ETH Zurich wants to run cement kilns as air-capture machines

Researchers at ETH Zurich have proposed operating cement plants as direct air capture facilities, using the limestone they already heat as a reusable carrier for CO2 drawn from ambient air ^[2][3][7]. The idea matters because cement is responsible for an estimated 5 to 8 percent of global CO2 emissions and has resisted the fixes that work elsewhere ^[1][5].

Cement makes CO2 twice. The first stream comes from burning fossil fuels to reach kiln temperatures; the second, and harder, comes from calcination, in which limestone breaks down into quicklime and CO2 when heated ^[4]. Because that second stream comes from the rock itself, swapping coal for cleaner electricity lowers emissions but does not eliminate them, which is why cement sits in the difficult-to-decarbonise category ^[5].

The study, published in Chem Circularity, is built on calcium looping ^[3]. Limestone is heated to quicklime and the released CO2 is collected rather than vented; water is added to make slaked lime, which absorbs CO2 from the surrounding air and reverts to limestone that re-enters cement production ^[7]. The calcium acts as a reusable carrier, and the more cycles it makes through the air-capture stage before becoming cement, the more atmospheric CO2 it removes ^[7][8]. That captured air CO2 can be compressed and stored underground, so in principle the plant becomes net-negative, removing more than the whole system emits ^[9].

The reason to host this in a cement plant is that both processes turn on the same material and the same act of heating limestone ^[7]. That shared chemistry is the substance behind reframing the industry as an asset rather than a liability: the kiln and the limestone supply already exist.

It is worth being precise about what the paper actually quantifies. Modelling indicates that electrifying the kiln and capturing CO2 directly could cut the climate impact of cement production by up to 78 percent by 2050 ^[10]. That is a reduction figure, not a cost figure; the source does not price this retrofit against standalone direct air capture, so the claim that cement plants are the cheapest available host is an inference the material supports in principle but does not measure. The outcome also hinges on power, because direct air capture is energy-intensive and the benefit depends on where the electricity comes from ^[11].

The researchers' own language is careful. Lead author Vittoria Bolongaro, a PhD student working with Bardow, describes the combination of direct air capture and cement production as "very promising" from a climate perspective, which is a claim about potential rather than deployment ^[6].

What to watch is whether anyone builds it, on what power, and at what cost. The 78 percent figure assumes a 2050 electricity system and an electrified kiln, neither of which exists yet at scale ^[10][11]. Until a pilot reports energy use and a per-tonne removal cost, the case for cement plants as the default DAC host rests on shared equipment rather than demonstrated economics.