Some Recent Findings On Marble Conservation By Aqueous Solutions Of Diammonium Hydrogen Phosphate
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Some Recent Findings On Marble Conservation By Aqueous Solutions Of Diammonium Hydrogen Phosphate Enrico Sassoni1, Gabriela Graziani1, Elisa Franzoni1 and George W. Scherer2 1
Department of Civil, Chemical, Environmental and Materials Engineering (DICAM), University of Bologna, Via Terracini 28, 40131, Bologna, Italy 2 Department of Civil and Environmental Engineering (CEE), Princeton University, 69 Olden Street, 08542, Princeton (NJ), U.S.A.
ABSTRACT Given the lack of satisfying treatments for consolidating marble affected by thermally induced grain detachment (the so-called "sugaring"), the use of aqueous solutions of diammonium hydrogen phosphate (DAP) has recently been proposed. The idea is to form a new binding mineral (hydroxyapatite, HAP) as the reaction product between the DAP solution and the calcitic substrate. In this study, we investigated the effects of adding small quantities of ethanol (EtOH) to the DAP solution, with the aim of favoring HAP formation. The results of the study indicate that, when a 0.1 M DAP and 0.1 mM CaCl2 solution in 10 vol% EtOH is used, complete coverage of marble surface with a crack-free coating with reduced porosity is achieved (whereas no coating is formed without EtOH addition). This is thought to be a consequence of the weakening of hydration shells of phosphate ions in the DAP solution, thanks to the presence of ethanol molecules. When used to restore mechanical properties of weathered marble, the treatment with 10 vol% EtOH was found to significantly improve the dynamic elastic modulus after a single application and to completely restore it after a second application.
INTRODUCTION Marble has been used in architecture and sculpture since antiquity. Unfortunately, the anisotropic thermal behavior of calcite grains that constitute marble, together with its very low open porosity, make marble highly sensitive to temperature variations [1]. As a result of repeated thermal cycles, calcite grains lose cohesion and start to detach, leading to the so-called "sugaring" and making marble more susceptible to other decay causes (e.g., frost). Examples of sugaring marble elements in the New York Public Library (XIX cent.) are illustrated in Figure 1. At the moment, no fully satisfying treatment for consolidation of sugaring marble exists, as organic treatments generally lack compatibility and durability with the substrate, while inorganic treatments generally have limited effectiveness. Recently, the use of aqueous solutions of diammonium hydrogen phosphate ((NH4)2HPO4, DAP) for consolidation of sugaring marble has been proposed [2,3]. The idea, first proposed for limestone consolidation [4], is to form hydroxyapatite (Ca10(PO4)6(OH)2, HAP) as the reaction product between calcite grains and the aqueous DAP solution. Thus-formed HAP is expected to bind loose calcite grains and also protect marble the surface from corrosion in rain, thanks to the much lower solubility and slower dissolution of HAP, compared to calcite [5].
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