Multidisciplinary characterization of gypsum crust on Lede stone (Belgium)

The Lede stone is an arenaceous limestone which occurs in marine sandy deposits (Lede Formation, Eocene, Tertiary; Belgium). Its framework consists of quartz grains, micro- and macrofossils, glauconite and a low percentage of dense minerals such as apatite and zircon. It is commonly characterized by a microsparitic cement of calcite. Since Roman times the Lede stone was used as a building material and is dominantly present in Gothic architecture. Nowadays, huge amounts of this stone are found in the Belgian built cultural heritage, thus being the subject of many restoration and renovation works. Since it has an substantial carbonate content, the stone is sensitive to sulphation. This causes formation of a gypsum crust and the dissolution of calcite cement towards the inside of the stone. At the same time cracks can develop during long-time atmospheric exposure. In this way, the subsurface can be divided in several zones, each with their degree of alteration. When a stone surface is cleaned during restoration works, it is important to know the structure of the gypsum crust and the cracks, the depth of the affected area and the stability conditions of each of the subsurface zones. A multidisciplinary case-study was performed on a Lede stone with a thin gypsum crust. The structure and composition of the superficial crust as well as the subsurface were studied with different techniques (X-ray tomography/radiography, X-ray fluorescence, scanning electron microscopy and optical microscopy) to determine the degree of weathering. Chemical alteration shows to be the most ‘common’ zonation with a superficial black crust layer, a second thicker gypsum alteration zone and the sound stone in the deeper part of the stone. Sub-horizontal cracks are present within both the gypsum crust and the sound stone.

Jaar van publicatie:2012