Gilles Fronteau

4D imaging and quantification of pore structure modifications inside natural building stones by means of high resolution X-ray CT.

Weathering processes have been studied in detail for many natural building stones. The most commonly used analytical techniques in these studies are thin-section petrography, SEM, XRD and XRF. Most of these techniques are valuable for chemical and mineralogical analysis of the weathering patterns. However, to obtain crucial quantitative information on structural evolutions like porosity changes and growth of weathering crusts in function of time, non-destructive techniques become necessary. In this study, a Belgian historical calcareous sandstone, the Lede stone, was exposed to gaseous SO(2) under wet surface conditions according to the European Standard NBN EN 13919 (2003). Before, during and after the strong acid test, high resolution X-ray tomography has been performed to visualize gypsum crust formation to yield a better insight into the effects of gaseous SO(2) on the pore modification in 3D. The tomographic scans were taken at the Centre for X-ray Tomography at Ghent University (UGCT). With the aid of image analysis, partial porosity changes were calculated in different stadia of the process. Increasing porosity has been observed visually and quantitatively below the new superficial formed layer of gypsum crystals. In some cases micro-cracks and dissolution zones were detected on the grain boundaries of quartz. By using Morpho+, an in-house developed image analysis program, radial porosity, partial porosity, ratio of open and closed porosity and equivalent diameter of individual pore structures have been calculated. The results obtained in this study are promising for a better understanding of gypsum weathering mechanisms, porosity changes and patterns on natural building stones in four dimensions.

Black-crust growth and interaction with underlying limestone microfacies

Abstract Black crust growth mechanisms on three French building stones are described using diagenetic models that reveal the close links between the crust–stone interfaces and the microfacies of the host limestone. Each limestone is representative of a specific sedimentary facies and displays mixed pore structure: crinoidal limestone (Euville limestone), oolitic limestone (Savonnières limestone) and bioclastic matrix-supported limestone (Courville limestone). The crinoidal limestone is mainly made of well-developed calcitic cement (spar syntaxial calcite) with low macrocroporosity (15–20 vol. %). The oolitic limestone is macroporous (30–40 vol. %), oolite nucleus being partially or completely dissolved. The third building stone studied is less porous (14 vol. %) but presents a significant microporosity. Weathering of the Euville limestone proceeds primarily through preferential exploitation of cleavages and microcracks and secondly by progressive recrystallization in the areas separated by previous gypsum fill-in (micro-box work). In the Savonnières limestone (oolitic limestone), gypsum recrystallization could occur without microcracks: elements are sometimes nearly totally weathered, while the palisadic calcitic cement surrounding the oolites was still preserved. In the matrix-supported limestone (Courville limestone), weathering could deeply affect the matrix while elements are not weathered. When a layer of microcrystalline calcite is observed on the surface of the limestone, however, the black crust growth seems to be limited to the external part of the stone. Porous characteristics of limestones directly depend on sedimentary and diagenetic phases developed. The pore network controls moisture movement and also determines the reactivity of the stone to gypsum recrystallization.

Holistic approach of pre-existing flaws on the decay of two limestones

This study aims to understand the influence of the microfacies and the determination of pre-existing flaws on the weathering behavior of two types of limestones. Therefore, both Lede and Noyant limestones were independently weathered by strong acid tests and freeze-thaw cycles. In order to characterize the weathering patterns inside the stones, a combination of high resolution X-ray CT, SEM-EDS and thin section microscopy was used. The advantage of high resolution X-ray CT is its non-destructive character and the obtained 3D structural information. By using this technique, a time-lapse sequence of the weathering patterns was obtained for both gypsum crust formation as well as crack formation due to freezing and thawing. This way, a clear link could be made with the initial non-weathered state. Thin section microscopy and SEM-EDS provided additional chemical information. The focus of this study lies in the processes that occur in the bioclast fragments in the stone and the influence of the surrounding cement or matrix. The results show that weathering patterns vary for both limestones although the causes of weathering were similar. In case of the Noyant stone, the weathering by crystallizing gypsum was mainly restricted to the microporous matrix of the stone, while in case of the Lede stone, several foraminifera and shell fragments were preferentially recrystallized. In general, the underlying microstructure determines the weathering pattern of the stone.

