Circular CO₂ Utilization Strategies for More Sustainable Concrete

The sustainability goals of the cement and concrete industry cannot be met by simple improvements to process efficiencies but instead demand innovative solutions. New processes have been developed to reduce the carbon footprint of ready mixed concrete through CO₂ utilization strategies concerning three components: binder, water and aggregates. The injection of an optimized dose of waste carbon dioxide into concrete reacts with the cement binder to form in-situ nanoscale calcium carbonate particles that can improve the compressive strength of the mix. The increased cement efficiency allows the concrete to be produced with less cement thereby realizing a GHG benefit through both the mineralized CO₂ and the avoided cement CO₂ emissions. Concrete wash water, a by-product of concrete production that is typically a waste stream and a challenge to reuse, can be treated with carbon dioxide. The CO₂ is mineralized in a reaction with the waste cement suspended in the slurry. The treated slurry can more readily be used as mix water in a new concrete batch. The performance benefit of using the recycled slurry, in particular the cementitious nature of the treated wash water solids, allows for a cement reduction. Finally, the performance of recycled concrete aggregate can be improved through a carbon dioxide treatment. The carbon dioxide reacts with the hydrated cement paste component of the crushed concrete to form CaCO₃. A combination of the three strategies can realize a net carbon benefit of around 76.9 kg per m³ of concrete including recycling of 50.3 kg of CO₂.