Alkoxysilane-based sols for consolidation of carbonate stones: Impact of the carbonate medium in the sol-gel processes

Abstract

Abstract This investigation addresses the impact of the carbonate medium in the sol-gel processes of stone consolidants and the possible detrimental effects in practical applications. The results demonstrate that the carbonate medium influences the sol-gel paths and that silicate materials developed in the carbonate stone pores can be structurally different from those formed within “inert” conditions, as a consequence of the combined effect of pH and ionic interactions between Si-O− and Ca2+. The trend is that the formation of the silica films around stone grains (silica growth mechanism through typical condensation) evolves into less advantageous growth processes (Oswald ripening and/or coagulation), resulting in a material that is unlikely to consolidate carbonate stone. Furthermore, pH measurements and different ratios between NMR Q3:Q4 species along stone depth showed that this effect seems more important at greater depths. The results underline the importance of the carbonate medium in the failure of alkoxysilane-based treatments and, accordingly, that an efficient design and development of new alkoxysilane-based consolidants must consider this effect.