Mechanistic investigation of the bond-slip behavior of brick-mortar interfaces: From mortar decay to interfacial deterioration under sulfate attack

The bond-slip behavior at the brick-mortar interface plays a pivotal role in the long-term structural integrity of ancient masonry pagodas. Sticky rice paper pulp mortar (SRPPM), a key traditional material, is susceptible to environmental erosion. However, the quantitative link between its degradation and the resultant interfacial mechanical deterioration remains poorly understood. This study investigates the degradation of the brick-mortar interfacial shear behavior under accelerated sulfate attack. Material-level tests, including uniaxial compression and Scanning Electron Microscopy (SEM), were combined with interface shear tests to reveal that interface deterioration of is governed by SRPPM degradation. The interface bond strength exhibited a non-monotonic evolution with cycles. It initial decrease (0–5 cycles) as pore enlargement and local stress concentration reduced interparticle interlocking. A partial recovery was observed during 6–12 cycles, corresponding to the refinement of crystals and local pore-filling, which enhanced local matrix stiffness and chemical adhesion at the interface. Subsequently, final strength loss (13–18 cycles) occurred due to progressive microcrack initiation and propagation under cyclic expansion stress. Besides, the interfacial friction coefficient decreased steadily, with a total reduction of 31.2% after 18 cycles. Energy dissipation analysis showed that the interface exhibited optimal elastic deformation capacity under a horizontal stress of 0.4 MPa. Based on the experimental data, a constitutive model for the bond-slip behavior of the interface degradation is developed. Additionally, a predictive model was established for the relative compressive strength of the SRPPM itself under sulfate attack. These findings provide a mechanistic understanding and a quantitative framework for modeling long-term interfacial degradation in historical masonry, supporting evidence-based conservation strategies.