Chemical and mineralogical investigations of lime plasters of medieval structures of Hampi, India

The historical lime plasters of World Heritage Site of Hampi were analyzed using X-ray fluorescence (XRF), petrological microscope, granulometric analysis, X-ray Diffraction (XRD), Fourier transform Infrared spectroscopy (FTIR), and Scanning Electron Microscopy (SEM). The studies indicated presence of aragonite and vaterite, i.e., the polymorphs of calcite in the lime platers of Hampi. The crystallization of calcite has significantly been influenced by self-healing of lime due to wet and dry cycle in the presence of magnesium that promoted formation of calcite polymorphs. The increasing solubility of metastable calcite led to loss of plaster by water dissolution. Granulometric analysis revealed inclusion of sub-angular to sub-rounded medium-sized aggregate grains of granitic origin and the sediments were medium transported before its deposition in fluvial river basin. The cementation index and mineralogical composition point use of air lime/sub-hydraulic lime admixed with non-hydraulic components sourced differently for Hampi plaster. Petrological analysis confirmed the plaster rich in essential minerals like quartz, orthoclase, plagioclase, and other clay components. The presence of chlorite and cristobalite revealed from SEM studies may be resultant minerals from the river sand of Tungabhadra. The scientific studies yielded information about the mineralogical, micro-structural, and chemical composition of the Hampi plasters useful to tailor compatible lime plaster for future conservation works.