What Are Supplementary Cementitious Materials (SCMs)?
Supplementary cementitious materials (SCMs) are fly ash, slag, silica fume, and calcined clay used to replace clinker. Types, replacement rates, CO₂ savings.
Supplementary cementitious materials (SCMs) are industrial byproducts or processed natural materials that partially replace Portland cement clinker in concrete, contributing to strength through pozzolanic or latent hydraulic reactions with the calcium hydroxide released during cement hydration. The four mainstream SCMs are fly ash, ground granulated blast furnace slag (GGBS), silica fume, and calcined clay; natural pozzolans (volcanic ash, opaline shale) make up a fifth category. Most blended cements (PPC, PSC: see OPC vs PPC vs PSC) are built on them.
The two chemistry families
SCMs work through one of two mechanisms. Pozzolanic materials (fly ash, silica fume, calcined clay, natural pozzolans) need the calcium hydroxide (Ca(OH)₂) released by Portland cement hydration to form additional calcium silicate hydrate (C-S-H). Latent hydraulic materials, GGBS being the type case, slowly self-hydrate once activated by the alkaline OPC environment. Both routes reduce the clinker factor of the finished binder.
Pozzolanic reaction. The reaction between a siliceous or alumino-siliceous material and calcium hydroxide in the presence of water, producing additional C-S-H. Named after the Roman volcanic pozzolana from Pozzuoli, Italy.
The four (plus one) SCMs
| SCM | Type | Replacement % | Primary benefit | CO₂ savings vs OPC | Source standard |
|---|---|---|---|---|---|
| Fly ash, Class F | Pozzolanic | 15-35% | Long-term strength, durability | 15-30% | ASTM C618; ACI 232.2R |
| Fly ash, Class C | Pozzolanic + mild cementitious | 15-40% | Early-age strength | 15-35% | ASTM C618 |
| GGBS | Latent hydraulic | 30-70% (up to 95% in CEM III/C) | Low heat of hydration, sulphate resistance | 40-65% | ACI 233R; EN 15167 |
| Silica fume | Highly reactive pozzolan | 5-12% | High strength, low permeability | 5-10% | ACI 234R; ASTM C1240 |
| Calcined clay (LC³-50) | Pozzolanic | up to 50% (with limestone) | Lowest-CO₂ SCM at scale | 30-40% | ASTM C618; emerging LC³ standards |
| Natural pozzolan | Pozzolanic | 10-25% | Regional availability | 10-25% | ASTM C618 Class N; EN 197-1 |
Ranges per cited ACI committee reports and standard practice. CO₂ savings are binder-level; project-level savings depend on mix design.
- Fly ash is the coal power station byproduct collected from precipitators or fabric filters. Class F (typically less than around 18% CaO, bituminous/anthracite) is the global workhorse; Class C (typically more than 18-20% CaO, lignite/sub-bituminous) is mildly self-cementing.
- GGBS is blast-furnace molten slag rapid-quenched in water and ground to cement fineness. Up to 95% replacement is permitted in CEM III/C per EN 197-1.
- Silica fume is condensed SiO₂ fume from silicon and ferrosilicon electric-arc furnaces. Very fine (BET 15-30 m²/g), highly reactive, used at 7-10% doses for high strength or low permeability.
- Calcined clay (metakaolin and broader kaolinitic clays) is the emerging fourth pillar. Clay calcined at 700-850 °C in a flash calciner or rotary kiln; the cement industry already operates that equipment. Standalone at 5-20%, or up to 50% clinker replacement in LC³-50 (see the LC³ piece).
- Natural pozzolans (volcanic ash, opaline shale, calcined diatomaceous earth) are the regional fallback where industrial SCMs are scarce. Roman concrete was 100% natural-pozzolan binder.
Why SCMs matter for decarbonisation
SCMs are the largest near-term lever for cutting cement CO₂. Cement emits roughly 2.4 Gt CO₂/yr, about 7-8% of global anthropogenic emissions (IEA, 2023). Around 60% of that is process CO₂ from limestone calcination; every kg of clinker substituted by an SCM is a kg of process CO₂ avoided. The GCCA Net Zero Roadmap (2021) targets a global clinker-to-cement ratio of 0.52 by 2050, down from around 0.72 in 2020. See also low-clinker cement.
Two supply constraints matter honestly: fly ash supply is declining as coal power plants retire; GGBS supply is bounded by global steel tonnage. The scale-up path depends substantially on calcined clay and natural pozzolans, both of which require new processing capacity rather than a recovered byproduct stream.
Common questions about this topic
Pozzolans are one of the two chemistry families within SCMs, but the terms are not interchangeable. Pozzolans (fly ash, silica fume, calcined clay, natural pozzolans) react with calcium hydroxide from cement hydration to form additional C-S-H. GGBS, by contrast, is latent hydraulic: it self-hydrates once alkali-activated. So GGBS is an SCM but not a pozzolan. ACI 232 covers fly ash; ACI 233 covers slag.
The split is by calcium content under ASTM C618. In practice Class F is the low-calcium pozzolan (typically less than around 18% CaO, from bituminous or anthracite coal); it is purely pozzolanic and contributes to long-term strength and durability. Class C is the high-calcium ash (typically above 18-20% CaO, from lignite or sub-bituminous coal) and is mildly self-cementing in addition to pozzolanic. Class C improves early-age strength but can show variable performance with sulphate exposure; mix qualification matters. ASTM C618 itself differentiates the classes primarily by the minimum sum of SiO₂ + Al₂O₃ + Fe₂O₃ (70% for Class F, 50% for Class C).
No. LC³ (Limestone Calcined Clay Cement) is a blended-cement system, not a single material. LC³-50 combines around 50% clinker, 30% calcined clay, 15% limestone, and 5% gypsum. Calcined clay is the SCM; the limestone is a fine filler that interacts synergistically with the calcined clay alumina, allowing 50% clinker replacement without strength loss (Scrivener et al., 2018).
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