Understanding the transport of nanolime consolidants within Maastricht limestone

tNovel nanomaterials, such as nanosilica or nano-titanium oxide, have been developed in the last decadefor the conservation of the built heritage. Among nanomaterials, nanolimes have acquired a considerablerelevance due to their potentialities as consolidant product. The so-called nanolimes, colloidal disper-sions of calcium hydroxide nanoparticles in alcohols, have been successfully applied as pre-consolidantson frescos and paper, and their use has later been extended to plasters, renders and stone. Nanolimeshave better potentialities compared to conventional inorganic consolidants based on limewater (e.g.faster carbonation rate and higher calcium hydroxide concentration). Moreover, nanolimes are consid-ered more compatible with CaCO3-based substrates than alkoxysilanes (e.g. TEOS), the most widely usedconsolidant products. Nanolimes can guarantee the recovery of the superficial cohesion of degradedmaterials. However, when a mass consolidation is required, like in the case of decayed stone, nanolimesshow some limitations. One of the problems is caused by nanolime accumulation at or just beneath thesurface of the treated material. In order to solve this problem, the transport mechanism of nanolimewithin porous materials, as stone or renders, should first be better understood. Commercial nanolimeswere applied on Maastricht limestone, a high-porosity yellowish limestone, used in the Netherlands andBelgium as traditional building material. The absorption and drying behaviour of nanolime in this lime-stone was measured and nanolime deposition in the stone was studied by optical and scanning electronmicroscopy. The results show that nanolime transport is strictly related to the properties of the solvent.The alcoholic solvent guarantees a stable dispersion that penetrates in depth in the material, but is par-tially back-transported to surface. The high volatility of the solvent and the high stability of the dispersionfavour the partial back-migration of lime nanoparticles to the surface during dryin