The nanolimes in Cultural Heritage conservation: characterisation and analysis of the carbonatation process

Water and milk of lime are usually adopted for conservative surfaces treatments thanks to the conversion of lime into calcium carbonate. Calcium carbonate is, as a matter of fact, very compatible with many carbonatic lithotypes and architectonic surfaces, because its characteristics are very similar to those of the materials to be restored. But there are some limiting aspects to treatments effectiveness: the reduced penetration depth and the incompleteness carbonatation process. In order to improve lime treatments, Ca(OH)2 particles with submicrometric dimensions (nanolimes) are recently introduced in Cultural Heritage conservation. Lime nanoparticles are typically produced by a chemical precipitation process in supersaturated aqueous solutions of the reactants (calcium chloride and sodium hydroxide). The aim of the present work is to analyse the nanolime carbonatation process in relation to some parameters, like time and the relative humidity conditions. For this scope, lime nanoparticles are therefore synthesised and characterised by X-rays diffraction (XRD), scanning and transmission electron microscopy (SEM-TEM), electron diffraction measurements (ED) and dark field images (DFI). The possibility to improve the nanolime carbonatation process is investigated using an alcoholic suspension and by adding a baking soda solution, in order to disaggregate particles and to increase CO2 content in the suspension respectively. The efficiency of the nanolime carbonatation process is reported. After that, the lime nanoparticles are applied on natural lithotypes (“Estoril” and “Pietra Serena”) and some tests are performed in order to estimate the superficial consolidating and protective effect of the treatment: “Scotch Tape Test”, capillarity and imbibitions tests. SEM analyses are performed to evaluate penetration depth and surface adhesion of nanolime treatments.