Problem Statement:
Rabʿ-e Rashidi is one of the most significant scientific, cultural, and architectural complexes of the Ilkhanid period in Tabriz, and its Great Tower—constructed in the post‑Ilkhanid era—possesses outstanding historical and identity-related values. Over time, the tower has suffered structural damage due to environmental factors as well as human interventions, particularly the application of incompatible modern mortars. Because of their physical and chemical incompatibility with historic materials, these new mortars have led to increased cracking and accelerated material decay, underscoring the necessity of an accurate characterization of the original historic mortar.

Research Objective:
This study aims to identify the composition, properties, and mechanical behavior of the original foundation mortar of the southern Great Tower at Rabʿ-e Rashidi, in order to provide a scientific and sustainable basis for conservation interventions on this historic structure.

Methodology:
Field sampling was carried out from various parts of the tower foundation. The samples were examined using laboratory tests including XRF, XRD, SEM–EDS, Raman spectroscopy, soluble and insoluble salt tests, moisture measurements, and particle-size (gradation) analysis, along with the determination of mechanical properties.

Conclusion:
The results indicate that the studied historic mortar is lime-based and incorporates coarse siliceous aggregates together with a proportion of wood ash. These constituents, through effective chemical interactions, have enhanced the mortar’s strength, cohesion, and long-term durability. The presence of charcoal/wood ash plays a key role in activating pozzolanic reactions, thereby improving mechanical performance. The measured compressive strength of the samples ranged from 2.5 to 3 MPa. These findings can serve as a robust scientific basis for the design and production of compatible repair mortars for similar historic buildings.