The addition of carbon dioxide to ready-mix concrete can improve its performance through in-situ mineralization of calcium carbonate. A model system of an oil-well cement paste mixed with and without additions of CO2 gas was investigated to characterize the impacts of in-situ CO2 mineralization on the first 15 min of hydration at 4 and 24 °C. Impacts were examined through aqueous chemistry, total inorganic carbon (TIC), stable isotope 13C/12C fractionation, loss on ignition (LOI), and scanning electron microscopy (SEM). It was demonstrated that the calcium and silicon concentrations increase immediately after the CO2 input and declined within the first few minutes. TIC testing confirmed that the mineralization was rapid and brief with no evidence of increasing carbonate formation in the minutes after the CO2 input. Isotope fractionation confirmed that the carbonates formed were attributable to the mineralization of gaseous CO2. The loss on ignition testing showed that the CO2-activated system had greater mass gain (bound water and bound CO2) over 15 min of hydration than did the reference. SEM observations identified carbonate reaction products <200 nm immediately after the CO2 injection. Gel products were formed over the ensuing 15 min. The temperature of the testing had little impact other than to increase the concentrations of Ca and Si in solution and increase the amount of mineralized CO2, as associated with the higher solubility of CO2 in water as the temperature decreases.