One September day, it started to snow inside MIT's Pierce Laboratory. Researchers depressurized a tank of liquid carbon dioxide (CO2), instantly freezing it and releasing solid flakes. These were blended into cement paste and pressed into disks roughly the size of a dime, each sealed with a thin layer of vegetable oil to keep water in and air out. The team trained lasers on each one, observing for the first time the transient chemical reaction that might explain why CO2-injected cement paste gains strength faster.

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