Carbonation–Rehydration Pathway Reconstruction for Structurally Engineered CaCO ₃ –C–S–H Composites Enabling Durable Arsenic Immobilization

High-volume utilization of arsenic-containing gypsum (AsGP) in cementitious systems remains challenging, primarily due to slow strength development and unstable arsenic immobilization under hydration-dominated reaction pathways. Here, a carbonation–rehydration pathway reconstruction strategy is proposed for a binder containing 50% ordinary Portland cement (OPC) and 50% AsGP. This strategy actively reconfigures the Ca–S–Si equilibrium. Consequently, it directs the formation of a composite skeleton. Controlled precarbonation induces selective decalcification of initial hydrates and in situ precipitation of aragonite-type CaCO 3 (up to 15.74 wt %) while simultaneously driving extensive silicate polymerization. Subsequent rehydration reactivates residual silicate units and promotes secondary calcium silicate hydrate (C–S–H) growth to fill the carbonate framework, yielding a compressive strength of 66.8 MPa after 7 days of carbonation followed by 28 days of hydration under the synergistic regime. 29 Si NMR reveals a highly polymerized silicate network with Q 3 + Q 4 species exceeding 81%, which is preserved after rehydration. Nanoindentation further indicates the development of a homogenized CaCO 3 –C–S–H composite phase with elastic moduli mainly concentrated in the 45–70 GPa range. Arsenic leaching remains below the EPA limit under all curing regimes, with leachate concentrations of 2.53–2.78 mg/L. This stability results from synchronized chemical incorporation, nanoscale confinement, and micromechanical stabilization within the reconstructed skeleton. This study shows that regulating the reaction sequence, rather than relying solely on hydration, yields cementitious matrices with high AsGP loading and durable arsenic immobilization.