Supplementary Cementitious Materials

Definition

Supplementary cementitious materials (SCMs) are inorganic materials used alongside portland cement in concrete and mortar to contribute to strength and durability through hydraulic or pozzolanic activity. They are added to the mix either as a partial replacement for cement or as an addition to the binder, and they react during hydration to form additional binding compounds.

The most widely used supplementary cementitious materials are fly ash, ground granulated blast-furnace slag (GGBFS), silica fume, metakaolin, and natural pozzolans such as calcined clays and volcanic ash. Many of these are industrial by-products, which is one reason SCMs are closely associated with lower-carbon concrete.

Pozzolanic and hydraulic behavior

SCMs work through two main mechanisms. Pozzolanic materials, such as low-calcium fly ash, silica fume, and metakaolin, have little binding ability on their own. In the presence of water, they react with the calcium hydroxide released by cement hydration to form additional calcium silicate hydrate (C-S-H), the compound primarily responsible for concrete strength. Latent hydraulic materials, such as GGBFS and some high-calcium fly ashes, can hydrate by themselves to a certain degree, and this reaction is accelerated by the alkaline environment created by portland cement.

Common reference specifications

Quality requirements for SCMs are defined in standards such as ASTM C618 for coal fly ash and raw or calcined natural pozzolans, ASTM C989 for slag cement, ASTM C1240 for silica fume, and the EN 450, EN 15167, and EN 13263 series in Europe. Project specifications usually reference one of these documents, and buyers should confirm material compliance through supplier test reports rather than assumptions.

Why it matters in concrete

SCMs influence nearly every stage of concrete performance, from fresh workability to long-term service life. Their effect depends on the type of material, its fineness and chemistry, the replacement level, and curing conditions.

Durability and pore structure

By converting calcium hydroxide into additional C-S-H and filling fine voids between cement particles, SCMs refine the pore structure of hardened concrete. A denser matrix reduces permeability, which slows the ingress of chloride ions, sulfates, and water. This is why SCM-rich concretes are commonly specified for marine structures, wastewater facilities, bridge decks, and foundations exposed to aggressive soils. Fly ash and slag are also used to help control alkali-silica reaction when reactive aggregates are present.

Heat of hydration and strength development

Replacing part of the cement with fly ash or slag lowers the heat released during early hydration, which reduces the risk of thermal cracking in mass concrete elements such as dams, thick rafts, and large piers. The trade-off is that early-age strength gain is often slower, while later-age strength at 56 or 90 days can match or exceed that of a plain cement mix. Silica fume behaves differently: its very high fineness and reactivity contribute to both early and long-term strength and are central to high-performance concrete.

Sustainability

Cement production accounts for a large share of the embodied carbon in concrete. Using SCMs to reduce clinker content is one of the most practical ways to lower the carbon footprint of a mix while also making use of industrial by-products that would otherwise require disposal.

Practical implications

Introducing SCMs changes how a concrete mix behaves, so mix design, admixture selection, and curing practice need to be adjusted accordingly.

Interaction with superplasticizers

SCMs affect water demand and admixture response in different ways. Spherical fly ash particles generally improve workability and can reduce the dosage of water reducer needed. Silica fume and metakaolin, by contrast, have very high specific surface areas and significantly increase water demand, which makes a high-range water reducer such as a polycarboxylate ether (PCE) superplasticizer practically essential to maintain workability at a low water-to-binder ratio. Unburned carbon in some fly ashes can adsorb admixture molecules and reduce their effectiveness, and slag may change slump retention behavior. For these reasons, superplasticizer dosage and type should be verified through trial batches using the actual cement and SCM sources planned for the project.

Setting time and curing

Mixes with high fly ash or slag content often show extended setting times, particularly in cold weather. This can affect finishing schedules and formwork stripping. Adequate moist curing is especially important for SCM concrete, because the pozzolanic reaction continues over a longer period and depends on available moisture. Insufficient curing can prevent the mix from reaching its intended strength and durability.

Sourcing and consistency

Because many SCMs are by-products, their properties can vary between sources and even between batches from the same source. Variations in loss on ignition, fineness, and chemical composition can affect color, air content, and admixture demand. Procurement teams typically request certificates of analysis for each shipment and include SCM variability in their quality control plan.

Related terms

Several glossary entries are closely connected to supplementary cementitious materials. The water-binder ratio replaces the traditional water-cement ratio when SCMs are part of the binder, since it accounts for the total cementitious content. Pozzolan describes the class of siliceous or aluminosiliceous materials that react with calcium hydroxide. Polycarboxylate superplasticizer refers to the high-range water reducers most often paired with silica fume and high-SCM mixes. Slump retention describes how long a mix maintains workability, a property that SCMs and admixtures jointly influence. High-performance concrete is a category in which SCMs and superplasticizers are used together to achieve high strength and low permeability.

Related guide

For a more detailed look at how to match admixtures with different binder systems, see our guide on superplasticizer compatibility with cement and supplementary cementitious materials. It explains how to plan trial batches, interpret slump and slump-loss results, and adjust dosage when fly ash, slag, or silica fume are introduced into a mix. The ADMIXTEC technical team can also review mix designs and help evaluate admixture performance with the specific materials used on your project.