The use of cement-based mortars for the repair of ancient masonry joints was quite common in the past and can account for a significant role in masonry decay, especially when highly porous building materials are present (bricks, sandstone, etc.). Mortar-induced degradation mechanisms in ancient masonry walls are quite complex and worthy of investigation, in order to properly assess the decay causes and to select suitable conservation strategies (removal, cleaning, substitution with compatible repair mortars, etc.). In the present paper, the case of the 17th century main portal of the Pio Palace in Carpi (Modena, Italy), where an outstanding differential decay between mortars and bricks is present, is discussed. Both bricks and mortars were characterized and the actual decay causes were detected, also in connection with the analysis of the environment surrounding the building and the historical evolution of the palace. The different microstructure of the original and repair materials was found to be responsible not only for different mechanical behaviour, but also for a different response to salt crystallisation, particularly intense in the palace due to the capillary rise of underground polluted water.
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