Nano CaCO3 seeding for improving properties of limestone calcined clay cement through in-situ carbonation

This study proposes an effective strategy to simultaneously enhance the mechanical performance and CO 2 sequestration capacity of limestone calcined clay cement (LC 3 ) incorporating low-grade calcined clay through aqueous carbonation. 25% of the cement fraction in LC 3 was subjected to aqueous carbonation for 10 to 40 mins with a water-to-solid ratio of 2.0, leading to the in-situ precipitation of nano-sized CaCO 3 . A maximum CO 2 uptake of 15.78% was achieved after 40 min of carbonation. After mixing with the remaining materials of the LC3 formulation, the synergistic dilution and nucleation effects of in-situ nano CaCO 3 promoted the hydration of silicate and aluminate phases, thereby refining the pore structure of LC 3 . At 3 days, the fraction of fine capillary pores (10-50 nm) increased remarkably, reaching 54% and 60% after 30 and 40 min of carbonation, respectively, and this refinement was largely preserved at 28 days. Consequently, the 28-day compressive strength of LC 3 mortars increased by 34.93% and 32.07% at carbonation durations of 30 and 40 min, respectively, compared with the control group. However, substantial consumption of portlandite during pre-carbonation constrained the later development of carboaluminate phases, which highlights a trade-off between enhanced early hydration and limited availability of secondary hydration products. These findings offer new insights into the role of in-situ CaCO 3 precipitation in modifying hydration and pore structure, demonstrating that aqueous carbonation is an effective route to enhance LC 3 performance while facilitating CO 2 sequestration.