The interfacial strength between clayey soil and substructures is of great concern to design engineers, particularly because of the very low frictional resistance offered by the former. To enhance the interfacial performance of structures erected in clayey soil, an interfacial polymer coating is proposed. The fundamental physical phenomena contributing to the interfacial adhesion between bisphenol-A-based epoxy resin-coated clay and cement substrates were investigated at multiple length scales using various techniques. The presence of macropores and electron donor oxygen sites on the clay surface were the driving factors for a higher interfacial performance at dry of optimum moisture content for clay. Furthermore, the effect of particle gradation- initiated porosity on interfacial adhesive strength was studied by considering clay-sand mixtures. A broader spectrum of pores was quantified by Xenon (129) NMR spectroscopy, and intrinsic details such as the critical pore entry radius were revealed using MIP technique. Clay-sand mixture at 50:50 ratio exhibited the highest interfacial strength, thereby confirming the role of accessible pores for epoxy penetration. Thickness of interfacial zone was quantified by SEM-EDS measurements.
In the second stage, epoxy- coated hydrating cement surface was subjected to interfacial slant shear and pull-off tests, and the effect of different parameters such as w/c, polymer thickness, and curing age were studied in detail. Epoxy showed a higher interfacial strength at w/c of 0.3 with no variation in strength at early and later ages of curing. Finally, clay-epoxy-cement sandwich specimens were tested for interfacial adhesive strength by simple shearing. Molecular dynamics simulations conducted on sandwich models revealed a greater affinity of epoxy for calcium cations in cement than for the sodium interlayer cations of clay. The interfacial adhesive strength deteriorated in the presence of the hydrating cement surface because of the preferential adsorption of the hardener component of epoxy on the hydrating cement surface, as revealed by XPS measurements. This study provides a comprehensive understanding of various mechanisms across multiple length scales that is imperative for future studies involving new polymer coatings on novel construction materials.