Alternative Cement
Concrete is the most-used material on earth after water, and the cement that binds it carries a carbon problem baked into its chemistry — released by the limestone itself, not just the fuel. The fix is not a cleaner kiln. It is using less of the guilty ingredient.
The effect compounds within years. Put it in place and it keeps working.
Origins
Cement is ancient. The Romans built a maritime concrete from volcanic ash and lime that has outlasted two thousand years of seawater — and, tellingly, their ash-and-lime binder was itself a low-clinker material by modern standards, because they had no way to reach the temperatures that make modern clinker.
Modern Portland cement — named for its resemblance to Portland stone — was patented in England in 1824, and its very virtue became its vice: by firing limestone and clay hard enough to form clinker, it produced a strong, reliable, universal binder that could be made anywhere from common rock. It built the modern world. It also became, by sheer volume, one of the largest industrial sources of carbon dioxide on earth.
The idea of stretching clinker with other materials is almost as old as the industry. For a century, cement makers have blended in supplementary cementitious materials — most importantly fly ash, the residue from coal power stations, and slag, the by-product of blast-furnace steelmaking. These worked well and cut emissions substantially. But they carried a hidden dependency: they were waste streams of the fossil economy. As coal power shuts down and steelmaking changes, fly ash and slag are dwindling — which is what makes the newer approach so important.
What it actually is
Concrete is everywhere: it is the second-most-consumed substance on the planet after water, the grey material of nearly every city, road, foundation, and dam. Concrete itself is mostly sand, gravel, and water. The trouble lives in the small fraction that binds it all together: cement, which is only ten to fifteen percent of concrete by weight yet carries almost all of its carbon footprint.
And cement’s carbon problem is unusually stubborn, because it comes from two sources and one of them is chemical, not energy. To make ordinary cement you heat limestone to around 1,450℃ to produce clinker. That does two things. It burns a great deal of fuel to reach the temperature — roughly 40 percent of the emissions. But it also drives carbon dioxide out of the limestone itself, chemically, as the calcium carbonate breaks down into lime and CO₂ — roughly 60 percent of the emissions, and a molecule you cannot avoid by switching to clean electricity or a cleaner fuel. The carbon is in the rock, and making clinker releases it by definition.
This is why you cannot decarbonise cement simply by cleaning up the kiln. As long as you make clinker, you release that chemical carbon. The only real answer is to make less clinker — to replace part of it with other materials that bind nearly as well but carry none of that chemical debt. That is what alternative cement is: the same concrete, with much of the guilty ingredient swapped out.
The numbers
The share of global emissions. Cement production is responsible for roughly 7 to 8 percent of global carbon dioxide emissions — a figure that would rank it, were it a country, among the top few emitters on earth.
Where the carbon comes from. About 60 percent of cement’s emissions are process emissions, released chemically from the limestone as clinker forms, and cannot be eliminated by changing fuels. The remaining 40 percent or so comes from the energy to heat the kiln.
The concrete paradox. Cement is only about 10 to 15 percent of concrete by mass, yet accounts for nearly all of concrete’s embodied carbon — which is why the whole problem, and the whole solution, sits in that small binding fraction.
What substitution achieves. Blends that replace part of the clinker can cut cement’s carbon substantially. The most promising scalable approach, limestone calcined clay cement, replaces up to around half the clinker with calcined clay and limestone and reduces CO₂ by roughly 30 to 40 percent, while matching conventional strength and improving some aspects of durability.
Why the newer approach matters. Calcined clay is fired at only 700 to 900℃, far below clinker’s 1,450℃, and the clays it needs are abundant across exactly the countries where cement demand is growing fastest — unlike fly ash and slag, whose supply is shrinking with coal and old-style steel.
Why it matters
We like this solution because it punctures a comfortable illusion — the idea that clean energy alone will fix everything — and replaces it with something more honest and, oddly, more hopeful.
You can run the whole cement plant on renewable electricity and green hydrogen and you will still emit the majority of the carbon, because it comes out of the rock, not the flame. That is a hard fact, and it is worth sitting with, because it teaches a general lesson: some emissions are chemical, structural, built into what a material is, and those cannot be waved away with a clean grid. They have to be engineered around. Pretending otherwise is how climate plans quietly fail.
But the honest version is also the encouraging one, because the engineering exists and it is not exotic. The leading answer, limestone calcined clay cement, is made from common clay and limestone, materials lying under the ground almost everywhere, and especially under the fast-growing cities of the Global South where most of the coming century’s concrete will be poured. It is, unusually, a low-carbon option that is also cheaper and more local than what it replaces. That is the rare and precious kind of solution: the greener path that is also the path of least resistance, so that adoption does not depend on virtue.
