The impacts of in-situ carbonate seeding on the early hydration of tricalcium silicate

The industrial injection of carbon dioxide into ready mixed concrete during mixing and batching can produce a measurable increase in hydration and a significant compressive strength increase. Physiochemical aspects of the mechanism were investigated through the analysis of a model tricalcium silicate system activated with a carbon dioxide addition immediately after hydration started. The attendant effects were examined through isothermal calorimetry, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Carbonate reaction products around 70 nm were observed to have formed within 60 s of the CO2 gas injection. Within 10 min the carbonates have been covered by new hydration products. The initial product formed appears to be an amorphous or poorly crystalline calcium carbonate. The overall reaction over 24 h was the same for the hydrated and CO2-activated cases although the latter instance included the formation of carbonate reaction products.