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AFF; International audience; Size effect is a known problem of mechanics affecting the extrapolation of laboratory results into conclusions regarding full structures. In the context of salt weathering, a gap exists between the understanding of the crystallization of a single crystal in a controlled environment and the damage of porous material due to a zone of coalescence of such process. The recent advances in quantitative x-ray micro-tomography analysis has been a step forward in the comprehension of precipitation damage in porous media. However, given the scale of the process, a search for ever higher resolution motivates the use of smaller samples. While the use of samples measuring few milimetres or even micrometers is useful for the visualization of different phases of the damage cycle, it might not be representative of the process taking place in full size pieces. We present data on the influence of sample dimension on the process of salt damage. Cylindrical samples of Lavoux limestone were prepared in four sizes (4x8 mm, 8x16 mm, 25x50 mm, 50x50 mm) and subjected to wet-and-dry cycles until the onset of damage. All samples were sealed in all faces except the upper side. During each cycle, a 3,4 weight % sodium chloride solution was dispensed on the free face in an amount equivalent to 60% of the volume of the respective porous space. The samples were kept for one hour at laboratory conditions (T=22ºC; RH=60%) allowing for the solution to penetrate the sample. Subsequently, a 23 hours drying step was carried out at 40 ºC and 40% relative humidity. At the end of the weathering, quantitative x-ray micro-tomographies were compared with prior scans of the samples in their unweathered state to assess the deterioration in all sizes of samples. The assessment of the relation between sample size and sample damage should highlight the limitations of the use of very small samples.

COM; International audience

AFF; International audience

COM; International audience; Environmental conditions are one of the most important factors that lead to the deterioration of salt contaminated stones or masonry materials. In particular, when environmental conditions such as humidity, exposure to rain or rising damp vary, salts in contact with water (liquid or vapour) can dissolve and cause damage to the material by crystallization upon drying. In this paper this challenging coupled problem is modelled improving the multiphase model developed in [1, 2]. The model - fully coupled, highly non-linear and time dependent - is referred to a Representative Elementary Volume, and takes as primary variables the relative humidity, the temperature, the mass fraction of the dissolved salt and the concentration of precipitated salt. This tool is used to simulate experiments [3] in which sodium chloride contaminated sandstones, put in contact with liquid water and water vapour until complete saturation, are dried at different humidities. The good agreement with experimental evidences proves the effectiveness of the proposed model.

COM; International audience

COM; International audience

ACT; International audience

ACT; International audience

We employ laboratory X-ray micro-computed tomography (μCT) during climate-controlled salt weathering experiments to acquire data on the kinetics of drying and salt precipitation and the distribution of crystals within the pore space of Mšené sandstone. For that purpose, a custom-designed setup was built at the UGCT’s scanners of the Ghent University Centre for X-ray Tomography (UGCT) that allows to acquire 4D scans while drying. Samples were initially capillary saturated with a saturated NaCl-solution and subsequently dried at 20% RH and at 50% RH, at room temperature. These RH-values are representative for winter and summer conditions for the salt NaCl, which is not temperature sensitive. Different salt precipitation dynamics result in different drying kinetics at the two RH’s. These crystallization and transport dynamics can be directly linked as revealed by the 4D X-ray μCT datasets.

AFF; International audience

COM; International audience

COM; International audience

COM; International audience

Experiments on evaporation of aqueous sodium chloride solutions from micro capillaries demonstrate for the first time that there is a metastability limit to the solubility of NaCl. The supersaturation of the solution reached at the onset of crystallization is found to be very high, almost twice the saturation concentration. At this concentration, we observe a peculiar form of crystal growth: the very rapid growth of a single Hopper crystal in which many crystallites grow outwards from a single central nucleus. The high supersaturations achieved at the onset of crystal growth are found to be independent of the size, shape and surface properties of the micro capillary. Keywords: metastability limit, NaCl, supersaturation, salt damage

COM; International audience

ACT; International audience