Evaluation of preventive performance of kaolin and calcium hydroxide nanocomposites in strengthening the outdoor carved limestone

The conservation of ancient Egyptian outdoor carved limestone sculpture is an important issue. This type of artifact is generally exposed to complex weathering processes, and the resulting physico-chemical decay causes unwanted changes in the stone structure and surface. The conservation of carved limestone is a difficult task, requiring the use of materials which need to be compatible to both the original components and those added as part of a previous conservation treatment. The polymeric-nanocomposite materials—which are more and more used on cultural heritage—show promising results for the conservation and preservation of stone monuments, particularly in preventing or reducing future damages. The aims of this study are to characterize limestone used in the outdoor historic sculpture and to examine the efficiency of polymeric nanocomposites in the treatment and preservation of exposed archeological carved limestone. This paper presents a comparative evaluation of the effect of kaolin and calcium hydroxide Ca(OH)2 nanoparticles to both the mechanical and the physico-chemical properties of an acrylic-based copolymer, in order to enhance significantly the polymer properties and its ability to protect and consolidate the weathered outdoor carved limestone. The polymer nanocomposites were prepared by direct melt mixing with a nanoparticle content of 5% (w/v). The stability and efficiency of the consolidation materials were tested by aging artificially the samples under different environmental conditions. The hydrophobic properties and the adsorption of the treatment materials by the stone were analyzed. Moreover, the properties of untreated and treated limestone samples were evaluated comparatively before and after aging by microstructural analysis (the surface morphology was studied using scanning electron microscopy), by measuring the color variances (using spectrophotometry), and by mechanical tests. The results showed that treatment with Ca(OH) nanoparticles/polymer nanocomposite enhanced the durability of stone samples exposed to artificial aging; moreover, it showed higher compatibility in physico-chemical and mechanical properties with the original stone material compared to the samples treated with polymer nanoparticles. The treatment with kaolin nanoparticles/polymer nanocomposite was also effective and showed results second to the Ca(OH) nanoparticles/polymer nanocomposite. The test for hydrophobic properties of both nanocomposites also has a positive outcome and no color change on the surface was observed.