Minero-petrographic, thermal and microchemical investigation of historical mortars used in Catania (Sicily) during the XVII century A.D.

Abstract

The combination of X-Ray Diffraction (XRD), Scanning Electron Microscopy-Energy Dispersive Spectrometry (SEM-EDS), Differential Thermal Analysis-Thermogravimetry (DTA-TG) and Optical Microscopy (OM) has been used to study several different ancient mortars recovered in the S. Nicola Church (built after the devasting earthquake of the 1693) and other historic buildings located in the historical center of Catania (Eastern Sicily). Investigations have been focused on the identification of raw materials mixed in the different mortars and their provenance as well as on the study of the relevant technological aspects of manufacturing processes. Minero-petrographic data have often shown that local volcanic raw materials have been used as aggregate fractions and, in particular, a new volcanic material, the so-called ghiara, has been largely adopted for the construction of the walls of the monuments after the earthquake. It is worthy to note that ghiara is a by-product of erupted magma during the impressive Etna eruption of 1669, which almost completely covered the centre of Catania. The following reaction with the soil gave rise to the formation of a layer of this typical ochre-coloured material. Moreover, the precise provenance of the ghiara has been determined trough geological surveys of the most important historic quarries. Results have allowed the identification of exploited sources of ghiara.

Furthermore, combination of SEM-EDS results and thermal information have indicated the hydraulic nature of the mortars due to the formation of hydraulic phases at the binder-aggregate interface. It, therefore, is due to the fact that the ghiara has slight pozzolanic properties.

Present data have an important role for any satisfactory reproduction of the ancient manufacturing techniques to be used for the restoration of the S. Nicola Church.