Enhanced consolidation efficacy and durability of highly porous calcareous building stones enabled by nanosilica-based treatments

Outdoor building stones are suffering from serious degradation. To restore internal cohesion and to alleviate the disintegration of decayed stones, consolidation treatment is necessary and significant. Up to date, no fully satisfactory consolidation agent and its application methodology are available, mainly due to the limited penetration depth of consolidants, low compatibility, or poor durability in environmental conditions. Herein, in this study, aiming to design an effective and enduring method for the consolidation of highly porous calcareous stones, nanosilica-based consolidation treatments were tested and compared with traditional compounds (tetraethoxysilane, alkylalkoxysilane, barium hydroxide). In order to evaluate their performance and compatibility, a series of standard tests, including the Drilling Resistance Measurement System (DRMS) test, surface color measurement, vapor diffusivity measurement, and peeling test, were carried out. Besides, the penetration depth and distribution profiles of consolidants were estimated by exploiting elemental raster scanning of SEM-EDS analysis performed throughout the thickness of samples. More importantly, by employing a climatic chamber, the artificial ageing test was also conducted by simulating harsh atmospheric conditions. After accelerated ageing, the performance of all consolidants was assessed again. Results demonstrated that the application of nanosilica (<10 nm in dimension) by cellulose poultice, followed by adding tetraethoxysilane with the classical method "wet on wet", is the best consolidation approach, in terms of in-depth consolidation efficacy, compatibility with the stone substrate, surface cohesion strength and performance durability in environmental conditions. Moreover, the treated surface is not hydrophobic, which allows further grouting and adhesion operations usually required for the restoration of historic buildings.