Complete guide to SCMs in concrete — fly ash, slag, silica fume, natural pozzolans and more
Understand what supplementary cementitious materials (SCMs) are, how they work, the types available, mix design proportions, performance benefits, and sustainability advantages for concrete construction in 2026.
Essential reference for concrete engineers, mix designers, specifiers, and contractors using SCMs in 2026
Supplementary cementitious materials (SCMs) are materials that, when used in combination with Portland cement in a concrete mix, contribute to its strength and durability through hydraulic or pozzolanic activity. SCMs include fly ash (a coal combustion by-product), ground granulated blast-furnace slag (GGBS) (from iron smelting), silica fume (from silicon production), and natural pozzolans such as calcined clay, volcanic ash, and metakaolin. They partially replace Portland cement in the mix — reducing clinker content, lowering carbon footprint, and enhancing long-term concrete performance simultaneously.
SCMs are used for two primary reasons: performance and sustainability. Technically, they densify the concrete microstructure through pozzolanic reaction with calcium hydroxide (Ca(OH)₂) produced during cement hydration, forming additional calcium silicate hydrate (C-S-H) — the binding phase that gives concrete its strength. This reaction fills capillary pores, reduces permeability, improves resistance to sulfate attack and chloride ingress, and controls alkali-silica reaction. Economically and environmentally, replacing a portion of Portland cement clinker — which generates approximately 820 kg CO₂ per tonne — with industrial by-products dramatically reduces the carbon intensity of concrete in 2026.
Modern concrete specifications increasingly mandate or incentivise the use of SCMs. The NCC and AS 3600 in Australia, Eurocode 2 and BS 8500 in the UK, and ACI 318 in the US all recognise blended cements and SCM additions as equivalent to or better than plain Portland cement for most exposure classes. The challenge for the mix designer is balancing early-age strength gain (which SCMs typically slow) against long-term durability (which SCMs improve). Understanding each SCM's reactivity, fineness, and chemical composition allows precise mix optimisation for any structural or durability requirement. See our guide on Air Entrained Concrete – Uses & Benefits for related mix design principles.
SCMs work through two distinct mechanisms, and understanding which mechanism applies to each material is fundamental to mix design. Pozzolanic SCMs — including fly ash, silica fume, natural pozzolans, and metakaolin — are not cementitious on their own. They react chemically with calcium hydroxide (Ca(OH)₂), a by-product of Portland cement hydration, to form additional C-S-H gel. This secondary reaction is slower than the primary cement hydration reaction, which is why pozzolanic SCMs tend to reduce early strength but improve long-term strength and durability. Latent hydraulic SCMs — primarily GGBS — are weakly cementitious on their own and are activated by the alkaline environment created by Portland cement hydration, producing C-S-H gel directly without requiring surplus Ca(OH)₂.
The practical result of both reaction types is a denser, less permeable concrete microstructure. Ca(OH)₂ is the most soluble and weakest phase in hydrated cement paste — by consuming it in the pozzolanic reaction, SCMs both strengthen the paste and reduce its vulnerability to leaching, sulfate attack, and carbonation. The finer particle sizes of silica fume (mean particle size ~0.1 µm) and metakaolin also contribute a physical filler effect, packing between cement grains to reduce initial porosity before any chemical reaction takes place. For more on concrete durability assessment, see our guide on Assessing Existing Concrete Structures.
The pozzolanic reaction converts weak, soluble Ca(OH)₂ (the vulnerable orange layer) into additional binding C-S-H gel — filling pores and densifying the concrete microstructure for superior long-term durability and reduced permeability.
Each SCM has a distinct chemical composition, physical form, reactivity level, and optimum dosage range. Selecting the right SCM — or combination of SCMs — requires matching the material's properties to the project's strength requirements, durability exposure class, pour size (heat of hydration), and concrete colour specification. The table below provides a comprehensive comparison of all major SCM types used in concrete construction in 2026.
