The Ceiling on Slag and Fly Ash
Limestone Calcined Clay Cement Takes the Lead
Alkali-Activated Materials and Geopolymers
Rebuilding the Chemistry of Clinker
Standards and Market Access
Where the Momentum Sits
References and Further Reading
Portland cement production accounts for more than 8% of global carbon emissions, and demand is expected to climb by roughly 25% by 2050.1
Image Credit: Cung kediro/Shutterstock.com
About half of that carbon comes from chemistry rather than from fuel, and is released when limestone breaks down into lime inside the kiln. Low-carbon cement work therefore targets the raw material as firmly as the furnace.
The International Energy Agency places the global clinker-to-cement ratio at near 0.71 in 2022 and models a decline toward 0.57 by 2050 in its net-zero pathway. Meeting that figure means finding binder materials that perform clinker's structural role at industrial volumes, in every region where concrete is poured.2
The Ceiling on Slag and Fly Ash
The established route to lower-carbon concrete involves replacing part of the clinker with supplementary cementitious materials, mainly ground granulated blast-furnace slag and pulverized fly ash. Both are industrial by-products with well-documented strength and durability behavior, which explains why engineers reach for them first.1
Supply sets the limit. Global slag availability is only five to 10% of cement production, with fly ash in a similar range, and both volumes decrease as coal plants close and steel recycling expands. Almost all UK production already goes into concrete, so specifying more of it shifts material between projects.1
Geography compounds the problem. Slag output concentrates in a handful of iron-producing countries while cement demand spreads across every economy. Alternatives that genuinely gain ground need raw materials that are widely available in quantities measured against billions of tons of annual cement output.1
Limestone Calcined Clay Cement Takes the Lead
Limestone calcined clay cement, known as LC3, currently holds the strongest position among alternatives beyond Portland. Clays rich in kaolinite become highly reactive when heated to between 700 and 850 °C, a temperature well below clinker burning, and they release no carbonate carbon during that step.1
A blend of 50% clinker, 30% calcined clay, 15% limestone, and 5% gypsum matches the mechanical performance of conventional CEM-I cement from seven days onward, with improved durability and roughly 30% lower CO2 emissions.
Demonstration projects in Cuba and India confirmed the recipe outside the laboratory, including a house in Jhansi that saved 15.5 tons of carbon.1
Research output has also grown rapidly. A report published in Heliyon traces a steady rise in LC3 publications and strong collaboration between research communities across Europe, Asia and Latin America, an early indicator that a material has moved past novelty and into engineering practice.3
Recent work also pairs LC3 with recycled aggregates. Testing reported in Scientific Reports found that LC3 could maintain useful mechanical performance while reducing unit cost and carbon emissions.
Recycled aggregates supported material reuse, although their environmental benefits depended strongly on aggregate quality and transportation distance. This matters for demolition-heavy markets where aggregate reuse is already common practice.4
Alkali-Activated Materials and Geopolymers
Alkali-activated materials take a different chemical route. An aluminosilicate powder reacts with a strong alkaline activator and hardens into a binder that carries load without any Portland clinker.
Read about the alkali-activated polymer binder that could be used to reduce carbon emissions.
