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

Pore structure evolution in andesite rocks induced by freeze-thaw cycles examined by non-destructive methods

In this paper, we compare the values of petrophysical properties before and after 100 freeze-thaw (F-T) cycles, as well as recorded length change behaviour and temperature development on a vacuum-saturated fractured andesite rock sample taken from the Babina Quarry in Slovakia using a specially-constructed thermodilatometer, VLAP 04, equipped with two HIRT-LVDT sensors. We also used non-destructive visualization of the rock pore network by µCT imaging in order to study the development of the pore structure and fracture network in pyroxene andesites during the freeze-thaw process. The results show that the andesite rock samples, due to good fabric cohesion, low porosity, and low pore interconnection, showed good resistance against frost-induced damage. However, it must be stated that the main process causing disintegration of this type of rock is fracture opening, which is caused by internal stresses induced by water-ice phase transition. The overall residual strain recorded after 100 F-T cycles was not significant, however, the increase of 31 pp in volume of the fracture showed us that repeated freezing and thawing can lead to long term deterioration in terms of subcritical crack growth in brittle-elastic solids like pyroxene-andesite rocks.

In situ Raman characterization of minerals and degradation processes in a variety of cultural and geological heritage sites

We test the capabilities of in situ Raman spectroscopy for non-destructive analysis of degradation processes in invaluable masterpieces, as well as for the characterization of minerals and prehistoric rock-art in caves. To this end, we have studied the mechanism of decay suffered by the 15th-century limestone sculptures that decorate the retro-choir of Burgos Cathedral (N Spain). In situ Raman probe detected hydrated sulfate and nitrate minerals on the sculptures, which are responsible for the decay of the original limestone. In addition, in situ Raman analyses were performed on unique speleothems in El Soplao Cave (Cantabria, N Spain) and in the Gruta de las Maravillas (Aracena, SW Spain). Unusual cave minerals were detected in El Soplao Cave, such as hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O), as well as ferromanganese oxides in the black biogenic speleothems recently discovered in this cavern. In the Gruta de las Maravillas, gypsum (CaSO₄·2H₂O) was identified for the first time, as part of the oldest cave materials, so providing additional evidence of hypogenic mechanisms that occurred in this cave during earlier stages of its formation. Finally, we present preliminary analyses of several cave paintings in the renowned "Polychrome Hall" of Altamira Cave (Cantabria, N. Spain). Hematite (Fe₂O₃) is the most abundant mineral phase, which provides the characteristic ochre-reddish color to the Altamira bison and deer paintings. Thus, portable Raman spectroscopy is demonstrated to be an analytical technique compatible with preserving our cultural and natural heritage, since the analysis does not require physical contact between the Raman head and the analyzed items.

Linking groundwater pollution to the decay of 15th-century sculptures in Burgos Cathedral (northern Spain)

Precipitation of salts-mainly hydrated Mg-Na sulfates-in building materials is rated as one of the most severe threats to the preservation of our architectural and cultural heritage. Nevertheless, the origin of this pathology is still unknown in many cases. Proper identification of the cause of damage is crucial for correct planning of future restoration actions. The goal of this study is to identify the source of the degradation compounds that are affecting the 15th-century limestone sculptures that decorate the retro-choir of Burgos Cathedral (northern Spain). To this end, detailed characterization of minerals by in situ (Raman spectroscopy) and laboratory techniques (XRD, Raman and FTIR) was followed by major elements (ICP and IC) and isotopic analysis (δ(34)S and δ(15)N) of both the mineral phases precipitated on the retro-choir and the dissolved salts in groundwater in the vicinity of the cathedral. The results reveal unequivocal connection between the damage observed and capillary rise of salts-bearing water from the subsoil. The multianalytical methodology used is widely applicable to identify the origin of common affections suffered by historical buildings and masterpieces.

Comparison between petrophysical properties, durability and use of two limestones of the Paris region

Most buildings of architectural heritage in Paris and its surroundings are built with Lutetian limestone. Several historic buildings of the ‘Vexin Normand’ region show Lutetian limestone in the upper parts of their walls, while the lower parts are built with a chalk known as ‘Pierre de Vernon’. The ‘Pierre de Vernon’ appears up to the first metre, although in exceptional cases it can reach the middle height of a building. Commonly, chalks exhibit low durability due to their high porosity. However, ‘Pierre de Vernon’ is supposed to have greater durability than other chalks because of its historic use for basement construction.

The objective of this research was to understand the use of the ‘Pierre de Vernon’ in the lower part of the constructions. A petrophysical characterization of Vernon chalk and Lutetian limestone was carried out, focusing mainly on the differences in porosity and water uptake. Salt crystallization tests were done to contrast their response to decay. Colour and roughness measurements and scanning electron microscope observations were performed.

Results show that the different porous networks of these two limestones lead to a high contrast in their hydric properties and responses to decay, and the use of Vernon chalk in the lower sections of buildings has been found to be appropriate.

A pore-scale study of fracture dynamics in rock using X-ray micro-CT under ambient freeze-thaw cycling

Freeze-thaw cycling stresses many environments which include porous media such as soil, rock and concrete. Climate change can expose new regions and subject others to a changing freeze-thaw frequency. Therefore, understanding and predicting the effect of freeze-thaw cycles is important in environmental science, the built environment and cultural heritage preservation. In this paper, we explore the possibilities of state-of-the-art micro-CT in studying the pore scale dynamics related to freezing and thawing. The experiments show the development of a fracture network in a porous limestone when cooling to -9.7 °C, at which an exothermal temperature peak is a proxy for ice crystallization. The dynamics of the fracture network are visualized with a time frame of 80 s. Theoretical assumptions predict that crystallization in these experiments occurs in pores of 6-20.1 nm under transient conditions. Here, the crystallization-induced stress exceeds rock strength when the local crystal fraction in the pores is 4.3%. The location of fractures is strongly related to preferential water uptake paths and rock texture, which are visually identified. Laboratory, continuous X-ray micro-CT scanning opens new perspectives for the pore-scale study of ice crystallization in porous media as well as for environmental processes related to freeze-thaw fracturing.

Replacement stones for Lede stone in Belgian historical monuments

The Lede stone (Lutetian, Eocene) is an important historic building stone used in the NW of Belgium. In Ghent, it is dominant in the post-Romanesque built cultural heritage. Its use was restricted several times by socio-economic constraints. Since quarrying and production started to cease from the seventeenth century, periodic revivals favoured the use of Lede stone for new buildings and restoration projects. Sulphation is the main threat for the Lede stone as black crusts are the most common degradation phenomena on this arenaceous limestone. Around the turn of the nineteenth century, the Belgian Gobertange stone was the most widely used replacement material. Throughout the twentieth century, the use of replacement material shifted towards French limestones. However, their colour, texture and petrophysical properties differ from the Lede stone, for which a natural yellow–brown patina is very characteristic. In order to solve this mainly aesthetic issue, several new stone types are used as replacement stone in the twenty-first century, while many others have been suggested. It remains, however, difficult to find a replacement stone that matches the visual and petrophysical properties of the Lede stone. One remaining Lede stone quarry pit has increased its activity since 2011, offering the opportunity to use new Lede stone as replacement stone.