Hydration–carbonation cyclic curing: Synergistic enhancement of CO₂ sequestration and mechanical properties of Portland cement via sequence-controlled alternating cycles

The cement industry faces stringent carbon emission reduction pressure, and CO₂ sequestration via carbonation of cementitious materials is a promising low-carbon strategy. Existing studies predominantly focus on single-stage or simple two-stage curing regimes, while the synergistic mechanism of dynamic hydration–carbonation cyclic curing remains insufficiently elucidated. This study systematically compares carbonation-hydration and hydration-carbonation cyclic sequences for Portland cement paste. Phase evolution, CO₂ sequestration performance, and compressive strength are characterized using TG–DTG, XRD, and compressive strength tests. Results demonstrate that cyclic curing significantly outperforms single carbonation curing, and the hydration-initiated sequence yields consistently better performance than the carbonation-initiated counterpart. The optimal hydration-carbonation-hydration-carbonation (HCHC) regime achieves a CO₂ uptake of 7.26%, a carbonation degree of 13.7%, and a compressive strength of 74.3 MPa - 52.5% higher CO₂ sequestration than 28-day single carbonation curing, and 5.4% higher strength than conventional 28-day water curing. A conceptual cascade amplification framework is proposed to interpret the underlying synergy. Initial hydration establishes a portlandite calcium reservoir and an interconnected capillary pore network that facilitates CO₂ ingress and diffusion; alternating rehydration cycles regenerate reactive calcium sources and mitigate excessive C–S–H decalcification, while carbonation promotes pore filling and microstructural densification. This sequence-controlled curing strategy offers a viable technological pathway for developing high-performance, CO₂-sequestering cementitious materials, particularly for precast concrete applications.