Fixing cement’s carbon problem

Start with why cement is a distinctive decarbonization problem rather than merely a large one. Cement is made in a long rotary kiln tilted at about five degrees; rocks tumble down it and two reactions happen on the way. At around 1,000 degrees Celsius limestone breaks into calcium oxide and CO2, which escapes as gas. Further down, at roughly 1,500 degrees, the calcium fuses with silicates into Portland cement. Ellis puts roughly half the emissions down to burning fossil fuel to reach those temperatures and roughly half to the limestone itself, which works out to about a ton of CO2 per ton of cement. The two halves are independent, and that is the whole difficulty. Solve the industrial heat problem and the chemistry is untouched; solve the chemistry and the heat is untouched. Kann puts it that way and Ellis agrees.

That independence is what makes each familiar pathway partial. Alternative fuels, burning tires or biomass instead of coal, address only the heat. Supplementary cementitious materials, the silicates blended into cement, work because hydrated Portland cement leaves more calcium floating around than the hardened phase needs; extra silicates soak it up and improve density and durability, which is why the industry blended them for decades on cost grounds long before carbon was the reason. But you can only go to about 30% before the silicates overshoot and performance dilutes, so you are still buying 70% Portland cement and, as Kann puts it and Ellis accepts, getting at best a 30% reduction. Fly ash, the most popular of these materials, comes from coal plants that are closing, and Ellis adds that whether it counts as sustainable depends on how you allocate its embodied carbon between the electricity and the ash. Curing concrete with injected CO2 puts some carbon back, and hardened concrete recarbonates naturally anyway, but only the limestone half is ever recoverable that way, the natural version takes decades to a century or more, and carbonation is simultaneously a degradation reaction. That leaves post-combustion capture as effectively the only route that takes Portland cement itself to zero, which is why it appears in every major cement company’s net-zero-by-2050 plan, and it hands you a CO2 stream that still needs pipelines and permanent storage.

Sublime attacks the process rather than its exhaust. Ellis’s background is electrochemistry, batteries specifically, and the company’s move is to dissolve calcium out of a calcium-bearing mineral and precipitate it in a second step, at ambient temperature, on electricity rather than flame, and tolerant of intermittent supply. Two feedstocks are possible and they lead to different places. Start from limestone and the calcium comes out without thermal decomposition, leaving pure cold compressed CO2 as an inherent byproduct, which Ellis says makes capture cost less than $10 a ton but still requires somewhere for the gas to go. Start instead from one of the non-limestone calcium-bearing minerals, largely silicates, and there is no CO2 at all, and the silicate you pulled the calcium out of is itself an ingredient of the cement.

The catch sits in what you get. Without the 1,500-degree step there is no tricalcium silicate, the phase responsible for Portland cement’s fast set and early strength, so the product is not Portland cement. Ellis’s claim is that the final hardened phase is the same, and it is the hardened phase that carries compressive strength and durability. Whether that is commercially sufficient is a question about specification rather than chemistry, which is why the most consequential part of the episode is about standards. Cement was historically specified by chemistry, meaning a required percentage of tricalcium silicate. That is giving way to performance-based standards: blended-cement standards permitting 50% Portland plus 50% of almost anything that passes the tests, and fully performance-based standards where the structural engineer asks only that it work. Kann draws the conclusion and Ellis endorses it, that if those standards take hold the market stops being defined by a formulation and a cement that never needed limestone becomes possible. The economics point the same direction, because every other decarbonization route is an energy adder and electric heat is inherently less efficient than direct combustion, whereas Ellis claims Sublime’s process matches Portland cement’s embodied energy today with a path to half. And a green premium on cement is small where it lands, since she puts labor at roughly 90% of installed cost and materials at about 5%, making the premium budget dust to a building owner while being one of the larger levers on a company’s scope 3 emissions.