Effect of acidic surface treatment of red mud on mechanical interfacial properties of epoxy/red mud nanocomposites

In this work, red mud (RM) was chemically modified by 0.1, 1, and 5 M H₃PO₄ solution to prepare epoxy/RM nanocomposites. The effect of chemical treatment on a RM surface was studied in terms of pH, acid-base values, N₂/77 K gas adsorption, equilibrium spreading pressure (π_e), and surface free energy. The mechanical interfacial properties of epoxy/RM nanocomposites was measured in the context of critical stress intensity factor (K_IC) and critical strain energy release rate (G_IC) measurements. From the experimental results, the acidic surface treatment led to a modification of the RM surface properties, such as the surface acid values, porosity, specific surface area, π_e, and London dispersive component (γ_S^L) of surface free energy of the treated RM as an increase of the treatment concentration. In the fracture toughness (K_IC and G_IC) measurements, the mechanical interfacial properties of epoxy/treated RM nanocomposites were intimately correlated with the improvement of interfacial adhesion between the RM surface and a matrix and the increase of γ_S^L of surface free energy of the RM due to the development of S_BET or porosity of RM surfaces.