MIT Researchers Directly Observe CO₂ Reaction in Cement for the First Time
Using Raman confocal microscopy, scientists visualized the entire 24-hour chemical process, confirming long-held theories about how CO₂ injection strengthens concrete.
For the first time, MIT researchers have directly observed the chemical reaction that occurs when carbon dioxide (CO₂) is injected into cement paste. Published in the Journal of the American Ceramic Society, the study provides high-resolution, real-time visual evidence of a process that had previously only been theorized.
The reaction unfolds in three distinct stages:
Calcium capture — CO₂ dissolves and reacts with calcium from the dissolving clinker. This forms calcium carbonate and temporarily slows normal hydration.
Silica gel formation — Once the calcium is consumed, the remaining silicates form a silica gel network throughout the paste.
C-S-H formation — After 4–5 hours, once the CO₂ is fully mineralized, normal hydration resumes. Calcium hydroxide reacts with the silica gel to form calcium silicate hydrate (C-S-H), the main binding phase, now distributed throughout the entire matrix.
"This study provides the first direct evidence for mechanisms that were previously only theoretical."
The findings may help explain a long-observed phenomenon: why CO₂-injected cement gains strength faster than conventional formulations.