On a September day inside MIT's Pierce Laboratory, researchers watched something almost magical unfold: liquid carbon dioxide depressurized into white flakes, blended into cement paste, and pressed into dime-sized disks, revealing the hidden chemistry that makes this building material 13% stronger in its early hours. For the first time, scientists have directly visualized the transient chemical reactions that occur when CO₂ meets fresh cement—a glimpse into a process that's already attracting commercial interest as a way to both store carbon and improve concrete performance.
The work matters because injecting CO₂ into cement products like concrete is one of the few methods we have to sequester the greenhouse gas permanently while creating useful materials. Companies are already commercializing CO₂-injected concrete mixes, yet until now, no one had seen the actual chemical sequence unfold. Previous research had pieced together a theoretical story from indirect evidence, but the reactions happened too quickly and disappeared too completely for conventional observation techniques. Enter Raman confocal microscopy—a method so elegant in its simplicity that it reveals molecular identity by illuminating materials with lasers and reading the scattered light's unique "fingerprint."
The team, led by MIT associate professor Admir Masic and first-authored by graduate student Marcin Hajduczek of the MIT Concrete Sustainability Hub and MIT Department of Civil and Environmental Engineering, conducted 24 hours of continuous scanning that captured a three-act chemical drama, now published in the Journal of the American Ceramic Society. Their collaborators included Santiago El Awad and Franz-Josef Ulm from MIT, researchers from IIT Jodhpur, and scientists from CarbonCure Technologies.
Act one unfolds in the first hour. When CO₂ is added to fresh cement paste, it dissolves into the pore solution and reacts with calcium released by dissolving clinker—the kiln-fired mineral mixture that forms cement's primary ingredient. This precipitation of calcium carbonate temporarily slows the normal hydration reaction, which ordinarily depends on available calcium. Meanwhile, the silicates released by the clinker dissolve into the pore solution and precipitate far from their source, linking together into chains that form an interconnected silica gel network throughout the paste. This amorphous, fleeting gel is the key to what comes next.
By four to five hours after mixing, injected CO₂ is fully mineralized and normal hydration resumes. Calcium hydroxide begins to precipitate, and crucially, it encounters the silica gel network that's been waiting. Their reaction produces calcium silicate hydrate—the compound that gives cement its binding ability. But here's what makes CO₂-injected cement distinct: this C-S-H forms not clustered around clinker particles as in conventional concrete, but distributed throughout the entire matrix wherever the silica gel had spread. That distributed binding network is what drives the strength gain.
Within eight hours, the silica gel is almost entirely consumed through a pozzolanic reaction with calcium hydroxide, as hydration reasserts itself and pH rises back to typical levels. The result is a stronger, denser microstructure formed in the critical early-set window—the 13% strength advantage that drew researchers to this puzzle in the first place.
"We've used Raman spectroscopy to better understand some of the most interesting materials in history, from the Dead Sea Scrolls to Ancient Roman concrete," Masic reflects. "Cement paste may seem less glamorous in comparison, but pointing a laser at CO₂-injected cement paste as it hardens allows us to visualize things that haven't been seen before." Now that the chemistry is finally visible, concrete companies have a clearer path to optimizing this promising carbon-storage technology.