Achieving uniform carbonation sequestration in cement-based materials with a potassium glycinate CO₂ carrier

Carbonation curing is a promising route for CO₂ sequestration in cement-based materials; however, challenges remain in achieving uniform carbonation and avoiding over-carbonation at the surface. This study explores an internal and external synergistic carbonation using a potassium glycine (KG) solution as an internal CO₂ carrier. Cement pastes were prepared with pure water (PC), CO₂-free KG (KG), and CO₂-enriched KG (KC) and subjected to external carbonation for 4–12 h. Compared with PC, KC reduced 3 d compressive strength by 30.6% but increased 28 d strength by 11.0% under non-carbonation curing conditions. TG-DTG analysis revealed that KC contained poorly crystalline carbonate phases, whereas PC predominantly formed calcite. Although the surface CO₂ uptake of synergistically carbonated specimens was slightly reduced, TG results indicated a higher total sequestration of 17.66 kg·m⁻³ in 1 m³ of concrete without external carbonation, exceeding that of externally carbonated samples (11.27 kg·m⁻³) due to enhanced internal carbonation. Mercury intrusion porosimetry indicated that synergistic carbonation produced more uniform pore refinement, while external carbonation resulted in higher total carbonate mass but localized densification. These findings clarify the trade-off between CO₂ sequestration efficiency and mechanical performance and provide guidance for optimizing KG-assisted internal and external carbonation curing.