Degradation and restoration of masonry walls of historical buildings: Fid-Bau Portal

Degradation and restoration of masonry walls of historical buildings

Collepardi, M.

Abstract Masonry walls have been tentatively classified according to all the possible combinations of the available original materials of historical buildings: lime, gypsum, hydraulic lime, stone, brick, etc. Fifty potential masonry walls of historical buildings have been assumed on this basis. The possible causes of chemical deterioration of masonries due to interactions among the original materials have been analysed: the potential reasons for chemical degradation are basically conductive to the alkali-amorphous silica reaction (in stones) and, above all, to sulphate salt reactions causing the formation of ettringite and/or thaumasite. In both cases, water plays a basic role in the course of the above-mentioned reactions: only in the presence of moisture-even occurring occasionally-have historical building masonries been subjected to significant deterioration. The recovery of historical buildings is a very delicate operation, since the materials employed in restoration work can interact negatively with some of the compounds that might be present in masonries. Owing to these reactions, the restoration operation can worsen the state of masonries. These reactions cause swelling, pop-out and falling of jointing and rendering mortars applied during the restoration work, as well as of cement grout injections for interior consolidation of structures. Again, ettringite and/or thaumasite may be found as deterioration products, provided that moisture is present in building masonries.