Decay of natural and artificial stones used in historical buildings: influence of mineralogy and microstructure and new restoration methods.

Among the natural stones used in historic buildings in Tuscany, sandstone and marble are the most widespread lithotypes since they outcrop widely in the region, especially along the Apennine and Apuan Alps mountain ranges, a determining factor for their extensive use in architecture as building materials throughout history. The issue of durability of stone materials remains a current problem. The main cause of stone degradation is in fact due to the action of water, which is activated through both physical and chemical mechanisms. The most important effect of this action on sandstone is arenization: an alteration caused by the presence of a high quantity of clay minerals that reduces the rock to sand; while in marble, the main cause of degradation is linked to the sulfation of carbonates with the formation of the so-called chalky crusts. In order to conserve the historical monumental heritage in stone material, it is essential to know thoroughly the characteristics of the rocks used and their behavior after the various restoration processes. In this work various types of rocks classically used in historical and monumental buildings with different compositional and textural characteristics have been selected: Sandstones of Matraia, Volterra stone ("Panchina" of Volterra and Montaione stone), "Panchina" stone of Livorno, Calcarenite of Gravina di Matera, and Apuan marbles. After a mineralogical, petrographic and petrophysical characterization, the samples were subjected to thermal aging processes and subsequent consolidation with different consolidating products, with particular attention to the effects of the application of nanoparticle-based materials. The water absorption values of the studied rocks show a wide variability among them. The high values of the saturation index found in most of the samples confirm the presence of a predominantly open-type porosity. From the capillary adsorption tests it is concluded that in general thermal degradation produces in the studied lithotypes, with few exceptions, an overall increase in the water adsorbed and a substantial increase in the capillary rise rate. The study of the absorption curves leads to the conclusion that in the processes of thermal degradation new detachments between the granules (in particular of calcite, where present) progressively occur, resulting in an increase in intergranular porosity, and consequent enlargement of the existing pores. The intragranular porosity remains substantially constant. As for the consolidating products used, ethyl silicate remains the best performing product for both silicate and carbonate rocks. The greater the porosity of the rock to be treated, the greater the grain size of the active ingredients of the consolidating product to be considered for the treatment of the material to be consolidated.