Investigation of the processes causing the degradation of the sandstone used on the Thorn church based on mineralogical analysis

Abstract

The rapid degradation of the Thorn-sandstone could be explained by the presence of organics or minerals such as swelling glauconite, oxidized glauconite or non-swelling clays sensitive to hydric variations despite being non-swelling. A variety of tests and observations have been carried out to identify the most likely process of degradation. Conventional optical microscopic observations, XRF and XRD analyses, Methylene Blue tests on rock powder, Methylene Blue staining of thin-sections, and Loss-on-Ignition have been conducted as well as more advanced observations using X-ray micro-computed tomography (micro-CT) and Scanning Electronic Microscopy (SEM). The micro-CT scans allowed the visualization of the grain arrangement in 3D along with the network of micro-fissures spread around within the material. The SEM combined with EDEX measurements allowed the visualization of grains in 2D in great detail along with determining chemical elements at the sub-grain scale. Combined results showed that the Thorn-sandstone consists of quartz grains along with some grains of hematite, rutile or ilmenite. Porosity (excluding fissures) is very low. The grains are tightly packed and surrounded by a very heterogeneous fine-grained brown material. This brown material consists of a mixture of minerals, possibly clays, chlorite , micas, weathered glauconite and feldspar as well as small grains of rutile, hematite or ilmenite. The aforementioned Micro-fissures run through the brown material. The sensitivity of the brown clayey material to water, when exposed to wetting and drying cycles, is likely to explain the degradation of the Thorn-sandstone. Other hypotheses could be formulated to explain the rapid deterioration of the Thorn sandstone.