In MIT's Pierce Laboratory, researchers depressurized a tank of liquid carbon dioxide (CO₂), instantly freezing it and releasing solid flakes. These flakes were blended into cement paste and pressed into discs about the size of a dime, sealed with a thin layer of vegetable oil to retain moisture and exclude air. The team trained lasers on each disc, observing for the first time the transient chemical reaction that explains why CO₂-injected cement paste gains strength faster. Injecting CO₂ into cement products like concrete is a method to store it and reduce atmospheric emissions, attracting commercial interest with numerous companies offering CO₂-injected concrete mixes.
A new open-access paper in the Journal of the American Ceramic Society, led by Associate Professor Admir Masic and graduate student Marcin Hajduczek from MIT's Concrete Sustainability Hub, describes the chemical sequence that unfolds after CO₂ meets fresh cement paste. Previous studies pieced together the chemical impacts of CO₂ injection from theory and indirect evidence, but key reactions were too fast and elusive for conventional techniques to capture. Raman confocal microscopy can illuminate molecules with a laser, revealing their identity through scattered light.
Act One: Capturing Calcium
Upon adding CO₂ to fresh cement paste, it dissolves into the pore solution and reacts with calcium released by dissolving clinker, precipitating as various forms of calcium carbonate. This occurs within the first hour, temporarily slowing the normal hydration reaction that requires calcium. Without CO₂, the released calcium supports the gradual formation of binding phases. The absence of calcium allows released silicates to dissolve and precipitate far from their source, forming an interconnected silica gel network throughout the paste.
Act Two: The Ghostly Gel
Once the injected CO₂ is fully mineralized (around four to five hours post-mixing), normal hydration resumes. Calcium hydroxide begins to precipitate into the pore space, interacting with the silica gel network to produce calcium silicate hydrate (C-S-H), the compound that gives cement its binding ability. This C-S-H forms distributed throughout the matrix rather than clustered around clinker particles. The CO₂ temporarily suppressed the paste's alkalinity, and the lower pH was crucial for maintaining the silica gel.
Act Three: A Rewired Matrix
With the silica gel consumed, the paste settles into conventional hydration, leaving behind a measurably different microstructure. The new binder's even distribution results in stronger and more uniform early-age strength, with CO₂-mixed paste showing an average 13% higher compressive strength at 24 hours compared to reference mixes. Masic states, "We've been injecting CO₂ into cement products for years without fully understanding what it was doing inside. Now that we can see it and understand the underlying mechanism that leads to improved performance, we can start to control it."
Future Research Directions
Understanding the mechanism allows researchers to pursue more specific questions. While the silica gel template explains the distribution of the new C-S-H, measuring its mechanical properties remains a next step. Additionally, the dosage of CO₂ is critical; too much can lock calcium into carbonate before the gel can form and react. Despite these open questions, the ghostly gel has been captured, and researchers can now explore the chemistry that unfolds in those first eight hours.