Cementing Carbon

To appreciate how fundamental a role cement plays in human society, one must first understand the importance of the carbon cycle in the evolution of the planet. Carbon dioxide (CO₂) in the atmosphere dissolves in seawater and gets metabolized by living corals and plankton that eventually die and decompose into ocean sediments. The sediments are compressed over millions of years until they become limestone – a natural storage vault for elemental carbon, like coal, oil and gas.

Through tectonic uplift of the earth’s crust over eons, that limestone rises above sea level to expose cliffs, mountains and other rock formations. But unlike most coal, oil and gas that must be extracted at considerable cost from deep underground, this limestone just sits there for the taking.

Using limestone as construction material dates back at least some 10,000 years, with archeologists unearthing lime mortar floors in buildings in areas spanning from modern-day Turkey to Israel. Limestone found its early lasting glory in the Great Pyramid of Giza, which is made up of 2.3 million blocks of limestone measuring about 1 by 1 by 1 meter and weighing around 2.5 tons each.

The ancient Romans began using a lime mortar, mixing lime with sand and water to build some of their eternal structures, including the Colosseum and Pantheon. They also discovered the utility of volcanic ash found near Pozzouli, on the outskirts of modern-day Naples in the shadow of Mount Vesuvius. Mixing lime with this ash made the cement stronger. And it turns out that making cement this way also releases less CO2 into the atmosphere, a fact that would form the basis for current attempts to displace clinker (processed limestone) with natural or synthetic “pozzolans,” named with a nod to Pozzouli’s ash.

The emphasis here is on processed. It was not until the 1800s that builders realized that burning limestone to a crisp at temperatures of around 1,450 degrees Celsius, about 2,600 Fahrenheit, made for much stronger building materials. At those temperatures, limestone (CaCO3) “clinkered,” turning it into lime or calcium oxide (CaO).

For those keeping track of the chemical symbols, we are left with an equation with one unfortunate byproduct:

CaCO3 + Heat = CaO + CO2

That’s the rub. Limestone burned in hot, rotating kilns makes one of the most fundamental building materials of the modern world, along with a byproduct that poses one of the most fundamental challenges to the survival of the modern world: CO2 released back into the atmosphere, where it all started.

Hard to Abate?

The carbon cycle is, of course, circular. CO2 in the air is eventually returned to its rightful place in nature. The catch is the time disconnect between the millions of years it takes for CO2 to be turned into limestone and the constant churn of over 3,000 cement kilns around the world producing around 4 billion tons of cement annually. Each ton of cement produced releases almost its own weight in CO2 into the atmosphere through the calcination process.

Worse, most of the heat powering the transformation in those kilns currently comes from burning coal, another major source of CO2. All told, global cement production is responsible for an astounding 5-8 percent of CO2 emissions globally – a lot less than fossil fuel consumption, but a significant part of what is often viewed as especially hard to abate.

In contrast to, say, internal combustion engines propelling cars forward where the carbon emissions are the product of burning fuels, most of the carbon emissions in cement- making come from the chemical process itself. Getting CaO out of CaCO3 releases CO2, and it’s been proven to be rather difficult to meddle with this fundamental chemistry.

Indeed, so much depends on how cement is made that many – though importantly, not all – construction standards for cement are based on what recipe is being used. Diverge a bit from that standard, for example, by substituting lower-carbon materials for CaO, and builders may reject the concoction lest they be held liable for diverging from tried-and-true recipes that have literally held up buildings for centuries.

For over 200 years that industry gold standard has been “ordinary Portland cement,” in which the CaO (aka clinker) gets ground up and mixed with gypsum and other common materials. The name, by the way, comes from its resemblance to stone found on the tiny Isle of Portland in the English Channel. It has nothing to do with the city in Oregon (or the one in Maine) – but you can be sure both cities have been built from it.

Almost all of the 4 billion tons of cement produced annually are of the Portland variety, making ordinary Portland cement the third most ubiquitous manufactured material on the planet. Number two? Sand and gravel. Mix cement with sand, gravel, and water, and you get the single most ubiquitous manufactured material: concrete.

