The Ledestone: deterioration and the need for replacement stones

The Lede stone, also known as the Balegem stone, is one of the most important natural building stones in the northern part of Belgium and the western part of The Netherlands. Occurring in the Belgian Eocene Lede Formation, this sandy limestone has frequently been used in monuments from the 13th century to the present. As a result, the Lede stone is prominently present in Flanders' cultural heritage (Nijland et al., 2006; Cnudde, 2009). The Lede stone has proven to have good constructive performances. However, since the Industrial Revolution, the Lede stone has suffered from something every limestone is sensitive to: the acidification of the urban atmosphere. The sulfur dioxide (SO2) concentration in the atmosphere has increased since the end of the 19th century through burning of fossil fuels. Only since 1990 SO2 levels are declining again in Europe as a result of sulfur control politics (Georgoulias et al., 2009). Sulfur dioxide dissolves in water to form sulpherous acid (H2SO3). In humid environments, this sulpherous acid enters the inside of the stones through its pores, where it reacts with the calcium carbonate (CaCO3) from the limestone to form a calcium sulphate solution as gypsum (CaSO4.2H2O). This calcium sulphate solution migrates through the stone and precipitates as a crust on the outer surface when the solution evaporates. CaCO3(s) + SO2(g) + 2H2O(l) + textonehalfO2(g) leftrightarrow CaSO4.2H2O(aq) + CO2(g) This process weakens the stone just beneath its surface trough dissolution of the calcite cement. In addition, the surface crust absorbs soot and fly ash out of the atmosphere and acquires a black colour. This decreases both the stone quality as well as its aesthetic expression. Renovation and restoration works become necessary and the Lede stone must often be replaced by fresh stones. Replacing Lede stone by Lede stone is best to retain the buildings' authenticity. Reserves however, become sparse since only one exploitation site, in Balegem, remains these days. Therefore, there is a need for alternative replacement stones. In the past, Lede stone has mostly been replaced by the French Massangis stone. However this stone has a white to pale gray color, whereas the Lede stone acquires a brown patina. The question thus arises whether no better alternatives could be found. For this, series of technical test are available to test whether stones comply some criteria and resemble one another. A couple of tests together with some possible replacement stones were selected in order to design a 'replacement stone series' from suitable tot non-suitable to replace Lede stone, based on well chosen stone characteristics. The stones selected for these tests are the Anstrude, Boisvillon, Massangis Roche Jaune, Massangis Roche Jaune Claire and Savonnières. Boisvillon and Massangis Roche Jaune Claire seem most suited for Lede stone replacement, whereas Anstrude (due to its pale appearance) and Massangis Roche Jaune (due to its weak frost resistance) should be avoided. Savonnières can be suited in specific cases. It is clear that better suited replacement stones can be found when the search for alternatives continues, based on a combination of representative laboratory tests.