Study on mechanical properties, hydration mechanism and carbon emission of phosphogypsum-based sustainable cementitious materials

Abstract

The excessive stockpiling of phosphogypsum (PG), rich in sulfates, phosphorus, fluorine, and heavy metals, threatens environmental safety if not properly handled. To address this issue, this study developed PG-based sustainable cementitious materials using PG, GBFS, NaOH, and Na₂SO₄. Analytical techniques such as XRD, SEM, and ICP-MS were employed to investigate its strength development mechanisms, hydration characteristics, microstructure, and leaching toxicity. The utilization rate of phosphogypsum is expected to reach 50 %, and the compressive strength is anticipated to exceed 40 MPa. The findings demonstrate that the optimal formulation for cementitious materials consists of 50 % PG, 47 % GBFS, 3 % NaOH and 3 % Na₂SO₄ (external). The compressive strengths at 3, 7, and 28 days are 2.27, 20.46, and 43.54 MPa, respectively, with a PG utilization efficiency of 50 %. An increase in PG content extends the second exothermic peak of the cementitious materials system. Concurrently, NaOH accelerates the hydration process, partially shortening the induction period and promoting the formation of hydration products such as AFt, and C–S–H, thereby enhancing the strength of the cementitious materials. In addition, the leaching concentrations of F- and PO₄³- in PG were 148.23 mg/L and 298.24 mg/L, respectively, which were reduced to 2.364–4.410 mg/L and 0.052–0.220 mg/L in cementitious materials, respectively, and the other pollutants were below the limits specified in the relevant standards. The use of PG-based sustainable cementitious materials significantly reduces the carbon emission index. Under the optimized proportioning, the carbon emission per unit is reduced to 46.8995 kg/m³, which is 394.1005 kg/m³ less than that of ordinary silicate cement, with obvious advantages in energy saving and emission reduction. Due to the presence of gypsum and AFt, phosphogypsum sustainable cementitious material systems have lower shrinkage compared to cement.