When the precursor contains little calcium, the product is usually called a geopolymer, and its aluminosilicate network replaces the calcium hydrates found in ordinary cement.1
Suitable precursors are widespread. Volcanic rocks across southern Europe, the Andes, and the Middle East; lateritic soils throughout the tropics; and kaolin-rich clays on every continent all supply the required silica and alumina in quantities that dwarf global cement production. Grinding and moderate heating turn them into usable powders.1
Metakaolin has become the best-studied precursor for clinker-free systems. Research indicates that metakaolin-based alkali-activated and geopolymer concrete have competitive compressive strength and refined pore structure. However, there are persistent challenges in workability control, activator handling, and mix consistency across different clay sources.5
Commercial progress remains uneven. Products such as Cemfree and Vertua reached the UK market but depend on slag as the precursor, which inherits the same supply constraint. A Northern Irish producer built a calcined clay geopolymer claiming 75% lower emissions before closing in 2019 for commercial reasons.1
Rebuilding the Chemistry of Clinker
A third group of alternatives changes how lime itself is made. Sublime Systems, for example, uses an electrochemical cell operating at ambient temperature to extract reactive calcium, removing the kiln from the process entirely. The company runs a pilot facility in Massachusetts and is developing its first commercial plant in Holyoke.6
Brimstone takes calcium from calcium silicate rocks rather than limestone, which avoids carbonate decomposition and produces alumina as a saleable by-product. Both approaches aim to produce lower-carbon materials capable of replacing ordinary Portland cement in conventional concrete applications.6
Policy exposure is real for this group. Both companies had federal demonstration awards terminated by the United States Department of Energy in May 2025, shortly after Sublime announced a procurement agreement with Microsoft. Capital intensity makes these technologies sensitive to shifts in public funding.6
Standards and Market Access
Codes decide what engineers can specify. European standards already permit ternary blends such as CEM II B-M with clinker content down to 65%, and a proposed extension lowers that threshold to 50%, though calcined clay does not yet appear as an approved constituent in that revision.1
Production capacity forms the second gate. Calcined clay availability falls far short of demand because the industry remains configured around Portland clinker, even though the raw clay reserves are effectively unlimited. Converting kilns and building clay calcination lines takes years of committed investment.1
The International Energy Agency notes that clinker substitution alone falls short of its net-zero requirements, pointing toward carbon capture and non-carbonate clinker sources as parallel measures. Reduced clinker ratios and novel binders therefore work together rather than competing for the same share of the reduction.2
Where the Momentum Sits
LC3 is gaining ground fastest. It uses abundant materials, fits within a modified version of existing plant equipment, delivers a verified 30% emissions cut, and has cleared enough field trials for structural engineers to specify it with reasonable confidence.3
Alkali-activated systems occupy the next tier. Their chemistry is proven, and their raw materials are global, while activator supply chains, standardization, and quality control still require sustained development before large structural applications become routine.5
Electrochemical and calcium silicate routes carry the largest potential reduction and the highest execution risk. Their trajectory depends on financing, offtake agreements from buyers willing to pay a premium, and the patience to reach plant scales measured in hundreds of thousands of tons per year.6
Engineers hold practical influence over which alternatives advance. Early specification builds order books, generates performance data, and gives standards committees the evidence they need. The cement industry of 2050 will likely run on a family of regional binders drawn from locally abundant, low-carbon minerals.1
References and Further Reading
- Sentucq, E. et al. (2024). Beyond Portland cement: Low-carbon alternatives. [Online] Institution of Structural Engineers. Available at: https://www.istructe.org/resources/guidance/beyond-portland-cement-low-carbon-alternatives/.
- Cement - Net Zero Emissions Guide. (2023). [Online] IEA. Available at: https://www.iea.org/reports/cement-3.
- Mañosa, J. et al. (2024). Research evolution of limestone calcined clay cement (LC3), a promising low-carbon binder – A comprehensive overview. Heliyon, 10(3). DOI:10.1016/j.heliyon.2024.e25117. .https://www.cell.com/heliyon/fulltext/S2405-8440(24)01148-4
- Yu, Y. et al. (2025). Mechanical, workability, economic and environmental properties of concrete with limestone calcined clay cement and recycled aggregates. Scientific Reports, 15. DOI:10.1038/s41598-025-97539-6. https://www.nature.com/articles/s41598-025-97539-6.
- Qader, D. N. et al. (2025). A systematic review of metakaolin-based alkali-activated and geopolymer concrete: A step toward green concrete. REVIEWS ON ADVANCED MATERIALS SCIENCE, 64(1). DOI:10.1515/rams-2024-0076. https://www.degruyterbrill.com/document/doi/10.1515/rams-2024-0076/html.
- Clancy, H. (2025). Low-carbon cement startups allied with Amazon and Microsoft lose federal funding. [Online] Trellis. Available at: https://trellis.net/article/low-carbon-cement-startups-lose-federal-funding/.
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.