Mechanical Behavior of Refractory Mortar Joints

Refractory mortar joints are among the most mechanically compliant and damage-prone constituents of masonry in high-temperature industrial vessels. Compared with refractory bricks, mortar joints generally exhibit lower strength and stiffness and are more susceptible to nonlinear deformation under various loadings, thereby acting as predefined locations for damage initiation and deformation localization in furnace linings. Despite their critical role in controlling structural integrity and service reliability, systematic investigations of refractory mortar joints thermomechanical behavior at elevated temperatures remain scarce. To address this gap, this study develops a comprehensive experimental¿numerical framework to characterize the shear, compressive, and creep responses of fireclay mortar joints with sandwich structures at elevated temperatures, elucidate their microstructural mechanisms, and investigate their influence on masonry performance from the joint scale to the mesoscale. An improved high-temperature slant-shear test with a movable testing plate and optimized specimen preparation enables reliable determination of temperature-dependent shear behavior and Mohr¿Coulomb parameters of mortar joints, which are interpreted based on phase evolution and microstructural changes governed by the competition between sintering and liquid-phase formation. The compressive response of mortar joints at room temperature is received by removing system compliance and substrate deformation from sandwich specimen, while microstructural observations demonstrate that substrate water absorption strongly affects joint microstructure and strength. High-temperature creep behavior is further characterized using pure alumina brick sandwich specimens, allowing isolated identification of mortar joint creep behavior; the resulting Norton¿Bailey parameters successfully reproduce creep curves, and combined with brick creep parameters, predict the behavior of fireclay brick sandwich specimens. Finally, a mesoscale Representative Volume Element model is used to quantify how mortar joint creep alters the macroscopic creep of masonry. Mortar creep induces lateral tensile stresses in adjacent bricks, significantly accelerating their creep deformation and amplifying overall masonry creep strain. When creep is assigned to either mortar joints or bricks alone, the macroscopic response exhibits pronounced orthotropy, which largely disappears when both constituents creep simultaneously. Through this integrated approach, the intrinsic behavior of mortar joints and their contribution to masonry deformation under high-temperature conditions are systematically clarified.