On the mechanism of using carbon dioxide as a beneficial concrete admixture

The addition of carbon dioxide into fresh ready mixed concrete has been observed to produce a measurable increase in hydration and a significant compressive strength increase. An optimal dose introduced during batching and mixing of ready mixed concrete imparts physiochemical changes to the early hydration. The mechanistic basis for macroscopic performance changes was investigated through the study of a model tricalcium silicate system. A C3S paste was treated with carbon dioxide immediately after hydration started. The carbon dioxide reaction products, and the attendant effects, were examined through isothermal calorimetry, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom probe tomography (APT). Carbonate reaction products around 80 nm formed within 60 seconds of the CO2 gas injection. The carbonates were intermixed with silicate reaction products.