Weathering of miocene ferruginous sandstone in ancient buildings in northeastern Belgium

Abstract In the Hageland and the Southern Campine in NE Belgium, goethite cemented ferruginous sandstone of local origin has been used in ancient monuments. Although this material, compared to limestone, may be considered as chemically inert, close examination of weathered building stone's revealed that features such as black crusts were present. Chemical and mineralogical analyses indicate the presence of gypsum, while nitrates and chlorides are practically of no importance. This was confirmed during investigation of thin sections. Gypsum is present in the surfacial zones of building stones, where it causes important damage due to its crystallisation in existing pore space. Many glauconite grains are broken up. Finally, the goethite cement itself is disrupted and the whole structure of the stone is fragmented. In buildings with mixed use of ferruginous sandstone and limestone, the source of the ions is obvious. However, considerable amounts of gypsum may be present in parts of monuments where limestone was not applied. Lime mortars are thought to be the major source of ions in that case. Crystallisation pressure of gypsum, provided from chemical weathering of calcareous material elsewhere in monuments, is thus found to be an important deteriorating agent of building stone.