Fly ash and granulated blast-furnace slag: the clinker substitute is running out just when we need it most

Cement accounts for roughly 7% of global CO₂ emissions, mostly from two sources: limestone calcination (CO₂ released by chemical reaction, not combustion) and the thermal energy needed to bring the kiln to 1,450 °C. Decarbonising cement has had, for decades, an effective shortcut: replacing part of the clinker (the emissions-intensive component) with supplementary cementitious materials (SCMs) that are either already reacted or react with the lime liberated by the clinker. The two traditional SCMs are coal-fired power station fly ash and ground granulated blast-furnace slag (GGBS or GGBFS). Both are by-products that the cement industry blends in. Both are heading for extinction in Europe because of the very decarbonisation of coal and steelmaking. The paradox is structural: the substitute we need most to decarbonise cement is the by-product of the industries we are also decarbonising.

One tonne of Portland clinker emits around 800-870 kg of CO₂: roughly 540 kg from the chemical decarbonation of calcium carbonate (CaCO₃ → CaO + CO₂) and the rest from fuel combustion required to reach clinkering temperature. One tonne of Portland CEM I cement contains around 95% clinker, so its emissions sit around 770-820 kg/t. If instead of 95% clinker we use 65% (CEM II/B) or 35% (CEM III/B with slag), emissions fall in proportion to the substitution rate, net of processing the additive.

SCMs add useful chemistry. Fly ash is a pozzolan: it contains amorphous silica and alumina that react with the portlandite (Ca(OH)₂) released by clinker hydration to form additional calcium silicate and aluminate hydrates that contribute to strength and durability. Granulated slag is mildly hydraulic by itself: properly activated, it sets on its own. The benefit for blended cements is that these reactions reduce concrete permeability, improve resistance to sulphates and alkalis and lower the heat of hydration, which favours large structural pours. SCMs are not a cheap fix: they are better chemistry for several applications.

Fly ash is captured in the electrostatic precipitators of pulverised-coal power stations. It is a powder of amorphous spherical particles between 1 and 100 µm. Throughout the twentieth and early twenty-first century it was, alongside slag, the most widely used SCM in the world. Spain operated around 14 GW of coal capacity in 2018 across fifteen plants, with annual fly ash production of several million tonnes. The energy transition dismantled that fleet: closures accelerated after 2018 and the last grid-connected coal plant stopped burning coal on 15 July 2025, according to Spanish grid operator Red Eléctrica.

The current paradigm is paradoxical. Galicia, one of the regions where fly ash had piled up in landfills for decades, has seen a market emerge that recovers stockpiled ash to supply cement makers. Spanish cement consumption, according to OFICEMEN, reached 16.57 million tonnes in 2025, up 11.3% year on year, and needs SCMs in growing proportion. Domestic supply of fresh fly ash has closed with coal. Heritage stocks at landfills have a finite horizon.

Granulated blast-furnace slag (GGBS or GGBFS) is the floating siliceous residue of the iron-from-ore process in the blast furnace. Quenched rapidly with water and ground, it becomes an amorphous glassy material with latent hydraulic properties. Blended with Portland clinker it replaces typical fractions ranging from 35% (CEM II/B-S) to 80% (CEM III/B and CEM III/C). High-slag CEM III/B emits around 250-350 kg CO₂/t, against the 770-820 kg of CEM I.

The future bottleneck is the technological shift in steelmaking. European steel is migrating from the classical blast furnace-basic oxygen route (BOF) to the electric arc furnace (EAF) recycling scrap and, eventually, to direct-reduced iron with hydrogen (DRI-H₂). The EAF does not produce granulated slag in the traditional sense. Every steel plant that abandons the blast furnace zeroes its contribution to the GGBS supply. The European Commission's JRC, in its Decarbonisation options for the cement industry report (Marmier, 2023), describes the scenario as simultaneously desirable (steel decarbonisation) and problematic for cement.

The Global Cement and Concrete Association's (GCCA) Concrete Future roadmap, published in 2021 with commitment from 80% of global cement volume outside China, sets a clinker/binder ratio decline from 0.63 today to 0.52 in 2050. Achieving it requires the aggregate volume of SCMs to grow by approximately 26% by 2050. The document's own internal arithmetic concedes that traditional SCMs are not enough: the GCCA lists calcined clays, recycled concrete fines, uncalcined limestone and new natural pozzolans as a mandatory complement. Pure substitution by conventional SCMs is physically and economically impossible.

This tension has three immediate consequences for the sector. First, prices: the premium on fly ash and granulated slag is rising structurally. Second, logistics: SCMs travel from countries that still produce them (Asia, parts of Africa) to demand markets, lengthening transport footprint. Third, quality: ash recovered from old landfills needs reactivation, drying and impurity removal, raising costs and adding variability.

