Chemical-physical agents and biodeteriogens in the alteration of limestones used in coastal historical fortifications

Columbu Stefano;Sitzia Fabio;Bacchetta Gianluigi;Podda Lina;Calvia G.;Coroneo Valentina;Pirinu Andrea;Mirão J. A. P.;Moita P. S. M.;Caldeira A. T.;Rosada T. I. S.

2018-01-01

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Abstract

The alteration of rocks is usually due to the chemical-physical processes that are initially established on the outer surface of the stone and gradually proceed towards the inner matrix. The chemical alteration generated by the interaction with atmospheric agents (weathering) involves the transformation of the mineral phases constituting the rock that are less stable in the current climatic conditions. That often leads to the formation of new secondary phases more stable with respect to the alteration. However, among these phases are often present some very soluble and hygroscopic phases (i.e., soluble salts, clay minerals) that cause inner degradation of the rock, due to their physical-mechanical actions (inner crystallization pressure, hydration dilation). In the case of carbonate rocks (limestone, sandstone with carbonate cement, etc.), the dissolution is the more frequent process, especially when the monuments were located within the urban cities, due to the acid meteoric precipitations (with H2CO3, H2SO4) that lead to the sulfation of carbonate matrix with formation of gypsum, very harmful to the stone. When the rock (e.g., clay-arenaceous limestones) naturally contains hygroscopic phases inside the matrix (i.e., marine salts, phyllosilicates) and they are also porous (> 20%), the physical degradation is accelerated, with decohesion of the mineralogical matrix (between the crystalline granules) and consequent disintegration of the stone. In the rock-atmosphere interaction often occurs the presence of biodeteriogens (plants, fungi, lichens, microorganisms, etc.), which negatively participate and in various ways in the processes of rock alteration. This paper aims to define the chemical-physical alteration factors on the limestones exposed to different climatic contexts (Mediterranean-Atlantic), taking two study-case monuments located in the Italian and in the Portuguese coasts. In this research the different presence of taxa present on stone surface (after their classification) and their different role in the degradation of limestone rocks have been also studied.