Semi-quantitative measurement of calcium carbonate in historic gypsum mortars, using diffuse reflectance infrared Fourier transform spectroscopy

This paper uses Fourier transform infrared spectroscopy to semi-quantitatively measure the amount of calcium carbonate in historical gypsum mortars. 17 historical mortar samples related to Arch of Alishāh in Tabriz-Iran, belonging to the Ilkhanid period, were evaluated. The methodology used in this study was diffuse reflectance infrared Fourier transform spectroscopy [DRIFTS]. Gypsum mortars containing 0-6.5% calcite were prepared and analysed using an FTIR spectrometer. The calibration curve was prepared based on the band intensity ratio of 1485 cm-1 related to carbonate vibrations in calcite to 660 cm-1 corresponding to sulphate vibrations in gypsum. Calculating the amount of carbonate in all historical mortars shows the presence of about 0.12 to 1.08% carbonate in gypsum mortar. According to obtained results, it can be acknowledged that due to the meagre amount of carbonate in the mixture of mortars, the existing carbonate has entered the mortar as an impurity through soil or dissolved salts. This can indicate that, contrary to the initial idea about some of these mortars, lime and gypsum mixture was not used as the main component in the making these mortars.