The Role of Air Pollutants and Sewage Waste in Acceleration of Degradation of the Islamic Cultural Heritage of Cairo

Abstract

The impact of gaseous CO2 and SO2 attains additional importance when examining the processes of alteration and decay in historical buildings made of calcareous stones. We selected the Al-Ghuri complex (Cairo, Egypt) for the study of black crust formation on stone building surfaces due to the presence of carbonaceous materials and sulfatation via CaCO3 conversion to gypsum (CaSO4.2H2O). Under dry deposition, the breakdown of calcareous stones depends on the relative humidity and the presence of catalysts, e.g., O3 and oxides of Cu, Fe and Mn. Depth profile analyses indicated the presence of sulfates to depths of 100 to 1200mum below the stone surface. Sulfatation was usually formed on the stone surface, and sometimes penetrated, the outer surface through micro-fractures and pores. NaCl crystals (in cubic form) were precipitated at stone surfaces and pores due to the presence of sewage water via capillary action. Polyhalite [K2Ca2Mg(SO4)4.2H(2)O] and epsomite (MgSO4.7H(2)O) were developed as minute crystals up to 12mum. This study reveals the influence of sulfatation and salt formation and microrganismal activities on accelerating the rate of monumental stone degradation. Such processes involve reaction of stone with the pollutants, transformation of CaCO3 to CaSO4.2H(2)O, and precipitation of NaCl, which under appropriate conditions crystallizes within pores and micro-cavities of stones. One possible consequence is the rupture and spilling of the upper surface of the stone due to crystal pressure.