Researchers at the Massachusetts Institute of Technology (MIT) have, for the first time, visualized the intricate chemical reactions that occur when carbon dioxide (CO2) is injected into cement paste, a process that enhances the material’s early strength. The findings, published in the Journal of the American Ceramic Society, explain a previously unseen mechanism involving a transient silica gel.
A New Look at Cement Chemistry
Injecting CO2 into cement products is a method for storing the greenhouse gas and has garnered commercial interest, with companies offering CO2-injected concrete mixes. However, the precise chemical processes involved were not fully understood due to the speed and transient nature of the reactions, which conventional techniques could not capture.
Using a technique called Raman confocal microscopy, which uses lasers to identify molecules by their unique light scattering patterns, the MIT team, led by Associate Professor Admir Masic, was able to observe these fleeting chemical transformations. They depressurized liquid CO2 into solid flakes, mixed them into cement paste, and sealed the samples. Lasers were then trained on the discs to observe the reactions as the cement hardened over 24 hours.
The Three-Act Chemical Drama
The process unfolded in three distinct stages. Initially, the injected CO2 reacts with calcium released from dissolving clinker, forming calcium carbonate. This step temporarily slows the normal hydration process, which requires calcium. In the absence of CO2, this calcium would remain available for gradual binding.
With calcium tied up in carbonate, silicate released from the clinker precipitates elsewhere, forming a widespread, amorphous silica gel. This gel serves as a template. Once the CO2 is fully mineralized, typically within four to five hours, normal hydration resumes. Calcium hydroxide then precipitates and reacts with the silica gel through a pozzolanic reaction. This interaction produces calcium silicate hydrate (C-S-H), the primary binding compound in cement.
Crucially, this C-S-H forms not just around clinker particles, as in standard hydration, but throughout the entire matrix where the silica gel had spread. The CO2 initially suppressed the paste’s alkalinity, which kept the silica gel intact. As hydration products form and pH levels rise, the gel is consumed, transforming into additional C-S-H within about eight hours. Researchers observed that the silica gel’s disappearance was a consistent feature in CO2-injected samples.
Rewired Matrix and Improved Strength
The result of this CO2-induced process is a measurably different microstructure. Because the new binder is distributed more uniformly, the resulting cement paste is stronger and more uniform at an early age. In the study, cement paste mixed with 1 percent CO2 by weight achieved an average of 13 percent higher compressive strength at 24 hours compared to reference mixes without CO2.
These findings also refine existing theories about CO2-injected cement’s higher early strength. Previously, calcium carbonate crystals were thought to actively seed C-S-H growth. The new research suggests these crystals are passive bystanders embedded in the silica gel template.
Understanding this mechanism opens avenues for further research, including directly measuring the mechanical properties of the newly distributed C-S-H. The researchers note that while the process could theoretically offset a significant portion of cement production’s carbon emissions, practical offsets are likely to be a fraction of that potential. However, even a partial offset could be substantial.
Steve Lopez is the Editorial Page Editor for News Raise. He covers Health. He has won more than a dozen national journalism awards for his reporting and column writing at seven newspapers and four news magazines.
