Abstract
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, UV aging, 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 UV aging, 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.
Highlights
Through the centuries, the exposed marble columns in historic buildings have deteriorated, due to physiochemical and mechanical factors such as rain water, sunlight, wind, frost, and excess loads (Winkler 2002)
Investigation of the microbial deterioration effect on historic marble columns The scanning electron microscopy (SEM) comparison investigations of infected areas in historic marble columns with areas where no fungi are present in the same environmental conditions (Fig. 4) have shown the presence of microorganisms living inside the marble structure and confirmed the role played by microorganisms in promoting stone deterioration through a variety of mechanisms chemically, mechanically, and esthetically as mentioned above (Lamenti et al 2000; Obuekwe et al 2005)
The micrographs of marble samples treated with acrylic polymer without ZnO nanoparticles (Fig. 6c and d and Fig. 7c and d) showed the growth of A. niger and Penicillium sp. in some areas on the surface, the microbial growth seem less than untreated samples, but the results revealed that the pure polymer could not completely prevent the growth of the fungus and still the spores and hyphae could still be seen
Summary
The exposed marble columns in historic buildings have deteriorated, due to physiochemical and mechanical factors such as rain water, sunlight, wind, frost, and excess loads (Winkler 2002). In the last five decades, conservation strategies have usually relied upon the use of the synthetic polymeric coatings to protect the stone surface, the bioreceptivity (Bracci and Melo 2003; Johnson and Mclntyre 1996), and on the application of biocidal products to inhibit biological activity. The improper interaction between the biocide and the water-repellent material may occur when separately applied to the substrate These challenges and drawbacks in polymeric materials have attracted the attention of conservation experts to use the modern techniques of other sciences to increase the efficacy of the conventional methods to achieve higher consolidation and protection efficiency (Malagodi et al 2000; Urzi and De Leo 2007; Muynck et al 2009)
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