Synergistic Effects of Polyphosphoric Acid and Montmorillonite Nanoclay on the Performance Properties of Asphalt Binder

Montmorillonite nanoclay has been used to improve asphalt binder stiffness, rutting resistance, and oxidation stability, but its moisture-induced swelling limits durability. This study evaluates polyphosphoric acid as a complementary additive to reduce swelling and enhance nanoclay performance. The acid intercalates into clay interlayers, creating a hydrophobic barrier against water ingress. Three binders were tested: neat PG 64 22, nanoclay modified, and a hybrid binder with both nanoclay and polyphosphoric acid. Performance was assessed using PG grading, multiple stress creep recovery, frequency sweep, swelling, and extended bending beam rheometer tests. The hybrid binder reduced swelling by nearly fourfold relative to nanoclay alone and showed improved rutting resistance and aging durability after thermal conditioning. However, both nanoclay and hybrid binders displayed greater physical hardening at low temperatures, indicating a trade-off between high-temperature durability and low-temperature flexibility. Overall, the hybrid system provides enhanced moisture protection and aging resistance while highlighting limitations in cold climates.