Could concrete absorb more CO2 than its production emits? An EU project is testing that idea

Eight percent. That is concrete's share of global CO2 emissions, driven primarily by one ingredient: Portland cement clinker. The carbon cost comes from two sources -- the energy needed to heat raw materials to roughly 1,450 degrees Celsius and, more significantly, the chemical deacidification of limestone that releases CO2 as a direct byproduct of the reaction. Every ton of clinker produced releases close to a ton of CO2. Given that the world produces about four billion tons of cement annually, the math is bleak.

A European consortium called C-SINC is working on a material substitution that could, in theory, flip that equation. Instead of merely reducing emissions from concrete, the goal is to make concrete a net carbon sink -- a material that stores more CO2 than its production releases.

Magnesium silicates as a clinker replacement

The approach centers on magnesium silicates, minerals that react with CO2 through accelerated mineralization to form magnesium carbonate. This carbonate can be used as a secondary cement additive, replacing part of the clinker in concrete mixes. The CO2 used in the reaction is captured from industrial exhaust gases -- effectively pulled from the atmosphere before it contributes to warming.

"By using CO2 that's extracted from industrial exhaust gases, not only can we lower emissions due to concrete, we can also make it work as a carbon sink," said Professor Frank Dehn, who heads the Institute of Concrete Structures and Building Materials at the Karlsruhe Institute of Technology (KIT). "The CO2 isn't just stored, it's chemically bound in a mineral. It remains firmly bonded, so it can't escape over very long periods."

The distinction between storage and chemical binding matters. Carbon capture and storage (CCS) typically involves injecting CO2 underground, where it can potentially leak. Mineral carbonation locks the carbon into a solid crystal structure -- the same process that naturally weathers silicate rocks over geological timescales, just accelerated to industrial speed.

From simulation to structural testing

KIT's contribution to C-SINC focuses on figuring out whether concrete made with these new binders actually works as a construction material. The team uses machine learning models to predict which concrete formulations are worth testing, then validates those predictions experimentally -- first on small samples, then on full-scale structural elements at KIT's materials testing facility in Karlsruhe.

"Using machine learning strategies and structural-mechanical modeling, we're investigating how the binding agent behaves in concrete, how to best mix the concrete, and how well it works in practice," Dehn said.

The key parameters are load-bearing capacity, durability, and safety -- the same criteria any structural concrete must meet before it can be specified for buildings, bridges, or infrastructure. A carbon-negative concrete that cracks under load would be of little practical use.

Why existing substitutes are running out

The construction industry already uses substitute materials for cement clinker, including fly ash from coal combustion and ground blast-furnace slag from steel production. These reduce the clinker ratio and lower emissions. But both materials are byproducts of industries that are being phased out or transformed. Germany's coal exit will eliminate fly ash supply. The shift from blast furnace steelmaking to electric arc and hydrogen-based processes will cut slag availability.

This creates urgency. The industry needs new supplementary cementitious materials that are abundant, effective, and compatible with existing construction practices. Magnesium silicates are geologically abundant, which gives them an advantage over the dwindling alternatives -- but only if they can be processed and incorporated into concrete at industrial scale and competitive cost.

Caveats worth noting

C-SINC is a four-year, EUR 4 million project funded by the European Innovation Council. It is still in the research and development phase. The energy cost of accelerating the mineralization process, the economics of CO2 capture at the volumes required, and the performance of the resulting concrete under real-world conditions all remain open questions. Making concrete a net carbon sink would require not just replacing part of the clinker but ensuring that the total lifecycle emissions -- including mining, processing, and transport of the magnesium silicates -- are accounted for.

The consortium includes KIT as the only German partner, alongside PAEBBL AB (Sweden), Delft University of Technology (Netherlands), KU Leuven (Belgium), the Spanish National Research Council, Prefabricados Tecnyconta (Spain), and Holcim Technology Ltd. (Switzerland) in a supporting role. KIT is also the only German university funded by the EIC in its Pathfinder program.