Characterization of C–S–H formed in coupled CO₂–water cured Portland cement pastes

The coupled CO₂–water curing technique has been used for curing Portland cement-based materials, enabling the cement/concrete products with rapid strength development and lower carbon footprints. However, there is still a lack of convincing understanding on the mechanism of the accelerated carbonation reactions of cement. This current work mainly focused on characterizing the C–S–H gel formed in the bulk cement paste prepared with a low water to cement ratio (w/c = 0.18) subjected to a coupled CO₂–water curing regime, by using solid state ²⁹Si NMR and infrared spectroscopy together with other common tools. The results indicated that the CO₂ curing process led to the formation of C–S–H gel containing more Q² species than that in the corresponding hydrated pastes. Prolonged CO₂ curing incorporated more Al tetrahedra into the C–S–H gel. Meanwhile, both the amorphous carbonates and calcite were formed during the accelerated carbonation reactions, and the calcite crystals could serve as nuclei to accelerate the cement hydration at the early age. Excessive CO₂ curing resulted in a deficiency of lime in solution, yielding a structurally modified C–S–H with the absence of interlayered Ca(OH)₂, and longer silicate chains as well as a higher polymerization degree when compared to the C–S–H formed in the normally hydrated cement samples.