--- title: Making better use of supplementary cementitious materials in concrete date: 2026-10-30T06:00:00Z modified: 2026-10-14T06:56:56Z permalink: "https://carbonaide.com/knowledge-base/making-better-use-of-supplementary-cementitious-materials-in-concrete/" type: post status: publish excerpt: Reduce cement costs and carbon footprint with SCMs. CO2 curing unlocks even higher replacement levels. wpid: 11801 categories: - Knowledge base language: - English timestamp: 2026-10-14T06:56:56Z tags: - Knowledge base - English --- Supplementary cementitious materials are used in concrete to partially replace Portland cement, reducing both the cost and the carbon footprint of the mix while maintaining or improving the performance of the finished product. They work alongside cement rather than replacing it entirely, contributing to strength development and durability through their own chemical reactions. The sections below address the most common questions concrete producers have about SCMs, from material selection to carbon footprint outcomes. ## What are supplementary cementitious materials used for in concrete? Supplementary cementitious materials, commonly called SCMs, are materials added to concrete mixes to partially replace Portland cement. They contribute to the binding properties of concrete either by reacting with water directly or by reacting with the calcium hydroxide that cement releases during hydration. The result is a concrete mix that uses less cement while still developing the strength and durability required for the intended application. The reasons concrete manufacturers use SCMs are practical as much as environmental. Portland cement is one of the most energy-intensive materials in construction to produce, and reducing its share in a concrete mix lowers both material costs and greenhouse gas emissions. SCMs often improve workability, reduce heat of hydration in large pours, and contribute to long-term strength gain. In precast concrete production, where mix designs can be tightly controlled, SCMs are a well-established tool for optimising both performance and the carbon footprint of the product. It is worth being clear about what SCMs are not. They are not admixtures such as plasticisers or accelerators, which affect the behaviour of the mix without contributing chemically to the binder system. SCMs are materials that become part of the binder itself, working with cement to develop the concrete’s structural properties over time. ## What types of supplementary cementitious materials are most commonly used? The most widely used SCMs in concrete production are ground granulated blast furnace slag, fly ash, silica fume, and natural pozzolans such as calcined clay. Each material has a different chemical composition, reactivity level, and practical behaviour in the mix, which means the right choice depends on what the producer is trying to achieve and what is available locally. ### Ground granulated blast furnace slag Slag is a byproduct of iron production. When ground to a fine powder, it becomes a latent hydraulic binder, meaning it can react with water on its own but does so slowly without an activator. In normal concrete production, slag is activated by the alkaline environment created by Portland cement hydration. It is widely available in regions with steel and iron manufacturing, and it can be used at relatively high replacement levels without significantly compromising strength development. ### Fly ash Fly ash is a byproduct of coal combustion. It is a pozzolanic material, which means it does not react with water on its own but reacts with the calcium hydroxide released by cement hydration to form additional binding compounds. Fly ash tends to slow early strength development but can improve long-term strength and workability. Its availability is closely tied to coal power generation, which is declining in many regions, making supply increasingly uncertain in some markets. ### Silica fume Silica fume is a highly reactive pozzolan produced as a byproduct of silicon and ferrosilicon alloy manufacturing. Its very fine particle size and high silica content make it effective at filling voids in the concrete matrix and reacting rapidly with calcium hydroxide. It is typically used at lower replacement levels than slag or fly ash, often to achieve high-strength or low-permeability concrete. ### Calcined clays and natural pozzolans Calcined clays, particularly metakaolin, are gaining attention as SCMs in markets where slag and fly ash are less available. They require thermal processing but draw on widely distributed raw material sources. Natural pozzolans such as volcanic ash have been used in construction for centuries and remain relevant where they occur locally. ## What limits how much SCM can replace Portland cement? The main factors limiting SCM replacement levels are reactivity, early strength requirements, and the availability of the alkaline environment that most SCMs need to activate. Because the majority of SCMs rely on calcium hydroxide from cement hydration to develop their binding properties, reducing cement content also reduces the activator available to the SCM. Beyond a certain replacement level, strength development slows to the point where the concrete no longer meets the performance requirements for its application. Early strength is often the binding constraint in precast concrete production. Precast elements need to reach a minimum stripping strength within hours of casting so that moulds can be turned over quickly. If SCM content is too high, early strength development may be too slow for efficient production, even if 28-day strength targets can still be met. This is one reason why SCM replacement levels in mainstream precast production tend to be moderate rather than approaching full replacement. Material quality and consistency also matter. SCMs vary in their chemical composition depending on their source, and variability in the SCM can make mix design optimisation more difficult. Producers need reliable supply chains and quality control processes to use SCMs effectively at scale. In some regions, availability of specific SCMs is simply limited, which constrains the options available regardless of technical suitability. Standards and product specifications set further boundaries. Concrete products must meet defined performance criteria, and the mix design must be validated against those criteria before production. This is not a barrier to SCM use, but it does mean that replacement levels need to be established through testing and documentation rather than assumed. ## How does CO₂ curing help activate supplementary cementitious materials? Carbon dioxide curing can activate SCMs that are