Investigation of nano-microstructural changes in Maastricht limestone after treatment with nanolime suspension

Nanolimes are dispersions of nanosized Ca(OH)2 particles in alcohols often used for the consolidation of various types of cultural heritage objects. The consolidation effect is based on the transformation of Ca(OH)2 into CaCO3 phases during carbonation process. The detection of microstructural changes consequent to a consolidating treatment (essential to evaluate its effectiveness) was approached adopting the innovative combination of two advanced techniques, covering a range in pore size from the nanometric to the millimetric scale: small-angle neutron scattering (SANS) and synchrotron X-ray micro-computed tomography (µ-CT). The changes in the 3D microstructure of samples of Maastricht limestone, a well-known weak stone material considered as a sort of ‘standard’ in cultural heritage conservation studies, pure and treated with nanolime dispersions, have been described in a fully non-invasive fashion, overcoming the limitation of previous approaches. The application of nanolime resulted to have a limited positive effect in reducing the fine porosity. Its time evolution was attributed to the progress of the carbonation reaction. On the contrary, the treatment produced positive effects on the porosity in the size range covered with µ-CT, reducing the pore accessibility between 30 and 65 µm, suggesting an improvement of the mechanical properties. The combined use of SANS and µ-CT represents and novel methodological approach in support of cultural heritage conservation works.