Stone surfaces and façades of historic buildings, due to their predominately outdoor location, suffer from many deterioration factors, including air pollution, soluble salts, relative humidity (RH)/temperature, and biodeterioration, which are the main causes of decay. In particular, the façades of the buildings deteriorate with direct exposure to these factors; deformation and disfiguration of superficial decoration and formation of black crusts are often observed on the stones. The development and application of self-cleaning and protection treatments on historical and architectural stone surfaces could be a significant improvement in the conservation, protection and maintenance of Cultural Heritage. A titanium dioxide nanoparticle has become a promising photocatalytic material, owing to its ability to catalyze the complete degradation of many organic contaminants and environmental factors. In this study, TiO 2 nanoparticles, dispersed in an aqueous colloidal suspension, were applied directly to historic marble stone surfaces, by spray-coating, in order to obtain a nanometric film over the stone surface. The study started with an investigation of some properties of TiO 2 nanoparticles, to assess the feasibility of the use of TiO 2 on historic stone and architectural surfaces. Scanning electron microscopy (SEM) was, coupled with energy dispersive X-ray (EDX) microanalysis, (SEM-EDX), in order to obtain information on coating homogeneity and surface morphology, before and after artificial aging; the activity of the coated surface was evaluated through UV-light exposure, to evaluate photo-induced effects. The changes of molecular structure occurring in treated samples were spectroscopically studied by attenuated total reflection infrared spectroscopy (ATR-FTIR); activity of the hydrophobic property of the coated surface was evaluated by Sterio microscopy, model Zeiss 2010 from Munich, Germany, equipped with photo camera S23 under 80X magnification. The efficacy of the treatments was evaluated through capillary water absorption, and colorimetric measurements, performed to evaluate the optical appearance. Results showed that TiO 2 nanoparticles are good candidates for coating applications on historic stone surfaces, where self-cleaning photo-induced effects are well evident; they enhanced the durability of stone surfaces toward UV aging, improved resistance to relative humidity (RH)/temperature and abrasion affect, reduced accumulation of dirt on stone surfaces when left in open air for 6 months, and did not alter the original features.

The marble columns at many historic sites represent one of the most important and fundamental architectural elements in a building. They are almost always subject to serious damage, whether in the base, middle, or crowns of columns by fungal infection. In most cases, the microbial deterioration affects the physical and mechanical properties of historic marble columns, which have in turn been affected by other damaging factors (e.g., weathering from the elements or mechanical damage), leading to their partial or total collapse. In this current study, researchers are turning to new technologies in order to find the ideal solution to inhibit fungal growth, and, in turn achieve the total protection of exposed historic marble columns. The photocatalytic inorganic nanoparticles of ZnO have been employed for the purpose of long-term protection of exposed marble columns by inhibiting microbial-fungal attack and forming a protective surface layer. ZnO nanoparticles were dispersed in laboratory synthesized acrylic polymer to create a combined biocidal and consolidating coating to be applied on historic marble columns substrate. The synthesized nanocomposite coating was characterized and applied to marble samples collected from various archeological sites in Egypt. The protecting effect of synthesized nanocoating against fungal attack by Aspergillus niger and Penicillium sp., in addition to RH/Temperature, UVaging, and mechanical deterioration, was studied. The consolidating action of the obtained mixtures was evaluated through microscopic examination and capillary water absorption. Further, colorimetric measurements have been performed to evaluate the optical appearance of the columns. ZnO nanocomposites displayed better performance when compared to the pure synthesized acrylic polymer. The coated ZnO nanoparticles enhanced the durability of stone surface to resist the fungal attack when subjected to inoculums containing Aspergillus niger and Penicillium sp. and improved the resistance to UVaging, relative humidity, and thermal effect compared to the samples coated with the acrylic polymer without ZnO nanoparticles. Self-protection properties were confirmed without any obvious color changes on marble surfaces.

Abstract:The unwanted changes in valuable historic calcareous stone monuments due to exposure to many physical and chemical effects may lead to its deterioration. The growing interest in the field of conservation of stone monuments encourages the development of consolidation and water-repellent materials. The aim of this study is to evaluate the effectiveness of CaCO 3 nanoparticles as a consolidation and protection material for calcareous stone monuments, when those nanoparticles used are dispersed in acrylic copolymer; polyethylmethacrylate (EMA)/methylacrylate (MA) (70/30), respectively. Samples were subjected to artificial aging by relative humidity/temperature to show the optimum conditions of durability and the effectiveness of the nano-mixture in improving the physical and mechanical properties of the stone material. The synthesis process of CaCO 3 nanoparticles/polymer nanocomposite has been prepared by in situ emulsion polymerization system. The prepared nanocomposites with 0.15 g CaCO 3 nanoparticles showed obvious transparency features and represent nanocomposites coating technology with hydrophobic, consolidating and good protection properties. Some tests were performed in order to estimate the superficial consolidating and protective effect of the treatment. The obtained nanocomposites have been characterized by TEM, while the surface morphology before and after treatment and homogeneous distribution of used consolidation materials on stone surface were examined by SEM. Improvement of stone mechanical properties was evaluated by compressive strength tests. Change in water-interaction properties was evaluated by water absorption capillarity measurements, and colorimetric measurements were used to evaluate the optical appearance. Taken together, the results indicate that CaCO 3 /polymer nanocomposite is a completely compatible, efficient material for the consolidation of artistic and architectural limestone monuments capable of enhancing the durability of limestone toward artificial aging and improving the stone mechanical properties compared to the samples treated with pure acrylic copolymer without Calcium carbonate nanoparticles.