The European Committee for Standardization approved in February 2021 the EN 197-5 standard (UNE-EN 197-5:2021 in Spain), adding two new categories to the five existing ones in the EN 197 series: composite Portland cement CEM II/C-M and composite cement CEM VI. Both allow lower clinker contents than the classical categories (between 35 and 65% in CEM VI) and allow several additions to be combined in a single cement (slag, fly ash, limestone, silica fume, natural pozzolans). In Spain, Royal Decree 320/2024 amends the Cement Reception Instruction (RC-16) to bring those cements into mandatory scope.

The standard effectively recognises what the GCCA roadmap signals: the path to 2050 means more SCMs and a more diverse catalogue. The operational translation matters to the designer. A CEM VI with 50% clinker, 30% slag and 20% fly ash can replace a CEM I in typical structural applications with an emissions discount close to 50% per tonne, with no documented loss of performance.

The most mature successor to the classical SCMs is LC3 (Limestone Calcined Clay Cement), conceptualised in 2005 by Karen Scrivener's group at the École Polytechnique Fédérale de Lausanne (EPFL) and developed as a formal project with the Swiss Agency for Development and Cooperation since 2014. LC3-50 replaces 50% of the clinker with a mix of 30% calcined kaolinitic clay, 15% ground limestone and 5% gypsum. The clay is calcined at 700-850 °C, well below the 1,450 °C of clinker, drastically reducing energy demand. Uncalcined limestone acts as reactive filler in the presence of calcined clay. Emissions reduction reported by Scrivener, Martirena, Bishnoi and Maity (2018) in Cement and Concrete Research is around 40% relative to a conventional Portland cement, at competitive cost.

LC3's advantage over fly-ash and slag dependency is that kaolinitic clays are abundant and geologically widespread. Spain, with deposits in Galicia, Catalonia and Andalucía, has natural geological capacity. Pilot plants exist in Colombia, Cuba, Ghana and India. The barrier is not technical: it is industrial (calcination capacity) and regulatory (full integration into EN 197).

Spain is going through a double transition. On the one hand, coal-plant closures have eliminated the domestic source of fresh fly ash. The stockpiled volumes at sites such as As Pontes, Andorra, Compostilla, Litoral and La Robla offer a finite horizon (15-20 years). On the other hand, Spanish steelmaking has been heavily on electric arc furnaces for decades: Sestao and other plants produce steel from recycled scrap with low granulated-slag generation. The consequence is that Spain is one of the European markets most exposed to aggregate scarcity of traditional SCMs.

The rational response combines three moves. First, exhaust and characterise fly-ash stocks in landfills as a transitional resource (15-20 years). Second, scale up domestic calcined-clay production: geological capacity is there, what is missing is investment in rotary kilns and grinding equipment. Third, fully standardise LC3 in RC-16 and in the Spanish Structural Code (CTE), so that designers can specify it with the same legal certainty as CEM II/A. While the first two require a market, the third only needs administrative coordination among IETcc, IECA and the Permanent Commission on Concrete.

For seventy years, cement blended with fly ash and slag was the main decarbonisation route for the most-used construction material on the planet. Decarbonising the energy and steel systems themselves is closing that route at the very moment pressure to decarbonise cement increases. The way out is not to artificially defend coal or the blast furnace: it is an orderly transition to calcined clays, LC3 cements and multi-component blends standardised in EN 197-5. The CEM I of the twentieth century was cheap because clinker was cheap. The CEM VI or LC3 of the twenty-first century will be, in the medium term, cheaper in total social cost (carbon included) than CEM I, if regulation passes the price of CO₂ through to the recipe. The question is not whether the transition will happen, but who finances it and who is left out of the catalogue when it does.

1. Scrivener, K., Martirena, F., Bishnoi, S., & Maity, S. (2018). Calcined clay limestone cements (LC3). Cement and Concrete Research, 114, 49-56. DOI: 10.1016/j.cemconres.2017.08.017
2. Global Cement and Concrete Association. (2021). Concrete Future: The GCCA 2050 Cement and Concrete Industry Roadmap for Net Zero Concrete. London: GCCA.
3. Marmier, A. (2023). Decarbonisation options for the cement industry. JRC Technical Report JRC131246. Luxembourg: Publications Office of the European Union.
4. UNE-EN 197-5:2021. Cement. Part 5: Portland-composite cement CEM II/C-M and Composite cement CEM VI. Madrid: AENOR.
5. Royal Decree 320/2024, of 26 March, amending the Cement Reception Instruction (RC-16), approved by Royal Decree 256/2016, of 10 June. BOE No. 76, 27 March 2024.
6. Spanish Cement Manufacturers Association (OFICEMEN). (2026). Statistical report of the Spanish cement sector for fiscal year 2025. Madrid: OFICEMEN.