A shot of carbon dioxide rewires how cement sets

Key Points:

• MIT researchers used Raman confocal microscopy to directly observe the rapid chemical reactions occurring when CO2 is injected into fresh cement paste, revealing a previously unseen sequence that explains faster strength gain.
• The study identified a three-stage process: CO2 reacts with calcium to form calcium carbonate, temporarily suppressing normal hydration; a transient silica gel forms and later reacts with calcium hydroxide to produce calcium silicate hydrate (C-S-H); and finally, a more uniformly distributed binder results in stronger early-age cement.
• CO2-injected cement paste showed an average 13% increase in compressive strength after 24 hours compared to traditional mixes, due to the even distribution of C-S-H throughout the matrix rather than seeding by calcium carbonate crystals.
• Understanding this mechanism allows for better control and optimization of CO2 injection in cement, potentially offsetting a significant portion of carbon emissions from cement production, though practical offsets will be less than theoretical maximums.
• The findings open new avenues for research into the mechanical properties of the silica gel template and the effects of CO2 dosage on cement chemistry and performance.