TEOS-based treatments for stone consolidation: acceleration of hydrolysis–condensation reactions by poulticing

Ethyl silicate (TEOS) is nowadays the most widely used consolidant for stone, due to its efficacy on silicate stones and also on limestones containing small amounts of quartz. However, because of the very long time required for TEOS curing reactions to be completed, the final mechanical improvement caused by the treatment is reached only after several months and, during this period, the treated stone is temporarily hydrophobic. This latter aspect is one of the most relevant drawbacks of TEOS, as it makes it impossible to perform water-based interventions after consolidation. Therefore, in this study the possible acceleration of TEOS curing reactions by prolonged contact with water, some time after consolidant application, was investigated. A commercial TEOS-based product was applied on Globigerina limestone, a porous stone from Malta containing also small amounts of quartz. After that preliminary tests indicated that prolonged contact with water is actually effective in both removing hydrophobicity and improving mechanical strength, a new consolidation cycle, based on TEOS application followed by application of a water-impregnated poultice was tested. The results of the study indicate that water application by poultice, after curing for 7 days, allows to remove the hydrophobicity and achieve higher mechanical properties than those obtained after curing for 4 weeks in laboratory conditions. Moreover, such mechanical properties are around 90% of those achieved after curing for as much as 7 months, which points out that the proposed method is a very promising technique for accelerating TEOS curing reactions.