Abstract: This study aims to investigate the influence of CO₂-mixing dose (mass fractions of 0.3%, 0.6%, and 0.9%) and prolonged mixing time on the fresh and hardened properties of cement pastes. The CO₂-mixing can act as coagulant in fresh cement mixtures, resulting in a significant reduction in workability associated with the formation of a rich calcium carbonate network on the surface of cement particles. The CO₂-mixing cement pastes were found to be much stiffer and more difficult to handle, place, and compact than the control mixture, which had a negative effect on the mechanical strength performance of the hardened pastes. However, prolonging the mixing time for 1 min (immediately after CO₂-mixing) can effectively improve the workability (by ~53%–85%) by breaking up the flocculation network of deposited calcium carbonates. As a result, the presence of detached calcium carbonate accelerated early cement hydration and densified the microstructure; this improved early-age compressive strength by ~6%–32%, depending on the CO₂-mixing dose used. Therefore, it seems that the CO₂-mixing dose should be controlled at ≤0.6% with the mixing time prolonged in order to attain satisfactory workability and mechanical strength.

Key words: CO₂-mixing; Calcium carbonate; Early cement hydration; Flowability; Microstructure

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