The role of environmental sulfur on degradation of ignimbrites of the Cathedral in Morelia, Mexico

We assess the damage on ignimbrite blocks used to build major architectonic monuments in Morelia, Mexico. Morelia belongs to the UNESCO Cultural Heritage and is located at the crossing of the Mexican Volcanic Belt, the Sierra Madre Mountains, the Coconuts and the Rivera plaques. The Cathedral and a beautiful Aqueduct are the main monuments built with blocks of ignimbrites. This study was performed on blocks of ignimbrites removed from the cathedral during a restoration campaign. We characterized the mechanical, chemical and physical properties of ignimbrite blocks from the four facades and the balustrade of the cathedral and compared them with the corresponding properties of “healthy” blocks from recently excavated nearby quarries. Techniques include mechanical testing with a sclerometer, Scanning Electron Microscopy and Energy Dispersive Spectroscopy, Petrography, sulfur and carbon analyses by combustion technique, X Ray fluorescence and X Ray diffraction. The blocks from nearby quarries have lower values of mechanical strengths than those used to built the cathedral, therefore, the former blocks cannot be used to replace particularly damaged blocks. We found a clear correlation between the contents of environmental sulfur and the degradation of the mechanical strength of the ignimbrites. Since the main sources of environmental sulfur in the city are motor vehicles and a paper factory in the southwest from where the main winds come from, we conclude that the damage is from anthropogenic origin.