Inorganic coatings for the protection of marble surfaces from deterioration

Abstract

The dissolution of marble, a calcitic natural stone is an important cause contributing to the weathering process of the building materials of monuments, statues and other artefacts of cultural heritage. Protection against dissolution may be effected either through the use of water soluble additives or by the development of coatings of sparingly soluble salts of inhibitors of dissolution with calcium. In the present study coatings were prepared by the reaction of anions known to protect marble against dissolution: phosphate, phosphonate and oxalate anions were used in acidic media for the development of the respective calcium salts. The present study is a contribution both to understanding the presence of coatings found on ancient monuments and to the development of novel methods for the protection of calcitic materials. For the phosphoric acid treatment, the increase of the calcium concentration resulted in the re-precipitation of the calcium phosphate dehydrate phase forming coatings of 100 ?m converting to the less acidic octa-calcium phosphate. The efficiency towards dissolution was demonstrated with dissolution kinetic experiments at constant undersaturation, which showed partial dissolution of the calcium phosphate coating. The use of (1-hydroxyethylidene)- 1,1 diphosphonic acid (HEDP) both in solution and in the form of coatings also resulted in reduced rates of dissolution for Pentelic marble slabs. Two different calcium-diphosphonate salts were formed depending on the appliced pH. Finally, oxalic acid treatment resulted in the formation of a layer of calcium oxalate monohydrate. The independence of the solubility of this salt on pH makes it a good candidate for the protection of marble against dissolution in polluted environment.Rastvaranje odnosno propadanje mermera (prirodne stene) uglavnom je posledica atmosferskog uticaja na materijal od koga su najcesce napravljeni spomenici, statue i drugi objekti od posebne kulturne vaznosti. Sprecavanje ovog nepovoljnog uticaja atmosfere moze se ostvariti bilo koriscenjem u vodi rastvornih aditiva ili razvojem i primenom pogodnih prevlaka koje nastaju od slabo rastvornih soli kalcijuma. Ove soli kalcijuma inhibiraju rastvaranje mermera. U ovom radu su pripremljene prevlake od kalcijuma u reakciji sa odgovarajucim anjonima (fosfati, fosfonati i oksalati) koji u kiseloj sredini formiraju odgovarajuce kalcijumove soli. Rezultati ovih istrazivanja predstavljaju doprinos, kako razumevanju prisustva slicnih prevlaka utvrdjenih da postoje kod nekih antickih spomenika ali i u razvoju novih metoda zastite kalciniranih materijala. Tako se tretiranjem povrsine mermera fosfornom kiselinom povecava koncentracija kalcijuma tokom procesa njegovog ponovnog talozenja u obliku dihidrata kalcijum fosfata koji stvara prevlaku debljine 100 ?m umesto manje kiselog okta-kalcijum fosfata. Efikasnost smanjenja rastvaranja je objasnjena eksperimentima koji su imali za cilj utvrdjivanje kinetike rastvaranja u uslovima konstantne, a manje od ravnotezne zasicenosti rastvora. Pokazano je da pri tome dolazi samo do delimicnog rastvaranja prevlake od kalcijum fosfata. Upotrebom (1-hidroksietiliden)- 1,1 difosfonske kiseline (HEDP), kako u rastvoru, tako i u obliku odgovarajuce prevlake, pokazano je da se smanjuje brzina rastvaranja ploce od Pentelskog mermera. Utvrdjeno je da stvaranje razlicitih soli kalcijum difosfonske kiseline zavisi od pH rastvora. Tretman sa oksalnom kiselinom dovodi na kraju do formiranja sloja kalcijum oksalat monohidrata. S obzirom da rastvorljivost ovih soli ne zavisi od pH rastvora proces nastajanja navedenih soli cini ih dobrim kandidatima za zastitu mermera od rastvaranja, a posebno u nepovoljnim uslovima zivotne sredine.