Modification of early-age microstructure and enhanced mechanical properties of cement paste via injecting low-dose CO2 at fresh stage

Most previous studies introduce high dose CO2 (> 0.05 %) into fresh cement paste during mixing (CO2-mixing) to sequester CO2. However, this strategy results in poor workability and lower long-term strength. This study attempts to use a lower CO2-mixing dose (0.017-0.083 wt.parts per thousand) to improve the performance of fresh and hardened cement pastes. The planar distribution, morphology, and evolution of the reaction products over time were investigated. The results showed that all the lower CO2-mixing fresh pastes displayed a good workability, and the compressive strength was increased by 0.3-21.7 % at 1-28 days. The enhancement in compressive strength was attributed to the formation of monocarbonate (Mc), deriving from the reaction of nano-CaCO3 covering the clinkers with the aluminum phase, which strengthened the interface bond between the clinker and the new hydration product. Moreover, the generated nano-CaCO3 also resulted in local high supersaturation on the clinker surface and promoted the formation of nano-Ca(OH)(2) embedded in the calcium silicate hydrate (C-S-H). As a result, the compactness of the hydration products was enhanced. The refinement in the microstructures of the low-dose CO2-mixed samples was translated into enhanced mechanical properties.