Lime-based mortar interlayer plays an important role in structural and architectural construction. Lime mortar undergoes repetitive changes in temperature when it is exposed to its service environment. Change in temperature has a significant effect on the thermal deformation of cement mortar and even affects directly the durability of century decade monuments and structure under extreme regions. This paper deals with thermal deformation of lime mortar, which is the prime constituent of cement and the oldest construction material by measuring its deformed geometrical configuration. In the first part, it is aimed to investigate the thermal deformation of mortar cube, its changes in length and mass of the specimens continuously measured during the heating and cooling cycles within the range of temperatures of −20 to 70 °C. The second part of this paper aims to investigate the pore formation and pore size distrubution in lime and lime mortar sample using optical microscopic images. Although a careful visual inspection can provide very good accuracy information, it has inherent limitations in assessing the structural integrity, and also, visual inspection can provide limited information to a damaged facility. In this paper, a detailed pore distribution analysis has done using digital image processing tool for better interpretation of thermal damages in lime mortar. The above-proposed tool can be used as aided tool for retrofitting and conservation of many architectural heritages. © Springer Nature Singapore Pte Ltd. 2020.