Thaumasite: evidence for incorrect intervention in masonry restoration

Thaumasite, as well as ettringite, are compounds which are increasingly found as deterioration products of cementitious materials subjected to sulfate attack. Thaumasite, and especially ettringite, have been abundantly reported in relation to concrete deterioration as well as, more recently, to the deterioration of cementitious mortars for masonry and for plasters. In particular, the problem appears serious in the field of repair of historical buildings, where the cementitious mortar can easily deteriorate just because of the formation of ettringite and thaumasite. However, although thaumasite is responsible for deterioration, in most cases, it may not be detected since it can be partially or almost completely removed by atmospheric agents. Many causes can be responsible for the presence of sulfates in masonry. At the same time, mortar and plaster are, in most cases, sources of calcium carbonate. Moreover, masonry is typically a porous material, which can be easily permeated by water, either rising groundwater or falling rainwater. Therefore, when cement based materials, which are in turn sources of calcium aluminates and calcium silicates, are used as binders, all of the ingredients necessary to cause thaumasite formation are present. Consequently, a compatibility issue emerges, which if not kept into proper account, will lead sooner or later to ineffective intervention.