| SCM Type | Source / Origin | Reaction Type | Typical Replacement % | Key Benefit | Key Limitation | Standard |
|---|---|---|---|---|---|---|
| Fly Ash – Class F (Low Calcium) | Coal combustion — bituminous/anthracite coal | Pozzolanic | 15–30% | Improved workability, reduced heat, ASR control | Slow early strength; quality variability | AS 3582.1 / ASTM C618 |
| Fly Ash – Class C (High Calcium) | Coal combustion — sub-bituminous / lignite coal | Pozzolanic + hydraulic | 10–25% | Better early strength than Class F; self-cementitious | Less effective for ASR; sulfate resistance lower | ASTM C618 |
| GGBS (Ground Granulated Blast-Furnace Slag) | Iron blast-furnace by-product — granulated + ground | Latent hydraulic | 30–70% (up to 80%) | Excellent durability, low heat, sulfate resistance, pale colour | Slower early strength; bleeding at high levels | AS 3582.2 / BS EN 15167 / ASTM C989 |
| Silica Fume (Microsilica) | Silicon / ferrosilicon alloy production — electrothermal furnace | Highly pozzolanic | 5–10% | Very high strength, ultra-low permeability, chloride resistance | High water demand; stiff mix; requires SP; costly | AS 3582.3 / ASTM C1240 / BS EN 13263 |
| Metakaolin | Calcined kaolin clay — thermally activated at 700–850°C | Pozzolanic | 10–20% | High reactivity, white colour, ASR control, early strength | Higher cost than fly ash; increases water demand | ASTM C618 (Class N) / BS EN 450 |
| Calcined Clay (LC3 — Limestone Calcined Clay) | Calcined low-grade kaolinite clay + limestone filler | Pozzolanic | 30–50% (combined LC3) | Very low CO₂; abundant raw materials; good durability | Variable raw material quality; emerging technology | Emerging — ASTM C618 Class N guidance |
| Natural Pozzolan (Volcanic Ash / Pumice) | Volcanic eruptions — amorphous silica-rich deposits | Pozzolanic | 10–35% | Locally available; low embodied energy; proven in ancient concrete | Variable quality and reactivity by source; coarser particles | ASTM C618 Class N / AS 3582 |
| Rice Husk Ash (RHA) | Controlled combustion of rice husks | Highly pozzolanic | 5–20% | Very high SiO₂ content; agricultural by-product; low cost in rice-producing regions | High carbon content if poorly burnt; variable quality | ASTM C618 Class N |
Each SCM modifies fresh and hardened concrete properties differently. The mix designer must understand these effects to compensate where necessary and to exploit the benefits strategically. The following stat cards summarise the key performance impacts of SCMs across the most important concrete properties — from workability and heat of hydration in fresh concrete to long-term strength, permeability, and chemical resistance in the hardened state.
SCMs generally reduce early-age strength (1–7 days) because the pozzolanic reaction is slower than primary cement hydration. At 28 days, fly ash concrete may reach 85–95% of the equivalent plain Portland cement strength. At 90 days and beyond, SCM concretes typically match or exceed plain cement concrete — GGBS and fly ash concretes continue gaining strength for months or years. Silica fume is an exception: its very high reactivity and filler effect can maintain or improve 7-day strength at dosages up to 10%. The water-binder ratio remains the primary strength-governing parameter — the same w/b ratio used with plain cement applies to SCM mixes.
One of the most valuable benefits of SCMs — particularly GGBS and low-calcium fly ash — is their significantly lower heat of hydration compared to Portland cement. Portland cement generates approximately 350–450 J/g of heat during hydration. GGBS generates 350–400 J/g but reacts more slowly, spreading heat release over a longer period. At 50–70% GGBS replacement, peak temperature in mass concrete pours can be reduced by 20–30°C — critical for preventing thermal cracking in large elements such as raft foundations, transfer slabs, and dam sections. For backfilling projects with concrete foundations, see our guide on Backfilling Around Concrete Foundations.
Fly ash is a highly beneficial workability aid. Its spherical particle shape (compared to angular cement particles) acts like ball bearings in the mix, reducing water demand by 5–10% for the same slump or enabling a reduction in superplasticiser dosage. GGBS also slightly improves workability. Silica fume has the opposite effect — its extremely fine particles (surface area 15,000–25,000 m²/kg) dramatically increase water demand and must always be combined with a high-range water reducer (superplasticiser). Metakaolin also increases water demand and stiffens the mix due to its platy particle morphology.
SCMs improve concrete durability significantly by reducing permeability through pore refinement and by chemically binding chloride ions in aluminate-rich phases. Fly ash and GGBS both contain aluminate phases that form Friedel's salt when exposed to chlorides, reducing the free chloride concentration in pore solution. This makes SCM concretes far more resistant to chloride-induced corrosion — the primary deterioration mechanism for reinforced concrete in marine and de-icing salt environments. GGBS at 50–70% replacement provides excellent sulfate resistance, equivalent to sulfate-resisting Portland cement (SRPC).
