During the last few decades, due to remediation procedures, SO2 emissions in the atmosphere have decreased, unlike NOx. Air pollution has changed. Indeed, the aim of this research is to assess the effect of NOx and their interactions with SO2 on stones, particularly on limestones used in Champagne-Ardenne (France) during the restoration processes. Three French building limestones (Courville, Dom and Savonnieres) and one reconstituted stone were exposed during 28 days to four strong acid atmospheres i.e. two H2SO3 solutions with different concentrations and two mixed atmospheres with different proportions of HNO3 and H2SO3. These tests produced an intensive acid attack on the stone, allowing the observation of short-term salt precipitation and the evolution of stone properties. Each day, one sample was removed from the acid atmosphere to measure the concentration of SO4(2-) and NO3(-) by ion-chromatography. The surface changes were assessed before and after the tests by 3D scanning and observations with electron microscopy. X-ray microtomography has been performed in the Centre for X-ray Tomography (UGCT) and the Department of Geology at Ghent University (Belgium) in order to observe the penetration of salts and the consequences in stones porosity. First observations showed that exposure to acid atmosphere, led to gypsum efflorescences. Obvious colour changes occurred in all tests. Salt crystallization entailed a change in the porous system, which was evidenced by 3D, mercury porosimetry and X-ray microtomography. Difference between weathered and fresh stone was highlighted by Ion chromatography analyses.
The St Bavo's Cathedral tower in Ghent, Belgium, is currently under restoration. This tower was built during the fifteenth and sixteenth centuries over a period of 73 years, using Eocene white stone. Restorations over the last two centuries introduced different Jurassic limestones from the Paris Basin. Documentation of these interventions, however, is very poor for the period before the twentieth century, and documentation on the original construction is non‐existing. Nevertheless, the use of the different stones in their historical context forms a typical example of the historical mindset on the use of white stone, representative for Ghent and by extension northwestern Belgium.
An increasing amount of imported natural building stones are being used in Western Europe, often as a replacement of more traditional, local building stones. Unlike for these traditional stones, which have been used under the prevailing climatic conditions in Western Europe, the durability of these imported stones is largely unknown. Therefore, it is essential to study their behaviour under these climatic conditions in order to predict their weathering resistance. The chemical and structural properties of these new building materials need to be determined and their behaviour under changing environmental conditions needs to be studied. When these materials are being used in Western Europe, they have to resist to significant mechanical stresses due to the imbibition of de-icing salt solutions. These de-icing salts are very frequently used during winter in Western Europe, while temperature fluctuates between freezing and thaw conditions. In this research, focus has been laid on the multi-disciplinary characterization of the compact Kandla Grey layered sandstone. This stone is recently frequently imported from India to Belgium. Besides traditional techniques, (according to European Standars for natural stone testing) highly advanced research techniques such as µ-XRF and HRXCT were used to characterize and monitor the changes under different external conditions such as freezing, thawing and salt crystallization. The results of this study demonstrate that the structural properties of the laminations inside Kandla Grey have an influence on the resistance of the stone to frost and salt weathering. Based on these results, it can be concluded that Kandla Grey can be vulnerable to these types of weathering under the current climatic conditions in Western Europe.
Abstract The relative permeability behavior of rocks with wide ranges of pore sizes is in many cases still poorly understood and is difficult to model at the pore scale. In this work, we investigate the capillary pressure and relative permeability behavior of three outcrop carbonates and two tight reservoir sandstones with wide, multimodal pore size distributions. To examine how the drainage and imbibition properties of these complex rock types are influenced by the connectivity of macropores to each other and to zones with unresolved small‐scale porosity, we apply a previously presented microcomputed‐tomography‐based multiscale pore network model to these samples. The sensitivity to the properties of the small‐scale porosity is studied by performing simulations with different artificial sphere‐packing‐based networks as a proxy for these pores. Finally, the mixed‐wet water‐flooding behavior of the samples is investigated, assuming different wettability distributions for the microporosity and macroporosity. While this work is not an attempt to perform predictive modeling, it seeks to qualitatively explain the behavior of the investigated samples and illustrates some of the most recent developments in multiscale pore network modeling.
Abstract Salt weathering is one of the most important causes of deterioration in the built environment. Two crucial aspects need further investigation to understand the processes and find suitable measures: the impact of different climatic environments and the properties of salt mixture crystallization. We demonstrate the importance of kinetics in quantifying crystallization and dissolution cycles by combining droplet and capillary laboratory experiments with climate data analysis. The results proved that dissolution times for pure NaCl are typically slower than crystallization, while thermodynamic modelling showed a lower RH eq of NaCl (65.5%) in a salt mixture (commonly found in the built heritage) compared to its RH eq as a single salt (75.5%). Following the results, a minimum time of 30 min is considered for dissolution and the two main RH eq thresholds could be applied to climate data analysis. The predicted number of dissolution/crystallization cycles was significantly dependent on the measurement frequency (or equivalent averaging period) of the climatic data. An analysis of corresponding rural and urban climate demonstrated the impact of spatial phenomena (such as the urban heat island) on the predicted frequency cycles. The findings are fundamental to improve appropriate timescale windows that can be applied to climate data and to illustrate a methodology to quantify salt crystallization cycles in realistic environments as a risk assessment procedure. The results are the basis for future work to improve the accuracy of salt risk assessment by including the kinetics of salt mixtures.
