Elsevier

Journal of Cultural Heritage

Volume 16, Issue 1, January–February 2015, Pages 94-101
Journal of Cultural Heritage

Original article
Hydroxyapatite-based consolidant and the acceleration of hydrolysis of silicate-based consolidants

https://doi.org/10.1016/j.culher.2014.01.001Get rights and content

Abstract

Limestone, composed of the mineral calcite, is susceptible to environmental weathering processes that cause weakening from disintegration at grain boundaries. This paper discusses the effectiveness of hydroxyapatite (HAP) as an inorganic consolidant for physically weathered Indiana Limestone compared to a commercially available silicate-based consolidant (Conservare® OH-100). A double application is also investigated, in which samples are coated with HAP followed by Conservare® OH-100. Finally, a technique to accelerate the hydrolysis reaction of the initially hydrophobic Conservare® OH-100 is also developed. The motivation for using HAP is its low dissolution rate and crystal and lattice compatibility with calcite. To artificially weather limestone, so that the damage found in nature could be mimicked in the lab, a reproducible thermal degradation technique was utilized. Then, a mild wet chemical synthesis route, in which diammonium hydrogen phosphate (DAP) salt was reacted with limestone, alone and with cationic precursors, was used to produce HAP microfilms to consolidate the grains. The effectiveness of Conservare® OH-100 is investigated by applying it alone, and by following up with an ethanol-water rinse to accelerate the hydrolysis reaction. Samples that were to be rinsed were left to hydrolyze naturally over two and seven weeks before being reacted in the ethanol-water mixture. The dynamic elastic modulus (a measure of stiffness) and water sorptivity of the treated stones were evaluated. HAP was found to be an effective consolidant for weathered Indiana Limestone, as it restored the modulus of damaged stones to their original values and exhibited superior performance to Conservare® OH-100. Rinsing the Conservare® OH-100-treated stones increased stone hydrophilicity significantly, although not to the level of DAP-treated stones, as determined by water sorptivity. The formation of the consolidants in the pores and at grain boundaries was confirmed by scanning electron microscopy (SEM) and energy-dispersive X-Ray spectroscopy (EDX).

Section snippets

Research aims

The first aim of this study was to develop an inorganic consolidant treatment for weathered limestone based on the mineral hydroxyapatite, as an alternative to conventional silicate and polymeric consolidants. The second aim was to investigate the ability of hydroxyapatite to act as a coupling agent for a silicate-based consolidant. Lastly, a technique to accelerate the reaction of the silicate-based consolidant was developed and the efficacy of the three treatments were investigated and

Materials

Indiana limestone (IL) is composed mainly of calcite (CaCO3, > 97%) and minute amounts of MgCO3, Al2O3 and SiO2. Most of the calcite is in the form of calcite-cemented oolites, with trace amounts of sparry crystals. IL has a porosity of approximately 14% [22]. IL samples used in this study were core-drilled cylinders with a diameter of 2 cm and a height of 5 cm.

The DAP (puriss. p.a. > 99%) and calcium chloride (CaCl2.2H2O, assay > 99.0%) were both purchased from Sigma-Aldrich, their concentrations

Microstructural analysis

The calcium phosphate layer coats the grain surfaces (see Fig. 3 of ref. [15]). Fig. 1a shows the limestone sample subjected to treatment IL 4. EDX spectra taken on the surface of one grain indicate the presence of carbon, oxygen, phosphorous and calcium (Fig. 1b). However, no nitrogen peaks were detected, thus confirming that there was no residual DAP or ammonium salts in the film. Ammonia odor was detected during the reaction, indicating that the ammonium carbonate produced had evaporated. No

Discussion

SEM and EDX analysis of limestone treated in 1 M DAP for two days indicated that a film of calcium phosphate had formed around the calcite grains, in pores as well as at grain boundaries. The film was not cracked and, given that the only species detected were Ca2+ and PO43−, the presence of damaging salts can be excluded. Chloride was not detected because the quantity used was minute (i.e., millimolar). However, chloride is sometimes detected by EDX even after extensive washing. Hence, it is

Conclusions

All of the DAP-based treatments applied were effective consolidants for weathered Indiana limestone, based on improvement in mechanical properties, retained water uptake and absence of harmful by-products. The DAP treatment's performance was significantly improved by the addition of millimolar quantities of CaCl2, which provides calcium ions for HAP formation. The double treatment of DAP and DAP-CaCl2 provided further improvement in mechanical properties over the single treatment, but

Acknowledgements

This work was supported by grant MT-2210-12-NC-08 from the National Center for Preservation Technology and Training, a unit of the National Park Service. Its contents are solely the responsibility of the authors and do not necessarily represent the official position or policies of the National Park Service or the National Center for Preservation Technology and Training. The authors also wish to thank the Kress Foundation for financial support. We are also grateful for the use of Princeton

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