The aqueduct of the Cairo Citadel is one of the significant Ayyubid structures in Cairo, which dates back to 914 AH - 1508 AD, it was built for the purpose of transporting the Nile water to the mountain citadel with a length of 2.2 km., Unfortunately, the aqueduct of the Cairo Citadel is exposed to damage and extinction for many natural and human reasons. As a first stage in the restoration process, it is necessary to assess the existing state and make the proper diagnosis of this significant monument, it is necessary to identify the type and condition of the building stones used in the construction process and to achieve this goal some stone samples were collected for testing of physical and mechanical properties, using polarizing microscope (PLM), scanning electron microscope coupled with Energy dispersive X-Ray unit (SEM-EDX), X-ray diffraction (XRD), and X-ray fluorescence (XRF).

The study showed that the type of stone used for construction is fossiliferous limestone (Biomicrite) very fine to fine-grained. Micro-fossils and fossil fragments are present in significant amount scattered in the very fine-grained matrix, a tiny amount of clay mineral are present. The stone samples show 11.68% water absorption, 2.02 g/cm3 Bulk density 23.62% apparent porosity and 9.65 MPa compressive strength. Stone samples suffers from black hard crust, micro-cracks, crystallization of salts, in addition to microbial infection with Halophilic fungi growth. Based on the present study results and studying the surrounding environmental conditions, an appropriate restoration plan can be developed.