Hard But Essential: The Case for Clean Cement & Concrete
Cement is made by heating limestone and clay in a rotating kiln to extremely high temperatures. As the materials move through the kiln, fossil fuels provide heat and limestone undergoes calcination, releasing CO₂ both from fuel combustion and from the raw material itself. The process produces clinker, which is cooled and then ground with gypsum to make cement, the main source of concrete’s carbon emissions.
Every skyscraper, every bridge, every mile of highway begins with the same silent — but significant — climate problem: cement. The key ingredient in concrete, cement is responsible for approximately 6% of global emissions — and demand is expected to rise in the decades to come. Despite its massive footprint, cement is difficult to replace because of its low cost and unparalleled performance, and difficult to abate because of the inherent chemistry of cement production. The good news? This monumental challenge is an equally massive opportunity. Innovation in green cement promises to resolve a looming climate liability while reinventing a trillion-dollar industry.
Using IPCC and GCAM data, Energy Innovation projected future “greenhouse gas emissions at stake” in 2050, assuming current policies remain in place. The resulting estimates overlap because different technologies may reduce the same emissions pool.
The Path We’re On
The material that built the world bears a heavy climate burden.
The world loves cement. It’s strong. It’s cheap. It’s durable. It’s the foundation of modern building. So it’s no surprise that we produce a lot of it each year — 4 billion tons of it, actually. And therein lies the problem. If cement were a country, it would rank third among the world’s top emitters, behind only China and the U.S.
Cement’s ubiquity makes it one of the most important materials to decarbonize. It’s also one of the hardest, largely because of the way it’s produced.
How is cement made?
First, a fossil fuel–fired kiln is used to heat limestone and clay to around 1,450 °C. More than half of cement’s carbon emissions come from the release of carbon dioxide contained in the limestone itself (which is 44% CO₂ by mass); the remainder comes from the fossil energy required to heat and drive the process.
Out of the kiln come clinker nodules — small, hard pebbles made primarily of calcium silicates. These nodules are ground into a fine powder and mixed with gypsum to produce Ordinary Portland Cement (OPC), which accounts for more than 95% of the cement used worldwide. When mixed with water, sand, and gravel, cement acts as the glue that holds concrete together.
On average, cement is composed of around 70% clinker — and that clinker is responsible for roughly 90% of concrete’s emissions. While supplementary cementitious materials like blast furnace slag and fly ash can reduce clinker content today, their supply is closely tied to carbon-intensive industries and will be constrained in a net-zero future.
Cement’s climate challenge is inseparable from how clinker is made. Producing clinker relies on limestone as a source of calcium. When limestone is heated, it undergoes calcination, releasing CO₂ as an unavoidable chemical byproduct. The remaining lime then reacts with other minerals to form clinker. This reaction requires extremely high temperatures and embeds emissions directly into cement’s dominant material recipe. Kilns, fuels, and heat recovery systems have already been optimized for decades, leaving limited room for further reductions through efficiency alone. As a result, deeper decarbonization will depend not just on improving the process, but on rethinking clinker chemistry, cement formulations, or both.
For every ton of clinker that’s produced, nearly the same amount of carbon dioxide is released into the atmosphere.
A New Way Forward
Paving a new path for cement
Clean concrete hinges on breakthroughs that can cut emissions without compromising the strength, cost, and reliability that make cement so indispensable.
The following imperatives and moonshots reflect two horizons of innovation: near-term advances that re-engineer today’s production methods to drive emissions toward zero, and longer-term breakthroughs that could transform concrete from a source of carbon pollution into a planetary carbon sink.
Innovation Imperatives
Produce the same material we use today, but without the emissions
The majority of cement’s emissions problem comes directly from the carbon released when limestone (which is 44% CO₂ by mass) is heated — an unavoidable problem with the current recipe. This imperative calls for changing the ingredients or production process, but not the final product. This will require either using non-carbonate feedstocks or capturing and storing emissions that can be processed into chemically identical cement without releasing CO₂. Because the final product is the same, this pathway can eliminate product risk, leveraging the longstanding trust in OPC — which makes it one of the most promising approaches for rapid, global-scale decarbonization.
Design concrete using cement with less than 50% clinker content through scalable supplementary materials, cement strengthening, and mix design
Clinker is the dominant source of cement’s emissions, making its reduction one of the most direct levers for cutting concrete’s carbon footprint. This imperative focuses on developing ways to significantly lower clinker content by expanding the supply of supplementary cementitious materials — including next-generation options like calcined clays and other pozzolanic materials — while maintaining or improving performance. Additional gains can come from strengthening cement and optimizing mix design to reduce the total amount of cement required. Because low-clinker approaches build on existing production methods and standards, they are among the most immediately deployable pathways to cleaner concrete, though their ultimate impact will be shaped by regional material availability, cost, and performance constraints.
Commercialize alternatives to Ordinary Portland Cement (OPC) with low or no carbon emissions and equivalent or superior performance.
It’s possible to design entirely new types of low-emissions binders or cements that match — or even exceed — the performance of OPC in every key metric, including strength, curing time, reinforceability, and long-term durability. To be adopted at scale, these new materials will also need to deliver the same degree of perceived risk, trust, and availability that OPC has provided for hundreds of years. If those barriers can be overcome, novel cements could unlock a new generation of high-performance building materials and fundamentally reshape the future of construction.
Moonshots
Negative emissions cement, aggregate, and supplementary materials to turn the built environment into a net carbon sink
Imagine a world where the built environment actively heals the climate, profitably sequestering gigatons of CO₂ within its walls. This moonshot envisions transforming concrete from a primary emissions source into a powerful carbon sink. The grand challenge is to invent carbon-negative building blocks that perform as well as today’s materials at a competitive cost and that can scale to billions of tons per year. This requires breakthroughs in one of two areas. The first option: developing processes to make OPC (or novel binders that are functionally equivalent to OPC) that absorb more carbon than it/they emit. The second: creating carbon-negative aggregates and additives, like biochar, that can replace traditional materials without compromising performance or price. Success with either option could fundamentally redefine construction, enabling us to build our way to a cooler planet by turning the foundation of civilization into a lasting reservoir for captured carbon.
The most viable solutions will:
Cement is cheap, which makes it cost-prohibitive to transport. As a result, cement markets are highly localized. Whatever feedstocks are used must be available at massive scales worldwide.
Cement often costs less than $100 per ton. Any decarbonized cement process or alternative will need to compete on price, especially in emerging markets where margins are thin and demand is rising most rapidly. Solutions with a high green premium will struggle to gain traction.
The world has built a vast industrial ecosystem around OPC — including standards, codes, and decades of performance data. Low-carbon solutions that preserve OPC’s strength, durability, and predictability can avoid added risk, cost, and complexity, making large-scale adoption more likely.
Solutions that can leverage already-built cement infrastructure will have a considerable headstart as compared with those that require construction of new facilities.
