SURFACE TREATMENTS USING NANOPARTICLES AND PHYSICOCHEMICAL METHODS FOR THE PROTECTION OF ANCIENT MONUMENTS BUILT OF CALCITIC MATERIALS

Building materials like marble, calcareous stone and mortars rich in calcitic content have been extensively used in ancient monuments. Stone conservation efficiency may be improved considering the mechanisms underly-ing deterioration due to wet precipitation. In the present work we have tested the possibility of treating calcitic materials with suspensions of amorphous calcium carbonate (am-CaCO3, ACC) and amorphous silica (am-SiO2) nanoparticles (NPs). Pentelic marble (PM) in powdered form, to maximize surface area and a mortar from the ancient Roman stadium of Patras consisting of calcium carbonate and silica served as test materials for the validation of the application of suspensions of NPs for protection towards the erosion in slightly acidic solutions undersaturated with respect to calcite (pH 6.50). All solids were characterized by physicochemical methods including X-ray diffraction (XRD), infrared spectroscopy (FTIR) and measurements of BET specific surface area and porosity by nitrogen absorption. ACC nanoparticles, with sizes 200-500 nm were prepared by the development of high saturation through the hydrolysis of dimethyl carbonate (DMC) in alkaline envi-ronment of calcium chloride solutions. am-SiO2 nanoparticles, with sizes between 15-25 nm were prepared by the hydrolysis of tetra ethyl orthosilicate (TEOS). ACC nanoparticles suspensions were rather stable over at least 9 days at room temperature and for over one month at -200C. am-SiO2 suspensions were stable for at least 6 months. ACC and am-SiO2 were deposited on the surface of the test specimens suspended in the synthesis solutions of each of the NP types. The rates of dissolution of the treated specimens decreased significantly. The effect was attributed to the chemical and structural affinity of the NP preparations with the treated build-ing materials