Instability Behaviour of Aggregate Pyrites in Cement Mortars of Old Plaster Coats

The paper describes the potentially harmful behaviour of aggregates that have appeared to be the main reason for damage observed in plaster and masonry of inner and outer walls of some old buildings. The evident signs of alterations, such as cracking, spalling and pop-out phenomena may be ascribed to the deleterious action of instable aggregates mixed in the mortar at the time of construction. By applying an extensive analytical procedure, using either simple traditional wet chemistry methods and common physicalchemical instrumental techniques, it has been possible to reconstruct the pathway that, starting from the precursor, had led to the actual degradation state. Common forms of ferrous sulphide as pyrites, probably embedded in the original aggregate extracted in a quarry near the location of the building, are most likely to be responsible for the observed staining and pop-out formations at the surfaces. The mechanism and the pathway by means the dangerous aggregate led to the observed damage have also been explained. The objective of this case study is to prevent further damages, plan correct procedures of repair and avoid the use of incompatible products.