Use of Coupled Passivants and Consolidants on Calcite Mineral Surfaces
- PDF / 401,779 Bytes
- 6 Pages / 404.46 x 644.22 pts Page_size
- 50 Downloads / 163 Views
ABSTRACT Deterioration of monuments, buildings, and works of art constructed of carbonate-based stone potentially can be arrested by applying a combination of chemical passivants and consolidants that prevent hydrolytic attack and mechanical weakening. We used molecular modeling and laboratory synthesis to develop an improved passivating agent for the calcite mineral surface based on binding strength and molecular packing density. The effectiveness of the passivating agent with and without a linked outer layer of consolidant against chemical weathering was determined through leaching tests conducted with a pH-stat apparatus at pH 5 and 25°C. For the range of molecules considered, modeling results indicate that the strongest-binding passivant is the trimethoxy dianionic form of silylalkylaminocarboxylate (SAAC). The same form of silylalkylphosphonate (SAP) is the second strongest binder and the trisilanol neutral form of aminoethylaminopropylsilane (AEAPS) is ranked third. Short-term leaching tests on calcite powders coated with the trisilanol derivative of SAAC, the triethoxy neutral form of SAP, and the trimethoxy neutral form of AEAPS show that the passivant alone does not significantly slow the dissolution rate. However, all passivants when linked to the sol consolidant result in decreased rates. Combined AEAPS plus consolidant results in a coating that performs better than the commercial product Conservare® OH and at least as well as Conservare® H. The modeling results indicate that there may be a threshold binding energy for the passivant above which the dissolution rate of calcite is actually enhanced. More strongly-binding passivants may aid in the dissolution mechanism or dissociate in aqueous solution exposing the calcite surface to water. INTRODUCTION Suitable protective coatings for monuments, sculptures, and buildings constructed of carbonate stone require improved adhesion between the carbonate substrate and alkoxysilane consolidants [1]. Here, we report on the efficacy of such adhesive agents designed through a combined approach of modeling their adhesive and passivating properties using computations at the molecular scale and testing those properties during simulated leaching tests in the laboratory. Three candidate passivant molecules were selected based on their potential bonding configurations with surfaces of calcite (CaCO 3): (1) silylaminocarboxylates (SAAC) with three carboxylate ligands, (2) silylphosphonates (SAP) with two strong P-O bonds, and (3) aminoethylaminopropylsilanes (AEAPS) with two nitrogen-based ligands. All three classes have a silanol tail that can combine with an alkoxysilane consolidant which can provide mechanical and UV stability (Figure 1). Passivants were applied to calcite powders in single or multiple coatings with and without a coating of an alkoxysilane consolidant in order to test their potential for retarding dissolution under simple simulated weathering conditions. Our goals were (1) to test the 301 Mat. Res. Soc. Symp. Proc. Vol. 462 0 1997 Materials Research Soc
Data Loading...
