Silica nanoparticles (SiO2): Influence of relative humidity in stone consolidation

The influence of relative humidity (RH) has been determined in SiO2 colloidal nanoparticles, to study their efficacy as a consolidating product by means of the physical changes in the hydric and mechanical properties produced in a siliceous-carbonate stone from a historic building (XVI century) with signs of degradation. Therefore, diverse analytical techniques have been used for the product characterization (TEM-EDS, ESEM-EDS, XRD, DTA-TG, spectrophotometry) together with micro-destructive (SEM, microdrilling resistance) and non-destructive petrophysical tests (hydric tests, ultrasonic velocity, adhesion and microhardness tests) for the characterization of the stone. The precursor water colloidal nanosilica, when is initially exposed to high RH environments, forms agglomerated spherical nanoparticles of amorphous silica, that holds a higher amount of adsorbed water and lower amount of silanol groups on the surface, compared to samples exposed to lower RH. This final product behaves in a similar way than a silica gel, when is exposed once again to lower and higher RH, as a reversible hydration-dehydration process of adsorbed water. Related to the efficacy as a consolidant, the results show differences both, in surface changes, decreasing the amount of released material from the substrate and increasing its surface hardness, as in the interior of the porous structure, increasing absorption and desorption water capillarity rates, ultrasonic velocities and drilling resistance, showing high efficiency and less adverse aspects at lower RH. However, this effect and the possible decay caused by repeated cycles of hydration-dehydration of the silica gel inside the porous of the stone should be assessed in order to determine its durability.