Sacrificial lime plasters for the prevention of salt damage in globigerina limestone walls : case study of the Nymphaeum, Argotti Gardens, Floriana

Abstract

This study is an investigation of the physical properties of eight sacrificial plasters designed for the Nymphaeum at the Argotti Gardens, Floriana. The Nymphaeum is an 18th century Globigerina Limestone structure which was originally decorated with a unique Baroque mosaic. Due to the main cause of deterioration, salt damage, most of the mosaic is lost, and the stone substrate is severely powdering and spalling. Therefore this study is proposing a sacrificial lime plaster treatment to reduce the salt damage currently affecting the original materials. The study started by on- site observations, and site analysis to help understand the materials and technology, the causes of deterioration, and how these are affected by the surrounding environment. A salt and moisture survey of the Globigerina Limestone walls was carried out to determine the salt types and their distribution in the walls, and petrographic plaster analysis was carried out to determine the composition of the original mosaic plasters. Furthermore, a one year environmental monitoring programme determined the temperature and relative humidity changes, since the climatic conditions of the site may be one of the instigators of deterioration. Based on the knowledge gathered on the site, and literature regarding sacrificial plasters, a list of working properties and performance criteria was established to be the requirements needed for the ideal sacrificial plaster treatment specifically designed for the Nymphaeum. Plaster components were selected due to their properties that presumably may aid in producing a good performing sacrificial plaster. These materials included lime putty as the non- hydraulic binder, Upper Coralline Limestone sand that served as an aggregate, Perlite thut was used both as an aggregate and as a pozzolanic additive, and Trass that served as a pozzolanic additive. From these materials, eight different plaster mixes were designed by changing the proportion of the aggregates and additives.