Degradation and conservation of clay-containing stone: A review

Significant progress in the understanding of the different degradation processes affecting building and ornamental stone has taken place in recent decades. However, some weathering phenomena still are not fully understood, which hampers the development of effective stone conservation treatments. One of these cases, stone damage associated with the presence of swelling clays, is reviewed here. Swelling clays cause damage when subjected to wetting/drying cycles, commonly resulting in stone scaling, flaking, spalling, exfoliation and/or delamination. Examples of this damage process and the mechanisms proposed for its development, as well as the key factors that control its origin, evolution and severity are presented and discussed. Alternative or complementary processes that may also lead to stone damage due to expansion/contraction cycles are also discussed. Finally, conservation treatments applied to halt or minimize clay swelling damage are reviewed. It is shown that traditional conservation materials such as polymers and alkoxysilanes applied to protect and/or consolidate damaged clay-containing stones typically have little effectiveness. Alternative treatments based on the application of clay swelling inhibitors, and a novel green conservation approach based on bacterially-induced formation of CaCO3 cement and associated production of exopolymeric substances, recently proposed to prevent/ control clay-related damage of tuff stone at the ancient Maya site of Copan, are presented and discussed.