Assessment of new protective treatments for porous limestone combining water-repellency and anti-colonization properties

Water is known to be the main deteriorating agent of building geomaterials. Owing to the presence of water, the alteration phenomena observed particularly on limestones are transportation of soluble salts, dissolution of calcite and development of micro-organisms. In restoration works, several chemical treatments can be used, such as biocides to remove existing biofouling and water-repellents to prevent the ingress of water and its deteriorating effects. The aim here was to combine these two properties in one treatment that could be applied at the end of the restoration work and to develop treatments based on natural products and that are safer for the users and the environment. Products with water-repellent and/or anti-colonization properties were developed and tested on a bioclastic limestone both in laboratory and outdoor conditions. They were compared with commercial water-repellents and a preventive anti-colonization treatment. Samples were exposed for 2 years in a forest environment, where the hydrophobicity and the colonization were evaluated every 6 months. In the laboratory, anti-colonization effects were determined through algae growth measurements. The product that confirmed an efficient hydrophobicity and anti-colonization effect will be applied on a restored site to test it under real conditions.

Laminar gypsum crust on lede stone: microspatial characterization and laboratory acid weathering

Gypsum crusts are typical decay forms on limestone in polluted urban environments. Their origin and relation to the stone facies have been thoroughly investigated in the past three decades. Here, we present the combined use of novel techniques for a microspatial structural, chemical and mechanical characterization of a laminar black gypsum crust on a sandy limestone. These techniques include i.a. X-ray computed microtomography, X-ray Fluorescence micromapping, permeability mapping and the scratch test. They reveal the typical architecture of a laminar gypsum crust, with an outer opaque layer, a subsurface gypsum crystallization layer and a deeper cracked zone passing irregularly into the sound stone. Gypsum crystallization is mostly restricted to an irregular outer zone with an average thickness of 500µm, while cracks are found deeper within the rock. These cracks decrease the rock strength to more than 27.5mm below the surface. Because of their surface parallel orientation and thickness of >10-100µm, they create the potential for surface scaling. This is shown by a laboratory acid test where the crack network extensively developed due to further exposure to an SO2 environment. The use of novel techniques opens potential for the study of different decay forms and can be used for stone diagnosis with regards to conservation studies.

Stone uses in Reims Cathedral: provenance, physical properties and restoration phases

Abstract Reims Cathedral is a major monument in the NE of France originally built with local Lutetian limestone. The recent closure of the last quarries has made restoration using the same stone more complicated. The restoration stones used currently are Lutetian limestones from the centre of the Paris Basin (Saint-Pierre-Aigle and Saint-Maximin stones). Mapping of the Cathedrals façades confirmed the data from ancient manuscripts: Courville stone was the original building stone, but several other local stones from various quarries and beds were also used. As a follow-up to this mapping, Lutetian limestones from five disused quarries were sampled for petrophysical characterization tests: thin section analyses, porosimetry, capillary and drying kinetics. The petrophysical properties of the limestones showed differences between the two main local stone types (Ditrupa limestone and miliolids limestone) and also between the quarries. This study addresses the difficulty of selecting new stone for restoration. Should ancient quarries be re-opened? If so, which ones?

