Conservation of historical buildings: use of traditional and innovative consolidants for natural stones

Over the last century, the decay of heritage buildings significantly increased due to rising air pollution and climate change, which accelerated and intensified the deterioration mechanisms of natural stones. So, the increasing concern about the unavoidable degradation of the cultural heritage, led researchers to focus on innovative and effective solutions for the conservation of the historical patrimony in order to guarantee its fruition. The recovery of physical and mechanical properties of a weathered stone often involves consolidation interventions. Usually, these treatments are carried out applying polymeric compounds in organic solvent or inorganic products. One of the key parameters in the science of conservation is the affinity and compatibility of consolidant with substrate; in this aim, the use of inorganic consolidating products for restoration of natural stones, demonstrated to have much more durability, compatibility with the weathered stones, efficacy and environmental sustainability if compared to the traditional and most widely used organic treatments. Recently, also the utilization of nanotechnology provided an interesting starting point for new promising research based on application of inorganic nanoparticles in the stone conservation. Among the inorganic consolidants, silica-based products, such as sodium or potassium silicate solutions and colloidal suspensions of silica made of nanometric particles, have been widely used on different stone substrates. The consolidating mechanism is due to the formation of amorphous silica, after solvent evaporation, which fills the accessible stone porosity. Also, nanoscale hydroxides, such as Ca (OH)2, Mg(OH)2 and Ba(OH)2 are considered traditional products used as consolidants. The consolidant action in this case, is based on the hydroxides carbonation reaction with the atmospheric CO2, resulting in carbonates formation. This paper aims to review and compare different consolidating treatments on natural stones by means of innovative inorganic products, assessing their role on the main chemical, physical and morphological properties of substrates.