<p>The preservation of heritage structures has been a critical task globally, but using repairs without determining their compatibility with the existing materials causes the structures to deteriorate. Therefore, the prime objective of this study is to assess the bonding performance of old mortar prevalent in Indian historical structures with lime sand mortar and cement sand mortar as repair materials. Lime surkhi, lime sand, mud sand, mud surkhi, cement sand and cement surkhi mortar have been selected as substrates to assess their bonding compatibility with lime sand and cement sand mortar. The composite specimens have been tested to evaluate their performance under shear and tension by split, bi-surface and slant shear strength tests. The results suggest that the slant shear strength test and bi-surface strength test are suitable tests for assessing the compatibility of the lime surkhi, lime sand, mud sand, mud surkhi, cement sand and cement surkhi mortars with lime sand and cement sand repair mortars. The materials exhibiting adhesion repair are suitable for repair purposes. It is also inferred from the present study that the repairs should be performed by preparing the surface so that the substrate interface forms a 30° angle with the repair interface for a stronger bond. The experimental findings are validated with numerical simulations using Ansys software, and strong coherence is established between the experimental and simulation results.</p>