GIS-based variability of building materials towards the Île-de-France cuesta (Paris Basin, France): inventory, distribution, uses and relationship with the environment

Abstract The Pays rémois in the eastern Paris Basin is an administrative area of 1394 km2; in the surroundings of Reims (France). Two main geological substrata are separated by the Île-de-France cuesta: the Tertiary substratum in the western part is composed of various types of geomaterials (clay, sandstone, limestone, burrstone), whereas the substratum in the eastern part is composed only of Cretaceous Chalk. A field survey in each commune of the Pays rémois identified 26 building materials documented in a Geographic Information System database (GIS-database) that includes information about them (lithology, petrographical and petrophysical data, weathering) and the corresponding buildings (e.g. town, georeferenced data, building type, position on the façade). The spatial analysis of the building materials’ distribution (Standard Deviational Ellipse) with GIS identifies their uses and the criteria established for the selection of the materials: availability, efficiency, workability and durability. Some lithologies were preferred for particular buildings or selected for their efficiency in specific positions. The study also defines the relation between the stones’ origin (local and non-local stones) and their application. This database is useful to establish stone replacement strategies in the Pays rémois.

Chlorophyll Fluorescence and Colorimetric Analysis for Monitoring the Algal Development on Biocide-Treated Stone

The present study used two non destructive technics, chlorophyll fluorescence and colorimetry to assess the Chlorella vulgaris growth on stone surface treated by experimental treatments for preventing biofouling. A protocol for laboratory tests was set up, consisting of inoculating treated slabs stone with a suspension of an algae culture (Chlorella vulgaris), that is a well known monument colonizing organism. The biofouling test was carried out under fluorescent lights for four weeks at room temperature which was 20°C and monitored by chlorophyll a fluorescence and colorimetry analysis. These techniques are rapid, non-invasive, and reliable. Treatments were introduced by mixing a tetraethoxysilane base with different ingredients: chitosan and/or silver nitrate for biocide effect and/or hydrophobic silica for water repellency. Results revealed four different patterns of algal development. The stones treated with the product containing chitosan and silver nitrate did not show any difference in their fluorescence signal as compared with the untreated stones (control). The stones treated with the product containing only silver nitrate showed biocide effect two weeks after the start of the experiment. The product containing silver nitrate, chitosan and hydrophobic silica at low concentrations completely inhibited algal development in the long term, the similar product with the same compounds at a higher concentration only delayed algal development by 1-2 weeks. This suggests that low concentrations of the products have a synergistic effect, that is lost if they occur in excess. In the present study, chlorophyll a fluorescence proved to be a valuable tool in detecting damage in the photosynthetic system of organisms and as a useful complement to other conventional measurements such as colorimetry.

Impact of lateral variations of geologic facies on water resources in homogeneous basins: example of tertiary plateaus in the Paris Basin

Spatial disparity of run-off conditions during low-water period in homogeneous watersheds depends on the degree of incision of valleys, but may also be influenced by a structural partitioning or some lateral variations in the aquifer lithology. For small basins (area <300 km2), often hydrometrically unsupervised, campaigns of serial flow measurements, gauging, conductivity and temperature are achieved. The Ardre basin, localised in the Tertiary formations of the Paris Basin is used as a workshop basin. Results validated by hydrological and physico-chemical profiles show increased hydrological yields in the basin which correspond to the increase in the thickness of aquifers and their porosity. These lateral variations of facies and of aquifer thicknesses are related to Eocene sedimentary conditions (Lutetian) for homogeneous basins, which explain the spatial disparity of water resources.

Geochemical characterization of “Lorraine limestones” from the Saint-Paul Cathedral of Liège (Belgium): assumptions for the true provenance of the building stones

We investigate the provenance of the ochre building stones called “Lorraine limestones” used for architectural specific elements of the Saint-Paul Cathedral in Liège (Belgium) between the thirteenth and fifteenth century. A multi-analytical approach, including archives study, petrography and Rare Earth Elements geochemistry, has been performed to characterize the stones used in the church and to compare them with ochre limestones outcropping in the north-east of the Paris Basin. Our study suggests that a very restricted geographical area near the ancient port of Donchery (Ardennes, France) including Dom-le-Mesnil and Hannogne ancient quarries of Bajocian limestones (Middle Jurassic) should clearly be regarded as the potential origin location for the stones used in the Saint-Paul Cathedral.