Indeed, there is only one other type of manufactured material that comes anywhere close to concrete in systemic importance to the world and in its impact on CO2 emissions: steel. The parallels are striking, right down to the fact that most CO2 resulting from its production comes from turning iron ore found in the earth’s crust into iron, which in turn is to steel what cement is to concrete.

The two industrial sectors share another fundamental characteristic: they are both low-margin commodity businesses, where competition is fierce, volume is a key to success, and innovation has been glacial – until, suddenly, it is not.

Dry Kilns

The basic recipe for ordinary Portland cement may have stayed the same for almost two centuries, but the industry has seen a dramatic shift in how the different ingredients get mixed together. The calcination of limestone into clinker – that is, getting from CaCO3 to CaO – happens in kilns that are massive, fiery, rotating tubes roughly the size of your average bus.

For the longest time, the industry standard was what has since been known as “wet” kilns, wherein raw materials are mixed with water to form a slurry. The high heat in the kiln easily evaporates the water, but the step adds to the energy bill. It is more energy-efficient to bypass the slurry step and grind the raw materials into a fine powder to form a raw meal.

In an industry that is as energy-intensive (and as competitive) as cement, even the smallest process improvements, once proven to work, spread like wildfire – and the savings in the shift from wet to dry kilns wasn’t small. The average dry kiln is 45 percent more energy-efficient than its wet cousin. Adding more technical refinement to the dry-kiln process adds another 10 percentage points for a total improvement of around 55 percent.

Input and Output Efficiency

All told, global cement production increased by some 160 percent in the first 15 years of this century. During that same time, carbon emissions from the cement industry grew by just 120 percent, implying modest but significant efficiency improvements.

One big reason for the improvements brings us back to the foot of Mount Vesuvius and Pozzouli’s volcanic ash. “Pozzolans,” whether natural or synthetic, can substitute for clinker in cement, in some cases even producing a better final product. Lucky is the cement company situated in the shadow of a volcano and able to reduce its use of clinker at low or even no cost.

Costless mitigation options are hard to come by for all the obvious reasons that have the makings of one of the classic economists’ jokes. Two University of Chicago – i.e., conservative – economists are out walking, and one finds a $20 bill on the sidewalk. The other economist dismisses his good fortune: “If the $20 bill were real, someone would have picked it up already.”

The joke works only if you suspend disbelief about the workings of the real world for a moment. Truth is, we all know that companies and markets do not always operate at 100 percent efficiency. If they did, those two Chicago economists would be the first ones to lose their jobs because Chicago’s business school, home to some of the most conservative of Chicago economists, would not need to exist. Why train people in how to run a business, if all businesses are already run perfectly?

Turns out, there are plenty of opportunities to increase efficiency in cement production, where blending with pozzolans can both increase strength and cut costs. The catch-all term for materials that can replace clinker is a mouthful: supplementary cementitious materials, or SCMs. Natural pozzolans top the list of seemingly environmentally benign SCMs that also improve the strength and durability of cement. Others are fly ash and steel slag, the byproducts, respectively, of coal-fired electricity generation and coal-fired blast furnaces producing primary iron.

Now, coal will remain a part of steel production for decades to come, but its days as a fuel for power plants are clearly numbered. That also means fly ash will soon be a dwindling commodity, making this particular SCM a temporary substitute.

Luckily, there is another type of blended cement that comes with even greater CO2- sparing benefits, plus its own acronym: limestone calcined clay cement. LC3 is the product of chemical optimization that showed how a mix of limestone plus calcinated clay makes it possible to swap out more of the energy-intensive clinker. Where traditional SCMs can cut CO2 emissions by around 15 percent, LC3 can double those emissions savings.

That puts this particular intervention on a par in terms of efficiency gains with the switch from wet to dry kilns. But, alas, LC3 has been around for years, and it has yet to take over the industry. One reason: large upfront capital costs relative to the expected savings. At (great) risk of overusing our economists’ joke, there may be $20 bills lying around, but you need an expensive tool to pick them up.

Yet technology that increases process efficiency in cement-making can’t do it all. At very best, it could cut carbon emissions by around 30 percent – nice but not a revolution for a global economy that’s going to need a whole bunch of technological revolutions to contain atmospheric warming to manageable levels. There is, however, an entirely different aspect of efficiency, one the cement industry may not appreciate: the potential for getting the same construction jobs done with a lot less cement.

