MIT researchers used real time Raman spectroscopy to capture a fleeting silica gel intermediate in CO2 injected cement, giving engineers a mechanistic handle on why the mix gains early strength.
The "ghostly gel" forms in hours and vanishes, but leaves a structural imprint that reshapes how engineers can design CO2-cured concrete. For a field that has been tuning CO2-injected mixes largely by trial and error, that imprint is the news: a new study from the MIT Concrete Sustainability Hub shows the chemistry directly, frame by frame, using a real-time imaging technique better known from art conservation and Roman concrete.
Cement production is one of the largest industrial sources of carbon emissions on the planet. For years, a small industry has been betting that injecting CO2 into fresh cement paste can both sequester carbon and produce concrete that sets faster, with less cement. The bet has produced commercial products, but the underlying chemistry has remained a black box. Engineers knew that CO2-injected mixes gained early strength, but not precisely why, and not how to tune the recipe from first principles.
The new paper, published in the Journal of the American Ceramic Society by graduate student Marcin Hajduczek, Associate Professor Admir Masic, and colleagues at MIT, IIT Jodhpur, and CarbonCure Technologies, changes that. Using a custom quartz-window stage and real-time Raman confocal microscopy, the team watched what happens inside fresh cement paste during the first roughly 24 hours after CO2 is introduced.
Three reactions unfold in sequence. Within the first hour, dissolved CO2 scavenges calcium from the dissolving clinker and precipitates calcium carbonate throughout the pore solution, briefly stalling normal cement hydration. Once calcium is locally depleted, silicates begin to dissolve, and an amorphous silica-gel network forms and spreads through the paste, the transient "ghostly" intermediate the team reports at roughly four to five hours post-mix, gone again by about eight hours. As pH rebounds, calcium hydroxide reacts with that silica gel in a pozzolanic reaction to form calcium silicate hydrate (C-S-H), the actual strength-building phase, distributed through the matrix rather than concentrated only around clinker grains.
The practical consequence is strength. In the lab, paste mixed with CO2 at 1% by cement weight showed on average 13% higher compressive strength at the 24-hour mark compared with reference mixes, a single dose at a single early timepoint rather than a sweeping claim. More importantly, the observation overturns a long-standing assumption. Calcium carbonate crystals, long suspected of seeding C-S-H growth, are not the active agents. They are bystanders embedded in a silica-gel template. Strength comes from where the gel directs the C-S-H, not from the carbonate itself.
That distinction matters because it gives the field something it did not have: a mechanistic handle on the chemistry. Instead of adjusting mix ratios empirically, engineers can now reason about dosage, timing, and the role of the silica-gel window. Too much CO2, the authors note, can lock calcium into carbonate before the gel can form, undermining the effect the technique is meant to produce. That is a tradeoff mix designers could not previously see.
The result also reframes a contested climate claim. MIT notes a theoretical upper bound of roughly 40% offset of cement-production carbon emissions from this chemistry, excluding kiln-fuel emissions, with the explicit caveat that practical offset is likely only a fraction of that figure. The new paper does not settle the broader question of whether CO2-cured concrete is a meaningful carbon storage strategy at commercial scale. It does, however, give researchers a way to study that question with sharper tools than they had before.
Open questions remain. The team flags durability, long-term performance variability, and at-scale carbon accounting as unresolved. The 13% strength gain was measured at one dose and one age; the chemistry may shift at other dosages, and the transient silica gel's mechanical properties have not been directly measured. CarbonCure, a co-author, already ships CO2-injected concrete commercially, which makes the mechanism finding immediately industrially relevant, but commercial performance and lifecycle claims are separate from the lab observation and would need their own verification.
For a field that has spent the last decade tuning CO2-cured concrete mostly by feel, the next round of work can be more deliberate. The reaction that was invisible is now a reaction they can watch, dose by dose, hour by hour.