Performance evaluation of mixing carbonated reactive magnesia slurry in Portland cement pastes

This study uses carbonated reactive magnesia (RM) as a partial cement replacement to improve the mechanical properties of blended cement pastes. To achieve more complete and homogeneous carbonation, 30 wt% RM was first mixed at a fixed 1.67 water-RM ratio for 0-90 min (aqueous carbonation). After this, the mixture was mixed with the remaining 70 wt% ordinary Portland cement (OPC) to achieve a final water-binder ratio of 0.5. During the first 10 min of aqueous carbonation, the pH of the RM slurry dropped significantly. After 40 min, needle-like nesquehonite was detected and predominated in the system at 90 min. Mixing the carbonated RM slurries with OPC (RMP cement paste) created more nano-calcite, which covered the nesquehonite surface and served as nucleation sites to accelerate the hydration of RMP cement paste. Consequently, the setting time and fluidity of RMP cement pastes decreased with carbonation time of RM slurries used. Upon one day of hydration, nesquehonite, calcite, and vaterite coexisted in the paste. At 28 days, vaterite gradually transformed into calcite and reacted with aluminum phases to form mono-carboaluminate, densifying the microstructure and boosting the 28day strength by 194 % compared to the control paste.