Novel hydroxyapatite-based consolidant and the acceleration of hydrolysis of silicate-based consolidants
- PDF / 2,080,841 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 86 Downloads / 144 Views
Novel hydroxyapatite-based consolidant and the acceleration of hydrolysis of silicate-based consolidants Sonia Naidu1, Chun Liu2, George W. Scherer3 1
Department of Chemical and Biological Engineering, Eng. Quad. E-226, Princeton University, Princeton, NJ 08544, USA 2 Department of Chemistry, University of California, Berkeley, CA 94720, USA 3 Department of Civil and Environmental Engineering, Eng. Quad. E-319, Princeton University, Princeton, NJ 08544, USA ABSTRACT This paper discusses the effectiveness of hydroxyapatite (HAP) as an inorganic consolidant for physically weathered Indiana Limestone, and as a coupling agent between limestone and a silicate consolidant. A double application is investigated, in which samples are coated with HAP followed by a commercially available silicate-based consolidant (Conservare® OH-100). To artificially weather limestone, a thermal degradation technique was utilized. Diammonium hydrogen phosphate (DAP) salt was reacted with limestone, alone and with cationic precursors, to produce HAP films. The dynamic elastic modulus, water sorptivity and tensile strength of the treated stones were evaluated. HAP was found to be an effective consolidant for weathered Indiana Limestone, and its performance was enhanced by addition of millimolar quantities of calcium chloride. However, HAP was not useful as a coupling agent; a double treatment with DAP is more effective than sequential treatment with DAP and Conservare®. INTRODUCTION A consolidant is intended to restore the mechanical integrity of deteriorated stones by binding the grain boundaries and fracture surfaces, either physically or chemically [1]. The ideal consolidant should retain the stone’s water transport properties and aesthetics, and the treatment should be reversible, or at least not hamper further treatment. Silicate-based consolidants are very effective on silicate stones, such as quartzitic sandstone, due to their chemical compatibility [2,3]. One such consolidant is oligomeric tetra-ethoxy-ortho-silicate (TEOS), -(Si(OC2H5)4)n- [4] (commercially available in the U.S. as Conservare™ OH-100) [5]. After application, atmospheric water gradually replaces the -OC2H5 groups with -OH groups. This process is known as hydrolysis, and is very slow, taking six to eight weeks to complete [2]. Once TEOS has hydrolyzed, the oligomers condense with each other to form a silica gel. The gel can then form covalent bonds with the silanol groups on the surface of sandstone [6]. Unfortunately, TEOS is not as effective on carbonate stones, as it can only bond mechanically, owing to the absence of – OH groups in calcite that would allow chemical interaction to occur [3]. To address the lack of an effective consolidant for limestone, hydroxyapatite (HAP) was tested, using the precursor diammonium hydrogen phosphate (DAP) and based on the following reaction [7]: 10CaCO3 + 5(NH4)2HPO4 → Ca10(PO4,CO3)6(OH,CO3)2 + 5(NH4)2CO3 + 3CO2 + 2H2O (1)
As indicated in the formula, HAP is usually found carbonated due to its interaction with carbon dioxide from
Data Loading...