Global cement production has plateaued in the past decade, and one big reason is that it is, indeed, possible to build taller and bigger with less cement. If wasting inputs are bad for cement companies, wasting cement is bad for construction companies and the planet alike. In part, the efficiency gains are linked to builders using better techniques to achieve the same outcome. Sometimes it is engineers and architects rethinking buildings from the ground up, with everything from 3D printing to construction that swaps out cement and concrete in favor of wood and other products.

Note, however, that using less cement does not always mean less CO2 emissions if builders end up using more steel and glass made with fossil fuel inputs. Consider, too, that thick concrete walls often provide better insulation than walls made of other materials, reducing a building’s lifetime emissions.

The one certainty: less cement means less output for the cement industry. So unless cement conglomerates expand downstream and produce other construction materials, cement companies’ shareholders are unlikely to appreciate this particular approach to reducing carbon emissions. But there is an entirely different type of change rattling the foundation of the cement industry.

From Lab to Scale

When you visualize smart, ambitious researchers out to change the world, you may picture Silicon Valley coders creating the next killer app or molecular biologists developing the next mRNA vaccines. But hardhats and kilns should have a place somewhere in this technology Valhalla.

Meet Cody Finke and Leah Ellis, 30-something PhDs, who have zeroed in on cement process technology, founding Brimstone Energy in Oakland, California, and Sublime Systems in Somerville, Massachusetts, respectively.

Every startup faces daunting cost barriers until they reach reasonable scale. Prototypes necessarily cost more to produce than the hundredth unit and far more than the millionth unit. The big question is whether there is a financially viable pathway to get to competitive scale, and to do so in the usual funding cycles and timescales that define the life of a startup.

The giants of the cement industry – the CEMEXs, Heidelbergs and Holcims – have the financial resources and industrial knowhow to scale new technologies fast. They even have internal venture capital shops seeking out new ideas – and, indeed, one of them (the Swiss-based Holcim) is now partnering with Sublime. But incumbents face a dilemma unique to them.

Amazon signing an agreement with Brimstone for one of its first sizeable batches of cement, or Microsoft doing something similar with Sublime, is one thing. Either cement startup turning into a runaway success would hardly affect Amazon or Microsoft’s core business. Cement incumbents face quite different incentives. If Brimstone, Sublime and other low-carbon technologies scale too slowly or fail as businesses before reaching scale, investing in them is wasted money and effort. If they scale too fast, they might render manufacturing capacities in legacy technologies prematurely obsolete, forcing incumbents to explain to their shareholders why they helped cannibalize their own businesses.

Finke and Ellis, for their part, are still far from having to worry about that eventuality. Their day-to-day worries mirror those of most other founders of fast-growing startups. Finke is currently choosing a site for Brimstone’s pilot plant. Ellis’s Sublime is farther ahead, with a pilot plant in operation since 2023. The first commercial plant is under construction in Holyoke, a former industrial town in western Massachusetts. The 40,000 residents of Holyoke – ground-zero to the first industrial revolution in the United States in the early 19th century – are more used to seeing manufacturing plants leaving town than knocking on their doors.

Process Versus Product

Brimstone and Sublime tackle the same underlying problem, but have devised fundamentally different paths in search of low-carbon cement. Brimstone’s Finke likes to emphasize how his company aims to produce the same old Portland cement that the industry has long been used to. Brimstone’s secret sauce: instead of calcinating limestone (CaCO3), a process that necessarily releases CO2 on the way toward producing clinker (CaO), Brimstone substitutes carbon-free silicate rocks. These silicate rocks are beyond abundant – silicates in general make up 90 percent of the Earth’s crust. And the “impurities” in them that need to be removed to get to CaO may also be valuable on their own.

Indeed, that co-production aspect is just what Brimstone focuses on in outlining the company’s value proposition, emphasizing how it produces three distinct products: CaO (of course), but also alumina found in its silicate rocks and copious amounts of SCMs.

