Search

In this research, the volcanic rocks belonging to the Sardinia Oligo-Miocene volcanic cycle (32 - 11 Ma) and building up the structure of the San Nicola church, one of the most representative churches of the Romanesque architecture, were studied. These stones were widely used in medieval architecture for the excellent workability, but they present some disadvantages, since they are greatly affected by alteration phenomena. The main objectives of this research are i) to focus the mineral, chemical and petrographic compositions of the San Nicola stones, ii) the chemical and physical alteration processes affecting these materials, and iii) to establish the exactly provenance of the volcanic rocks. Furthermore, a comparative study between the rocks from the ancient quarries and those forming the structure of the church was performed. In the ancient quarries, where presumably a more advanced alteration occurs due to the vertical alteration gradient, different facies of the same volcanic lithology, characterized by macroscopical evidences of chemical-physical degradation degree, were sampled. Petrographic, geochemical (both major elements that the traces) and physical-mechanical features of the collected samples were determined to highlight the compositional differences (density, porosity, water-absorption kinetics, mechanical resistance) as a function of the different alteration degree. Moreover, chemical-mineralogical analysis of the sample surfaces from the church, was performed, to highlight possible presence and nature of secondary newly-formed phases (e.g., salt efflorescence). Several methodologies were applied to carry out physical-chemical and petrographic analysis: X-Ray fluorescence (XRF) and Inductively Coupled Mass Spectrometry (ICP-MS), X-Ray Diffractometry (XRD) for chemical and mineral composition; Optical and Scanning Electron Microscopy (SEM) for textures, mineral assemblages and microstructures studies; He-picnometry, water-absorption and mechanical tests for physical properties. In the paper some significant result will be presented, to better illustrate the methodological approach and the results achieved in this field. The results indicate that the volcanics of church have a mainly dacitic composition and come from local quarries. The XRD and optical analyses mainly consisting of K-feldspars and plagioclases), show the presence of an important glass fraction (about 40% vol.), and newly-formed minerals occur in different amounts. The most important are: smectite-group minerals (mainly montmorillonite) up to 30 wt%, gypsum up to 10 wt%, illite up to 5 wt%. Celadonite, glauconite, K-Ca-Mg sulfates, are occasionally detectable and generally occur in traces. In some samples of the church, the chemical alteration on the surface is not manifest since operate slower than the physical degradation. This latter generally occurs as macroscopic forms of pitting, exfoliation and alveolation. The chemical analysis of the outcrop-samples show a mobilization increasing degree of alteration, with an consequent leaching for some elements. At the same time it shows a clear decrease in the mechanical strength, due to the increase of the total porosity and a decrease in the apparent density. These variations of the porosimetric characteristics (with total porosity between 20-50%) also determine an increase of the water absorption, with a potential increase in the interaction between the circulating-fluid and the rock

The stones and historic mortars of the Santissima Trinità di Saccargia Romanesque Basilica (Sardinia, Italy): a multi-analytical techniques' approach for the study of their features and provenance Stefano Columbu (1), Marcella Palomba (2), and Fabio Sitzia (1) (1) Department of Chemical and Geological Sciences University of Cagliari, Italy (columbus@unica.it), (2) Istituto di Scienze dell'Atmosfera e del Clima (ISAC) Unità Operativa di Supporto (UOS) - C.N.R. of Cagliari Abstract: A research project devoted to the study of building materials of the Romanesque churches in Sardinia is currently underway. One of the objectives of the project is to focus the mineral, chemical-physical and petrographic characterisation of the construction materials, as well as the alteration processes. To make a contribution to the preservation of Sardinian monuments, we suggests a new approach to define the different alteration-modes of rocks in function of their local exposure to the weather, studying: 1) the changes of physical properties on surface of stone (porosity, water absorption, micro-morphology) determined through laboratory tests and photogrammetry observations, 2) the alteration phases present on surface (e.g., secondary minerals, soluble salts) determined by mineralogical and chemical investigations. This methodological approach will allow to select appropriate, suitable and compatible materials for replacing the original altered one's, and to plan appropriate strategies devoted to the restoration work. In this paper the geomaterials used for construct the Santissima Trinità di Saccargia Basilica have been investi- gated. The church, finished in 1116 over the ruins of a pre-existing monastery, is the most important Romanesque site in the island. Have been studied the chemical alterations and physical decay of two different stones, as volcanic rocks (i.e. basalt) and sedimentary rocks (i.e. limestones) used in bichromy on the Basilica. The main purpose is to observe the different modes of alteration of these two lithologies with different petrophys- ical characteristics, placed in the same conditions of weathering. Macroscopic evidences show that the limestones, while not having a high porosity, they were strongly affected by alteration phenomena, especially in the outer surface of ashlars, due to the solubilization of the carbonate matrix. The basalt rocks show no obvious physical alteration. Occasionally, in some ashlar located in basal zone of the monument structure, where the samples have a greater porosity, patinas of soluble salts on their surface can be observed. The features of a selected set of samples, including the stones and historic mortars, has been determined by the application of different techniques, as X-Ray Diffraction (XRD), Thermal Analysis (simultaneous TG and DTA analysis), Optical Microscopy, X-Ray Fluorescence (XRF) and Inductively Coupled Mass Spectrometry (ICP-MS). Mineral data, both from XRD and petrographic studies, show that limestones' ashlars consist of pure calcite (100 wt%) or to include other mineral species, especially dolomite (up to 15 wt%), as well as minor fractions of illite and quartz. Newly-formed minerals occur in different amounts, both in limestones (gypsum up to 10 wt%, illite up to 5 wt%) and basalts (smectite-group minerals up to 10 wt%, K-Ca-Mg sulfates in traces). Original masonry mortars show a qualitatively constant mineral association, mainly consisting of calcite (from 50 to 70 wt%), quartz (10-20 wt%), plagioclase (10-15 wt%), k-feldspar (5-10 wt%), gypsum (5-40 wt%), and illite (traces). The heterogeneous composition and high porosity of study mortars strongly affected its durability. The weathering processes have seriously damaged their texture and consistency, also contributing to the instability of other building materials in contact.