Calcium and magnesium alkoxides for conservation treatment of stone and wood in built heritage

In the frame of the EU-project NANOMATCH new consolidants based on metal alkoxides were developed and tested. New compounds were prepared with different synthetic pathways to overcome the oligomerization issue, which can strongly influence solubility and therefore application of alkoxides as conservation materials for built heritage. Calcium alkoxides react in presence of humidity and carbon dioxide to give CaCO₃ and alcohols. This peculiar behavior -in atmospheric conditions- makes this class of chemicals suitable for consolidation of carbonate stones, plasters and wall paintings and also for pH buffer to avoid acidification of cellulose based materials such as wood and paper. Within the frame of the project, it has been demonstrated that the developed calcium compounds are more soluble than the corresponding commercial Ca-bearing consolidants and, after reaction with air, they evolve into different calcium carbonate structures, i.e. amorphous carbonate or crystalline calcite and vaterite. The ratio among these forms can be oriented by environmental conditions and treatment with water, in order to force a carbonate phase in place of other ones. Commercially available magnesium alkoxides solutions were also tested as consolidants precursors. Carbonation of magnesium compounds brought to precipitation of low crystalline Mg hydroxide and carbonate coatings. Moreover, hydrated Mg carbonate phases were identified. Many of them are salts sensitive to water and prone to ion-exchange, possibly evolving to soluble magnesium salts deleterious for stone. These experimental evidences led to the decision to discard magnesium alkoxides for conservation purposes.