Development of high-performance phosphogypsum-based cementitious materials through CO2-assisted alkali activation

Phosphogypsum (PG)-based cementitious materials often suffer from low strength and poor water resistance, especially when incorporating a high volume of PG. This issue arises mainly from the high solubility of CaSO₄·2H₂O. This study develops a novel CO₂-assisted alkali activation method for phosphogypsum-based cementitious materials (HPCM), improving compressive strength to 49.3 MPa (+50.83 %) and enhancing water resistance (softening coefficient: 0.97). The presence of alkali created a favorable environment for the carbonation of PG, forming a core-shell structure with PG as the core and calcite and C-S-H forming the inner layer and outer shell, respectively. This core-shell structure effectively mitigated sulfate leaching during alkali activation while enhancing the chemical bonding within the matrix. Furthermore, the SO₄²⁻ released from PG reacted with the dissolved Al phase to form AFt, which, combined with C-(N)-A-S-H, further strengthened the bond between PG and the matrix. This approach integrates carbonation and alkali activation in preparing PG-based cementitious materials, providing a synergistic method for the resource utilization of PG. The CO₂-assisted alkali activation of PG significantly reduced the leaching of hazardous elements, with P and F concentrations decreasing from 146.5 mg/L and 87.63 mg/L in raw PG to approximately 1 mg/L in HPCM. Heavy metals were effectively immobilized within AFt and C-A-S-H phases, demonstrating the potential of HPCM for safe and sustainable utilization of PG in construction.