But what might count as an advantage for Brimstone to scale quickly into a crowded industrial space, may also be its biggest limitation. If everything else stays the same, avoiding the process emissions caused by the CaCO3-to-CaO transformation caps Brimstone’s maximum emissions reduction at 60 percent. Using ordinary kilns heated to 1,450 degrees Celsius means that the emissions associated with getting kilns to that temperature won’t change.

To be clear, the heat for those kilns does not need to come from coal. Electric resistance heating powered by 100 percent renewables, or any other kind of zero-emission electricity, is perfectly capable of getting to these temperatures. But no matter how you slice and dice it, it is expensive to generate this much heat.

Ellis and Sublime thus have the seemingly more creative solution: change the process to consume less energy. Why not use electrochemistry to revolutionize a centuries-old sector from the ground up, doing away with 1,450-degree kilns altogether, substituting a process where temperatures no hotter than a hot cup of coffee will suffice? Such is the promise of Sublime’s electrochemistry.

Ellis, then, is altering the product as well as the process. She takes the resulting challenges in stride, arguing that her cement already passes other construction standards that are based on functionality rather than the exact ratio of CaO to gypsum.

Even if the final product performs better than what came before, it is hard breaking into a construction supply chain dominated by old boys’ networks and deeply ingrained institutions. That goes as far as to include insurance companies writing policies for contractors and builders. Using tried-and-true methods in cement production makes it easier to get plug-and-play contracts for the resulting building. Change the process, and insurers might balk.

Technology, Interrupted

For all their concerns about the economics of scaling their emissions-saving technologies to the break-even point and, in Sublime’s case, selling what is effectively a new product to the conservative-minded construction industry – the startups thought they had an ace in the hole. Both had won hefty grants from the Department of Energy as part of the Biden Administration’s green industrial policy push: Brimstone $189 million, Sublime $87 million.

But what the Biden administration gaveth, the Trump administration taketh away. Both companies lost their grants in June. And while both are pressing forward without the federal money, the challenge of proving their technologies and reaching profitable scale relying entirely on funds from private investors is now even more daunting.

The Trump effort to reverse many of President Biden’s signature efforts also hit a $500 million grant given to cement giant Heidelberg for a proposed new plant in Indiana that would have gone all-in on carbon capture, utilization and storage (CCUS). The size of that grant alone speaks volumes, showing how much emphasis both cement incumbents and the Biden DOE put on the technology.

The reversal on that $500 million grant might have also come as a bigger surprise than the cuts to Brimstone and Sublime because the main forces driving CCUS are typically seen to be more closely allied with the current occupant of the White House. Indeed, heavy lobbying by fossil fuel interests have helped preserve and even expand generous Biden-era tax credits for CCUS. To see why, listen, for example, to Occidental Petroleum CEO Vicki Hollub, who has argued that direct air capture could give the fossil fuel industry “a license to continue to operate for the 60, 70, 80 years that is … going to be very much needed.”

To its credit, the Global Cement and Concrete Association has an ambitious net-zero goal by 2050. The group’s Net Zero Roadmap looked at a number of different efficiency and technological levers proposed to meet this goal, but the “net” still does significant work in form of the single biggest lever: CCUS accounts for around 36 percent of the total reductions. The American Cement Association has its own net-zero plan. There, the role for CCUS? Just over 50 percent.

* * *

All that leads us back to Brimstone and Sublime, and technologies that promise to revolutionize cement production.

Neither company’s success is guaranteed – far from it. Such is the life of startups that must spend huge amounts of money to find out whether their technologies are commercially viable, and now must manage the job without assurance of financial help (or even goodwill) from Washington.

Possibly the highest praise and hope for change comes from the industry’s oldest lobby. The American Cement Association had been known as the Portland Cement Association for the first 109 years of its existence. It changed its name this past May, justifying the move by arguing that “the new name better represents the diversified range of materials produced by our members.”

Brimstone and Sublime still have a long way to go before filling out the membership form and joining the big cement players. But the door at least has been opened a crack.

Gernot Wagner is a climate economist at the Columbia University Business School and faculty director of its Climate Knowledge Initiative.

This essay was first published by the Milken Review under the title "Cementing Carbon" (22 January 2026). It is the third in a mini-series; the prior essays are titled "Taming Carbon" (23 October 2023) and "Tipping Carbon" (24 July 2024).