Injecting carbon dioxide (CO₂) into fresh cement paste increases its early strength by 13 percent, according to new research. This process, which also stores CO₂, creates a stronger and more uniform microstructure in the cement. Scientists used Raman confocal microscopy to observe the chemical reactions in real time.
The research, published in the Journal of the American Ceramic Society, details the chemical sequence that occurs when CO₂ is added to cement. The study was led by Associate Professor Admir Masic and graduate student Marcin Hajduczek from the Massachusetts Institute of Technology (MIT). Co-authors included researchers from IIT Jodhpur and CarbonCure Technologies.
When CO₂ is introduced, it dissolves into the cement's pore solution and reacts with calcium. This reaction forms calcium carbonate and temporarily slows the normal hydration process. Without available calcium, silicates from the clinker dissolve and form an interconnected silica gel network throughout the paste.
After four to five hours, the injected CO₂ is fully mineralized, and normal hydration resumes. Calcium hydroxide precipitates and reacts with the silica gel network, producing calcium silicate hydrate (C-S-H). This C-S-H is distributed more evenly than in conventional hydration, leading to a stronger matrix.
The silica gel disappears within eight hours as it converts into C-S-H. This rapid transformation during the early setting period is crucial for the improved strength. The study found that cement paste mixed with one percent CO₂ by cement weight achieved 13 percent higher compressive strength at 24 hours compared to reference mixes.
This understanding of the chemical mechanism allows for better control over the CO₂ injection process. Researchers can now explore optimizing CO₂ dosage and further enhancing cement performance. The findings also clarify that calcium carbonate crystals are passive bystanders, not active participants, in the C-S-H formation.
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