Under Our Buildings, this is the quiet foundation — literally. The material our civilisation is built from can be rebuilt to carry far less carbon, using the earth beneath our feet, without asking anyone to build less.
What it actually takes
Cut clinker, don’t just clean the kiln. Because most of cement’s carbon is chemical, the central move is replacing clinker with supplementary materials. Everything else is secondary to this.
Scale calcined clay. As fly ash and slag dwindle, calcined clay is the one supplementary material available in the quantities the world needs, sourced regionally almost everywhere. Building the kilns and supply chains to calcine clay is the key industrial task.
Update the standards and codes. Much of the barrier is not technical but regulatory: building codes and procurement rules written around old-style Portland cement have to be updated to permit and prefer low-clinker blends.
Use less concrete, and use it longer. The greenest cement is the cement not needed: smarter structural design, reuse of existing structures, and longer-lived buildings all cut demand at the source.
Pair with efficiency and, eventually, capture. Cleaner kiln fuels and, for the irreducible process emissions, carbon capture at the plant will be needed to reach the last mile — but clinker substitution is the large, cheap, available first step to take now.
Where it matters most
Alternative cement matters most where the most concrete is about to be poured — the rapidly urbanising regions building the cities of the coming century, which also happen to sit on the clay the new cements require.
Across the Western Ghats and the wider Indian subcontinent, cement demand is enormous and rising, and the kaolinite-rich clays that limestone calcined clay cement depends on are abundant — a near-ideal match of need and resource.
The North China Plain anchors the largest cement industry on earth by a wide margin, so even incremental shifts in how its clinker is blended move the global figure.
Across the East African Rift and the Sahel, where urbanisation is accelerating and much of the built environment of 2100 does not yet exist, there is a chance to build it from low-clinker cement from the start rather than retrofitting later.
And in the high, growing cities of the Andes, local clay and limestone offer the same opportunity to pour the coming decades of concrete with a fraction of the carbon.
How to tell it’s being done well
Does it reduce clinker, or just the fuel? Because most of cement’s carbon is released chemically from limestone, the real test is how much clinker has been replaced, not how clean the kiln burns.
Does the substitute actually scale? Fly ash and slag help but are dwindling. Calcined clay is the one material available in the volumes required, so scalable solutions lean on it.
Does it match strength and durability? A low-carbon cement is only useful if the structure lasts. The leading blends match or exceed conventional performance, and that should be demonstrated, not assumed.
Do the codes allow it? Real adoption depends on building standards and procurement rules that permit and prefer low-clinker cement. Regulatory readiness is part of doing this well.
Is demand being cut too? The best programmes pair cleaner cement with using less of it, through efficient design, reuse, and longer-lived buildings.
Where this matters most
Who is working on this
We are researching which organizations in our directory of 8,493 actively work on this solution, and we only list an organization once we have verified it. That research is ongoing. In the meantime, search the directory yourself:
Questions
Why can't we just make cement with clean energy?
Because roughly 60 percent of cement's emissions are chemical, not from fuel. Making clinker means heating limestone until it releases carbon dioxide from the rock itself, a reaction that happens regardless of how clean the heat is. Clean energy addresses only the other 40 percent. The rest has to be engineered around by using less clinker.
What is alternative cement?
Cement in which much of the clinker, the carbon-intensive ingredient, is replaced by other materials that bind nearly as well but carry none of the chemical carbon. Traditional substitutes are fly ash and slag; the most promising scalable one is limestone calcined clay cement, or LC3.
What is LC3 and why does it matter?
Limestone calcined clay cement replaces up to about half the clinker with calcined clay and limestone, cutting CO2 by roughly 30 to 40 percent while matching strength. It matters because its raw materials are common clays fired at far lower temperatures than clinker, and they are abundant exactly where cement demand is growing fastest.
Why not just keep using fly ash and slag?
Because they are by-products of coal power and old-style steelmaking, both of which are shrinking. As those industries change, the supply of fly ash and slag is dwindling, which is why a scalable new supplementary material like calcined clay has become essential.
How big is cement's climate footprint?
Cement production accounts for roughly 7 to 8 percent of global carbon dioxide emissions. If it were a country, it would rank among the largest emitters on earth, which is why even partial substitution of clinker delivers a very large absolute benefit.
Sources
- Project Drawdown - Alternative Cement / Materials sector Framework and classification. Cited, not reproduced.
- GlobalABC - Calcined clay limestone cements (LC3)
- IEA - Cement technology roadmap and tracking
- World Economic Forum - ways to make cement more sustainable
The solution taxonomy follows the framework popularised by Project Drawdown. The analysis above is our own; for their carbon modeling and rankings, visit them directly.