Phosphogypsum as a warm mix asphalt additive: Preparation and performance

Phosphogypsum is a typical industrial solid waste that presents significant environmental and economic challenges worldwide. This study aims to exploit the crystalline water in phosphogypsum to develop a novel warm-mix asphalt additive, thereby achieving high-value utilization and addressing the variability in the performance as a warm-mix additive. Initially, the atmospheric acidification method was employed to synthesize a phosphogypsum warm-mix additive (PWMA) with high purity and minimal impurities. Subsequently, the PWMA was characterized through X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TG). Finally, the laboratory-prepared WMA mixture containing PWMA and traditional hot-mix asphalt (HMA) mixture underwent comprehensive engineering performance tests, including moisture susceptibility test, wheel-tracking test, beam bending test, and four-point bending test. The experimental results demonstrated that the atmospheric acidification method effectively reduced the impurity content in phosphogypsum, yielding a PWMA with calcium sulfate dihydrate purity exceeding 99 %. The infrared absorption peaks at 3390 cm⁻¹ and 1620 cm⁻¹ correspond to hydroxyl (O-H) vibrations, indicating the presence of crystalline water molecules in PWMA and confirming its water-releasing capability. The PWMA exhibited its initial weight loss stage between 50–100 ℃, with a 0.29 % mass loss, attributed to the adsorption of atmospheric moisture containing free water molecules. The second weight loss stage commenced at 100 ℃, with a 20.7 % mass loss at 150 ℃, attributed to crystalline water release. PWMA-added WMA mixtures demonstrated overall superior performance compared to HMA, with slightly reduced low-temperature and water stability, yet remaining comparable and satisfactory. This study confirms the viability of phosphogypsum as an effective and sustainable WMA additive.