Preliminary Results of the Use of Hydroxyapatite as a Consolidant for Carbonate Stones
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Preliminary Results of the Use of Hydroxyapatite as a Consolidant for Carbonate Stones Enrico Sassoni1, Sonia Naidu2 and George W. Scherer3 1
Dipartimento di Ingegneria Civile, Ambientale e dei Materiali, Università di Bologna, via Terracini 28, 40131 Bologna, Italy 2 Department of Chemical Engineering, Princeton University, Eng. Quad. E-226, Princeton, NJ 08544, USA 3 Department of Civil and Environmental Engineering, Princeton University, Eng. Quad. E-319, Princeton, NJ 08544, USA ABSTRACT The effectiveness of using hydroxyapatite (HAP) as a consolidant for carbonate stones was evaluated. HAP was chosen as a consolidating agent since it is notably less soluble than calcite and has a similar crystal structure and a close lattice match to it. Among possible methods for forming HAP, the reaction between the calcite of the stone and a solution of diammonium hydrogen phosphate (DAP) in mild conditions was chosen. Indiana Limestone samples, artificially damaged by heating to 300°C for 1 hour, were treated with a 1 molar DAP solution by partial immersion and capillary absorption for 48 hours or by brushing until apparent refusal and wrapping with a plastic film for 48 hours. After washing in deionized water for 3 days and drying under a fan at room temperature until constant weight, the improvements in dynamic elastic modulus and tensile strength were evaluated. The formation of calcium phosphate phases was observed by scanning electron microscopy (SEM) and the phase characterization performed by energy dispersive X-ray spectroscopy (EDS) and electron back-scattered diffraction (EBSD). The water absorption modification after the consolidating treatment was then assessed. Results show that treated samples experienced significant increases in dynamic elastic modulus and tensile strength, as a consequence of crack reduction and pore filling consequent to HAP deposition at grain boundaries. The sorptivity of the treated samples is reduced by 26-44% (based on treatment technique), so that water and water vapor exchanges with the environment are not blocked. INTRODUCTION The main consolidants currently used for strengthening carbonate stones – e.g., limebased, silicate and polymeric consolidants – show some limitations, related either to their effectiveness or durability. Lime-based consolidants, in spite of their chemical compatibility with the substrate, are characterized by a reduced penetration depth, an extremely slow rate of conversion of lime into calcium carbonate and a limited solubility of lime in water, which may cause chromatic alteration of stone surface [1, 2]. Silicate consolidants, although being highly effective when applied to silicate stones, as they can chemically bond to the stone grain surface, are much less effective on carbonate stones, as the bonding to the stone is merely physical [3, 4]. Finally, the use of polymeric consolidants may lead to problems related to their penetration depth and durability, particularly threatened by yellowing and biodeterioration [1, 5-7].
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