New insights into value-added application of phosphogypsum in asphalt mixture through chemical stabilization of polymeric methylene diphenyl diisocyanate

Abstract

Phosphogypsum (PhG), a high-volume industrial byproduct, has considerable potential for use in asphalt mixtures. Its utilization is of great significance for mitigating the excessive consumption of natural mineral fillers. However, the high water absorption and wet expansion of PhG significantly compromise the moisture-induced resistance and durability of asphalt mixtures, thereby limiting its broader engineering application. In this study, PhG was used as a full replacement for conventional mineral filler, and polymeric methylene diphenyl diisocyanate (PMDI) was introduced as an asphalt binder modifier to prepare PhG asphalt mixtures (PhGAM). The fatigue and freeze-thaw (F-T) resistance of PhGAM were evaluated using semicircular bending (SCB) fatigue and F-T cycle tests, respectively. After simulating thermo-oxidative aging, the long-term service performance of PhGAM was systematically assessed through wheel tracking, low-temperature indirect tensile, Marshall immersion, and F-T cycling tests. The SCB test results demonstrated that 4% PMDI, by weight of asphalt binder, can markedly improve the fatigue life of PhGAM, whereas further increasing PMDI content provides limited additional benefit. F-T cycle test results indicated that the incorporation of PMDI can notably enhance the indirect tensile strength and water damage resistance of PhGAM, enabling it to withstand at least four cycles, whereas the unmodified PhGAM/PMDI0 fails after only one. After long-term aging, aged PhGAM/PMDI mixtures exhibit significantly higher resistances to permanent deformation at elevated temperature compared to unaged ones. The dynamic stability of aged PhGAM/PMDI4 reaches 5736 passes/mm, compared to 2921 passes/mm for aged PhGAM/PMDI0. Furthermore, aged PhGAM/PMDI4 still exhibits better resistance to low-temperature cracking and moisture-induced damage, especially compared to aged PhGAM/PMDI0. Overall, a 4 % PMDI content is optimally recommended for blending with the asphalt binder for enhancing the engineering performance of PhGAM and facilitating the high-value utilization of PhG in the construction of more durable asphalt pavement.