Image digital analysis for the characterization of the alteration degree of Italian natural building materials

The study is part of a research programme which is being carried out by the authors, aiming to characterize from the mechanical and physical points of view the natural materials used since ancient times in Italy both as building and ornamental materials. The greater and greater necessity of recovering and conservatory interventions on historical buildings claims for an ever greater knowledge of the physico-chemical properties of the materials used in their construction. Samples of rocks used in the past as building materials are being tested in laboratory with both traditional techniques for the characterization of their alteration state and using innovative methods based on image analysis. For a better test control, fresh rocks having known mineralogical and chemical features were used for the following standard tests: i) freezing-defrosting cycles, ii) warm-cold cycles, iii) wet-dry cycles and iv) simulating reaction tests to some atmospheric agents. Each set of samples underwent a different number of cycles. The technical behaviour of samples was determined depending on the state of alteration achieved. At the same time, during the various experimental phases - in particular, at the end of each of them - image analysis techniques were used in studying sample zones characterized by mineralogical associations, textures and structures taken as particularly significant for an evaluation of the alterability characters of materials. The obtained results, in particular those from non-destructive and non-invasive techniques such as image analysis, are discussed in order to evaluate the possible application of such techniques in all those situations not allowing a removal of portions of the material to analyse using classical methods - for instance, when one has to evaluate the alteration degree of rocks used as ornamental stones in buildings of great historical and archeological interest