematite pig iron): SiO₂ 34–36%, Al₂O₃ 10–12%, CaO 38–41%, MgO 7–10%, FeO/Fe₂O₃ 0.16–0.2%. (confidence 0.93; sources: CIT-SLG-01)
    • From composition table in Wikipedia Slag (sha256 verified). Specific to hematite pig iron slag; other ore types produce different compositions.
  • Rapidly water-quenched (granulated) blast furnace slag is 90–100% glassy and acts as a latent hydraulic binder, forming calcium silicate hydrates (C-S-H); slow-cooled slag is crystalline and largely non-reactive. (confidence 0.95; sources: CIT-SLG-02)
    • Directly stated in Wikipedia GGBS (sha256 verified).
  • GGBS is classified as a latent hydraulic binder (Type II addition) under EN 206:2013 and as CEM III component under EN 197; used at 30–85% replacement levels for Portland cement clinker. (confidence 0.95; sources: CIT-SLG-02)
    • Directly stated in Wikipedia GGBS (sha256 verified).
  • A basic blast furnace slag (CaO/SiO₂ > 1) removes sulfur from the pig iron melt; alkali removal is favored by a more acidic slag (CaO/SiO₂ < 1), creating an operational trade-off that furnace operators must manage. (confidence 0.92; sources: CIT-SLG-01)
    • Directly stated in Wikipedia Slag (sha256 verified), including the ‘hook test’ and the sulfur/alkali trade-off description.

Needs verification

Slag melting temperature 1350–1550°C. (non-blocking)

Cited from CIT-BF-01 (Blast Furnace Ironmaking node prose, step 5). The Wikipedia Blast Furnace article was the source; the specific passage was in the BF node’s existing text and not independently re-verified in this cycle (snapshot was truncated in fetching).

Alkaline leachate pH >12. (non-blocking)

General civil engineering knowledge; not verified from a fetched source in this cycle. Non-blocking informational claim.

Expanded/foamed slag and pelletized slag forms described in common_forms. (non-blocking)

Standard industry practice but Wikipedia Slag article was truncated and these specific forms were not confirmed in the fetched text. Non-blocking contextual description.

Future cluster connection: GGBS is a major supplementary cementitious material used in Portland cement concrete (Portland cement cluster not yet built). This node's CLM-SLG-04/05 lay the groundwork, but a MANUFACTURED_BY edge from a future GGBS Processing procedure node, and a REQUIRES_INPUT edge into a Portland Cement Concrete Procedure, should be added when that cluster is developed. (non-blocking)

Cross-domain linkage to Portland cement cluster not yet in graph. Tracked here so future Planner cycles can pick it up.

Connections

Outgoing

  • Manufactured by → Blast Furnace Ironmaking — Blast furnace slag is co-produced during Blast Furnace Ironmaking as a secondary output. Inverse of the PRODUCES edge (5f04fca9 → Blast Furnace Slag). Allows the Material node to trace its manufacturing origin without scanning all Procedures, per schema convention.

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.

Sources

  • CIT-SLG-01 · (2026) Slag — Wikipedia. sha256:261ddb4882d5bed5fbc08a9ba185ea27c9e1bed346bab69fb1e5188ed0fc3587. https://en.wikipedia.org/wiki/Slag — Verified 2026-05-20. Confirms: slag is a by-product/co-product of pyrometallurgical smelting; major components are metal oxides + SiO₂; density ~2.4 g/cm³; for modern BF with iron-rich ores: 180–350 kg slag per tonne pig iron; extremes 100 kg/t (charcoal) to 1300 kg/t (poor ores); blast furnace slag composition table (hematite pig iron): SiO₂ 34–36%, Al₂O₃ 10–12%, CaO 38–41%, MgO 7–10%, FeO 0.16–0.2%; basicity index definition i = CaO/SiO₂; ‘hook test’ description; sulfur and alkali removal trade-off; enthalpy of fusion ~1800 MJ/t = 3.5% of BF energy balance. Snapshot stored on disk.
  • CIT-SLG-02 · (2026) Ground granulated blast-furnace slag — Wikipedia. sha256:ecf336812d5073a6fcb655788d0d9abc9e4702e940bb5e4644583404724e9b09. https://en.wikipedia.org/wiki/Ground_granulated_blast-furnace_slag — Verified 2026-05-20. Confirms: GGBS obtained by quenching molten BF slag in water/steam to produce glassy granulate, then drying and grinding; latent hydraulic binder forming C-S-H on contact with water; composition range CaO 30–50%, SiO₂ 28–38%, Al₂O₃ 8–24%, MgO 1–18%; glass content 90–100%; classified as CEM III in European norm EN 197; replacement levels 30–85% of Portland cement; slow heat of hydration benefit; higher durability vs. chloride and sulfate attack. Snapshot stored on disk.
  • CIT-BF-01 · (2026) Blast furnace — Wikipedia. sha256:5babca653f71416e0b7f987dfe26e847394756940b04bae8aeb5a8fd3fd476d6. https://en.wikipedia.org/wiki/Blast_furnace — Already cited in Blast Furnace Ironmaking node. Confirms slag melting temperature ~1350–1550°C (composition-dependent), slag density less than liquid iron allowing separation, CaO/SiO₂ basicity target, GGBS cement end use. Snapshot stored on disk.

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