Calcite polymorphs in historic plasters of India’s arid region – execution technique, composition and characterization

In this work, the microscopic observation of thin section was utilized to analyse the geological and fine morphological features of the 16th century Amber fort lime plaster -a World Heritage site. FTIR, XRD and SEM photomicrographs showed the stabilization of different crystalline phases of calcite polymorphs. Chemical analysis and SEM-EDX data revealed dolomitic limestone was probably sourced for plaster works. The high magnesium present in the raw material, the environmental conditions during the application, pH, etc. have stabilized calcite polymorphs in the plaster. The calcite meta-stable phases have undergone dissolution with time making the plaster weak. This has increased porosity, permeability, and resultant lowering of plaster's mechanical strength. Thermal analysis and cementation index revealed use of non-hydraulic, binder-rich, air-lime for plaster works. The granulometric study showed the mixing of reddishbrown sand-size grains, and the aggregates were probably sourced from the same location for major construction activity. The low proportion of aggregates led to reduced mechanical strength, and the plaster is vulnerable to damage.