otherwise non-reactive or only weakly reactive in conventional concrete production. When CO₂ is introduced into the curing environment, it reacts with calcium-bearing compounds in the mix, forming calcium carbonates and releasing silica. This creates reaction pathways that certain SCMs can participate in even without the alkaline activation that cement hydration normally provides. A well-documented example is gamma-dicalcium silicate, a compound found in some slags produced by iron and steel processes. In normal curing conditions, gamma-dicalcium silicate is essentially inert and cannot contribute to concrete strength. In the presence of CO₂, however, it becomes reactive and functions as a binder. This means that carbon dioxide curing opens up a wider range of slag types and other calcium-rich industrial byproducts as viable SCMs, including materials that would otherwise have no useful role in concrete production. There is a second mechanism at work as well. During carbon dioxide curing, CO₂ reacts with compounds in the mix to form ultrafine calcium carbonate crystals. These act as nucleation sites that accelerate the early stages of strength development. The result is that concrete cured with CO₂ can reach target early strengths faster than conventionally cured concrete, which in turn means that higher SCM content can be used without sacrificing the stripping times that precast production depends on. This combination of broader material activation and faster early strength development is what makes carbon dioxide curing particularly relevant to SCM optimisation. Producers can work with materials and replacement levels that would not be practical in conventional production. The [Carbonaide CO₂ Curing System](https://carbonaide.com/wp-content/uploads/wp-mfa-exports/page/solutions.md) applies this principle in a controlled, measurable way, enabling precast producers to integrate a wider range of SCMs into their mix designs while maintaining the production efficiency their operations require. ## Can concrete with high SCM content still have a negative carbon footprint? Yes, concrete with high SCM content can achieve a negative carbon footprint when carbon dioxide curing is used alongside the mix design. The combination works through two distinct mechanisms: reducing the amount of Portland cement in the mix lowers the embodied emissions from raw materials, and permanently mineralising CO₂ into the concrete structure adds a carbon removal component to the product’s lifecycle calculation. Portland cement is the dominant source of emissions in conventional concrete production. Replacing a significant share of the cement with SCMs such as slag reduces those raw material emissions considerably. When CO₂ is then mineralised into the concrete during curing, that carbon is permanently stored as carbonate minerals within the product. The stored CO₂ does not return to the atmosphere even if the concrete is later demolished or recycled, because the mineralisation process converts it into stable solid compounds. The calculated carbon footprint of a concrete product reflects both the emissions associated with its production and any carbon that has been permanently removed from the atmosphere and stored in the product. When the stored carbon exceeds the residual emissions from production, the product’s carbon footprint becomes negative. This outcome is more achievable when SCM content is high, because the baseline emissions from cement production are already lower, and the CO₂ mineralised during curing represents a larger share of the total lifecycle calculation. It is worth noting that achieving a negative carbon footprint requires careful mix design, process control, and verified measurement of the mineralised CO₂. The result is not automatic and depends on the specific materials used, the replacement levels achieved, and the amount of CO₂ successfully stored during curing. ## What should concrete producers consider when adopting SCMs at scale? Adopting SCMs at scale requires attention to supply chain reliability, mix design validation, production process compatibility, and documentation requirements. Each of these areas involves practical decisions that affect both the feasibility and the economic return of increasing SCM content in production. ### Supply chain and material consistency SCM availability varies significantly by region and by material type. Slag supply depends on proximity to steel and iron production; fly ash availability is tied to coal power generation, which is declining in many markets. Producers considering a significant shift toward SCMs need to assess whether reliable, consistent supply is available at the volumes their production requires. Variability in SCM chemistry between batches or suppliers can affect mix performance and requires robust quality control processes to manage. ### Mix design and performance validation Increasing SCM content changes the behaviour of the concrete mix, particularly in the early hours after casting. Early strength development typically slows as cement content decreases, which can affect production cycle times in precast operations. Mix designs with higher SCM content need to be validated against the performance requirements of each product type, and this validation takes time and testing. Carbon dioxide curing can help address early strength limitations, but the interaction between the curing process and the specific SCMs used needs to be understood and controlled. ### Measurement and reporting If the goal of increasing SCM content includes reducing the reported carbon footprint of concrete products, producers need accurate measurement and documentation of the changes. Environmental product declarations and carbon reporting require verified data on material inputs and any carbon stored in the product. The [Carbonaide Service Platform](https://carbonaide.com/wp-content/uploads/wp-mfa-exports/page/carbonaide-service-platform.md) provides the measurement, data management, and certification support that producers need to report carbon storage accurately and meet the requirements of carbon markets and product declarations. ### Economic case The economic return from increasing SCM content depends on the cost difference between the SCM and the cement it replaces, the production volumes involved, and whether carbon credits or other environmental value can be captured from the process. In markets where cement prices are high and suitable SCMs are available at lower cost, the economic case for higher replacement levels can be strong. Where SCMs are scarce or expensive to process, the calculation is less straightforward and requires careful assessment against actual production conditions.