Prospective Study 2: El-Merdani Mosque, Cairo, Egypt

Prepared by: Aachen Univ. of Technology Geological Institute, Working group "Natural stones and weathering"

conclusions

El-Merdani Mosque in Cairo was selected for investigation within the frame of the Prospective Study 2 (PS2) for reasons of its historical importance, characteristic stone materials and considerable stone damages.

El-Merdani Mosque was built in the XIV century as one of the finest examples of Islamic architecture in Cairo. Porous limestones from quarries near Cairo were used for construction. These limestones represent the main construction material of monuments in historical Cairo. The type of porous limestone has been used in construction of numerous historical monuments in the whole Mediterranean area.

Bad condition of El-Merdani Mosque was started at the end of the XIX century. Restoration works like reconstruction, structural reinforcement and renovation of walls, were carried out in the period 1896-1903 by the Arab Antiquities Conservation Committee. Today, El-Merdani Mosque is again in need of intervention. Especially, damages at the lower parts of the monuments are striking. This concerns walls at the outer façades and walls in the interior of the monument in the same way. Comparable situation can be found at many other monuments in historical Cairo. Thus, the PS2 studies at the selected pilot monument have contributed to assess a very characteristic damage situation at ashlar monuments in the center of Cairo.

Interdisciplinary studies on stone degradation due to weathering at the mosque were carried out by the project partners from Egypt, Germany and France by means of joint application of modern, complementary in situ methods and analytical procedures in laboratory. In situ investigations were executed in the frame of three field campaigns of the project partners. Ins situ investigations and laboratory analyses were aimed at characterization of stone materials and at characterization and quantification of weathering forms, weathering products and weathering profiles which jointly characterize the state of degradation due to weathering and which provide information on weathering factors and weathering processes. Subsurface conditions at El-Merdani Mosque were studied and considered additionally in order of comprehensive damage diagnosis.

The studies on stone materials have shown a significant variation of petrographic and mechanical properties. The results reveal, that the type of porous limestones used in historical Cairo since ancient times until today comprises a considerable number of very different subtypes. The results obtained from the PS2 studies mean an important contribution to assessment of the range of limestones at monuments in Cairo and in nearby quarries and they have improved awareness of this important objective from scientific sand practical point of view regarding differentiated characterization, interpretation and rating of stone degradation and quality management of monument preservation measures. Continuation of systematic survey and analysis of limestones in Cairo is recommended.

Monument mapping was applied as a well-experienced and internationally accepted non-destructive method for precise registration, documentation and evaluation of weathering forms, which represent phenomenological results of weathering processes. As basis for monument mapping, a detained classification scheme of weathering forms was worked out, tailored to objective and reproducible applicability at all Cairo historical ashlar monuments constructed with limestones. Damage categories and damage indices were established as practical tools for quantitative rating of damages at these Cairo historical monuments. a correlation scheme "weathering forms — damaged categories" was developed in the frame of PS2 for all historical limestone monuments in historical Cairo. The consequent approach "weathering forms — damage categories — damage indices" was applied in the frame of PS2 at an Egyptian monument for the first time. This approach can now be transferred to all Cairo historical monuments constructed with limestones.

Well-directed in situ measuring procedures and laboratory analysis by means of modern analytical procedures have provided important additional quantitative information on state of degradation, especially as concerns weathering profiles and weathering products. numeric structure-modeling analysis was applied as modern tool for assessment of structural stability.

The interdisciplinary co-operation and the consistent methodological concept have guaranteed information on all scales of stone degradation ranging from nanoscale (<mm) to microscale (mm to cm), mesoscale (cm to m) and macroscale (façades, entire monument).

For the studied lower parts of El-Merdani Mosque remarkable damages were stated. The damages are characterized by:

- loss of stone material - recession of the stone surface amounts to 10 cm at maximum;

- recent detachment of stone material - many different types, often high intensities,

- deposits - soiling end especially salt deposits in form of efflorescences / subflorescences and salt crusts,

- structural discontinuities/instabilities - fissures, break out, displacements.

Results of in situ measurements and laboratory analyses confirmed considerable stone degradation, i.e. textural weakness of the stone material and loss of cohesion in the surface zone of the walls. For all three-investigation areas high proportion of severe or even very severe damages were stated. The calculated damage indices quantify and advanced state of stone degradation and indicate need of intervention.

The results obtained fro El-Merdani Mosque reveal a significant example of weathering damages at limestones due to salt weathering. Different sources of the salts can be distinguished:

1. Increase of air pollution in Cairo as consequence of the rapid expansion of the city has increased significantly. This results in deposition of pollutants from the atmosphere at the stone surface of monuments with subsequent salt formation, especially gypsum, on the stone surface or in the pore space of the limestones near to the surface.
2. The water table (ground water, subsoil water) has significantly risen during the last decades. It can be observed, that in extreme cases the water table has reached the ground floor of monuments. Insufficient or defect sewage systems have caused increasingly pollution of the water. Salt solutions from subsurface intrude into the walls of the monuments and salts are precipitated, especially halite, on the stone surface or in the pore space of the limestones near to the surface.
3. Lime mortar and plaster — the latter especially used at the walls in the interior of the mosque — must be considered as further sources of salts.

Detailed quantitative information on air pollution, subsurface conditions and subsurface water quality in the historical center of Cairo — which could not be obtained in the limited time frame of PS2 — are desired for further improvement of knowledge and for environmental management in the context with monument preservation activities.

Cycles of salt accumulation and detachment of stone material resulting in loss material are characteristic for the studied parts at El-Merdani Mosque. The susceptibility of the limestones to degradation increases with number of such cycles and, thus, weathering progression accelerates. Considering the age of the monument and restoration works a century ago, weathering progression must be rated as high.

A clear correlation between extent of salt load and degree of damages was stated. Most severe damages were found at that zone of the walls that corresponds to main level of salt precipitation from ascending salt-polluted ground water/subsurface water. It was found that salt loading of the limestones y salts which origin from rising humidity is higher and, therefore, has initiated and still initiates more harmful salt weathering processes at the lower parts of the monument than salt loading by salts due to deposition of pollutants from the atmosphere.

Within the frame of PS2 a consistent methodological approach to the scientific damage diagnosis has been demonstrated. The interdisciplinary co-operation with consequent synthesis of results has improved knowledge of interactions between the stone materials and environmental influences at the monuments in historical Cairo. Beyond this, the co-operation was very fruitful for the project partners as concerns of scientific and practical knowledge, investigation strategies and complementation of results.

Thje results represent a very good basis for future studies on stone degradation at monuments in historical Cairo. At the same time they mean an important contribution to decision, planning, execution and control of effective and economic monument preservation measures.

Preservation measures such as elimination/reduction of subsurface water influences, desalinization, stone repair, fixation of loose stone material, mechanical stabilization, structural reinforcement and stone replacement are under consideration. Environmental management and rehabilitation aspects will have to be considered additionally.