Alkali-silica reaction (ASR) — the expansive gel-forming reaction between alkali hydroxides in cement pore solution and reactive silica in aggregates — is effectively suppressed by SCMs. Low-calcium fly ash at 25–30% replacement and GGBS at 40–50% replacement reduce the alkali content of pore solution and dilute the Portland cement fraction, sufficiently suppressing ASR expansion below the damage threshold. Silica fume at 10–15% is also effective. Metakaolin and natural pozzolans provide moderate ASR suppression. ASTM C1567 accelerated mortar bar testing is used to confirm ASR mitigation effectiveness for a specific cement–SCM–aggregate combination.
Portland cement clinker production is responsible for approximately 8% of global CO₂ emissions — roughly 820 kg CO₂ per tonne of clinker. Replacing 30% of cement with fly ash or GGBS reduces the carbon intensity of the binder by approximately 25–30%. At 50% GGBS, the binder carbon footprint is halved. GGBS has an embodied carbon of approximately 50–80 kg CO₂/tonne (a by-product allocation); fly ash is similarly low at 4–27 kg CO₂/tonne. For large infrastructure and high-volume concrete projects in 2026, SCM use is not just best practice — it is increasingly a contractual requirement under low-carbon specification frameworks such as the UK's ICE Carbon Database targets and Australia's Green Star rating system.
Modern mix design increasingly uses ternary blends combining two SCMs with Portland cement to exploit the complementary benefits of each. A common high-performance ternary blend is Portland cement + GGBS + silica fume: the GGBS reduces heat, improves long-term durability, and lowers cost, while the silica fume tightens the microstructure and boosts early strength — compensating for the slow early gain of GGBS alone. Another effective combination is Portland cement + fly ash + GGBS, used widely in mass concrete and marine structures. For ASR-affected aggregates, Portland cement + low-calcium fly ash + GGBS provides excellent ASR suppression at high combined replacement levels of 50–60%. Ternary blends require careful testing to confirm performance before use.
Designing a concrete mix incorporating SCMs follows the same fundamental approach as plain Portland cement mix design — establishing the required water-binder ratio for strength and durability, then selecting paste volume, aggregate grading, and admixture dosages — with additional steps to account for the SCM's reactivity, its effect on workability, and its influence on setting time and early strength development. The following steps apply to a typical structural concrete mix incorporating SCMs.
The most frequent error is applying the same early stripping programme to SCM concrete as to plain Portland cement concrete. SCMs — particularly fly ash and GGBS at high replacement levels — can significantly delay strength gain in cold weather, and stripping on elapsed time rather than on cube test results has caused failures. A related mistake is reducing curing duration on SCM concrete because it "sets" later — SCM concretes are in fact more curing-sensitive than plain cement concrete, and inadequate curing produces a soft, dusty, permeable surface layer. Never use a high-calcium (Class C) fly ash in sulfate-resistant concrete specifications designed around the assumption that fly ash is Class F — the higher calcium content of Class C ash offers inferior sulfate resistance. Finally, always confirm the SCM source's loss on ignition (LOI) — high LOI fly ash absorbs admixtures unpredictably and can cause severe air entrainment problems and strength variability.
Supplementary cementitious materials are governed by national product standards: AS 3582.1 (fly ash), AS 3582.2 (GGBS), and AS 3582.3 (silica fume) in Australia; BS EN 450 (fly ash), BS EN 15167 (GGBS), and BS EN 13263 (silica fume) in the UK; and ASTM C618 (fly ash and natural pozzolans), ASTM C989 (GGBS), and ASTM C1240 (silica fume) in the US. Concrete specifications reference these standards to confirm SCM suitability and define permitted replacement levels for each exposure class in 2026.
Air Entrained Concrete Guide →In 2026, reducing embodied carbon in concrete is a critical industry priority. SCMs are the single most impactful tool available to reduce concrete's carbon footprint at scale — replacing 30–70% of Portland cement with fly ash, GGBS, or calcined clay eliminates 25–60% of the binder's CO₂ intensity. Major infrastructure clients, green building rating systems (Green Star, BREEAM, LEED), and government procurement frameworks increasingly mandate minimum SCM replacement levels and maximum embodied carbon thresholds for concrete works on capital projects.
Concrete Assessment Guide →Modern durability-based concrete design uses service-life modelling to determine the minimum SCM type, replacement level, and water-binder ratio needed to achieve a target design life in a given exposure environment. Models such as fib Model Code 2020 and the Life-365 software tool use diffusion coefficients and binding capacity parameters derived from SCM concrete testing to predict chloride penetration rates and rebar depassivation times for 50–100 year service life design. Specifying SCMs based on durability modelling rather than prescriptive replacement levels is increasingly adopted in Australia, the UK, and Europe for marine and infrastructure concrete in 2026.
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