Physics

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The drying of porous media is of major importance for civil engineering, geophysics, petrophysics, and the conservation of stone artworks and buildings. More often than not, stones contain salts that can be mobilized by water (e.g., rain) and crystallize during drying. The drying speed is strongly influenced by the crystallization of the salts, but its dynamics remains incompletely understood. Here, we report that the mechanisms of salt precipitation, specifically the primary or secondary nucleation, and the crystal growth are the key factors that determine the drying behaviour of salt contaminated porous materials and the physical weathering generated by salt crystallization. When the same amount of water is used to dissolve the salt present in a stone, depending on whether this is done by a rapid saturation with liquid water or by a slow saturation using water vapor, different evaporation kinetics and salt weathering due to different crystallization pathways are observed.

We investigate the origins of salt damage in sandstones for the two most common salts: sodium chloride and sulfate. The results show that the observed difference in damage between the two salts is directly related to the kinetics of crystallization and the interfacial properties of the salt solutions and crystals with respect to the stone. We show that for sodium sulfate, the existence of hydrated and anhydrous crystals and specifically their dissolution and crystallization kinetics are responsible for the damage. Using MRI and optical microscopy we show that when water imbibes sodium sulfate contaminated sandstones, followed by drying at room temperature, large damage occurs in regions where pores are fully filled with salts. After partial dissolution, anhydrous sodium sulfate salt present in these regions gives rise to a very rapid growth of the hydrated phase of sulfate in the form of clusters, that form on or close to the remaining anhydrous micro crystals. The rapid growth of these clusters generates stresses in excess of the tensile strength of the stone leading to the damage. Sodium chloride only forms anhydrous crystals that consequently do not cause damage in the experiments. KEYWORDS. Salt damage, crystallization, porous media, crystal growth Salt weathering is a major cause of deterioration of rocks and building materials. Many

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

We study the spontaneous nucleation and growth of sodium chloride crystals induced by controlled evaporation in confined geometries (microcapillaries) spanning several orders of magnitude in volume. In all experiments, the nucleation happens reproducibly at a very high supersaturation S~1.6 and is independent of the size, shape and surface properties of the microcapillary. We show from classical nucleation theory that this is expected: S~1.6 corresponds to the point where nucleation first becomes observable on experimental time scales. A consequence of the high supersaturations reached at the onset of nucleation is the very rapid growth of a single skeletal (Hopper) crystal. Experiments on porous media reveal also the

In the title compound, C 38 H 30 N 2 O 2 , the acenaphthylene ring is close to being planar [maximum deviation = 0.1047 (11) Å ]. The dihedral angles between the three benzene rings and the acenaphthylene system are 39.47 (3), 37.65 (3) and 44.47 (3). An intramolecular O-HÁ Á ÁN interaction forms an S(5) hydrogen-bond ring motif. In the crystal, molecules are linked into [101] chains by a set of C-HÁ Á ÁO interactions.

The study of the behavior of sessile droplets on solid substrates is not only associated with common everyday phenomena, such as the coffee stain effect, limescale deposits on our bathroom walls , but also very important in many applications such as purification of pharmaceuticals, deicing of airplanes, inkjet printing and coating applications. In many of these processes, a phase change happens within the drop because of solvent evaporation, temperature changes or chemical reactions, which consequently lead to liquid to solid transitions in the droplets. Here we show that crystallization patterns of evaporating of water drops containing dissolved salts are different from the stains reported for evaporating colloidal suspensions. This happens because during the solvent evaporation, the salts crystallize and grow during the drying. Our results show that the patterns of the resulting salt crystal stains are mainly governed by wetting properties of the emerging crystal as well as the pathway of nucleation and growth, and are independent of the evaporation rate and thermal conductivity of the substrates.

In this study we show that the key to understanding why the same salt can cause damage in some conditions and not in others is the kinetics of recrystallisation. Salt-contaminated porous materials are known to deteriorate with environmental fluctuations. Salts can be naturally present in the materials used for construction or can be derived from external sources. With changes in climate, entrapped salt crystals can once again form a salt solution after contact with liquid water (dissolution) or with water vapour (deliquescence). The solution is mobilized in the porous network and then salts re-crystallize with drying. We present both macroscopic and microscopic experiments assessing the recrystallisation dynamics of NaCl in sandstone with wetting-drying and humidity cycling. Advanced techniques such as high resolution X-ray computed tomography and Scanning Electron Microscopy are used to study the recrystallisation process in the porous network in parallel with the drying kinetics. ...

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 evi...

Salt crystallization is a major cause of damage in porous building materials. Accelerated salt weathering tests carried out in the laboratory are among the most common methods to assess the durability of material to salt decay. However, existing standards and recommendations for salt weathering tests have limitations in terms of effectiveness and/or reliability. In the framework of the RILEM Technical Committee 271-ASC, a procedure has been developed which proposes a new approach to salt crystallization tests. It starts from the consideration that salt damage can be seen as a process developing in two phases: accumulation of the salt in the material and propagation of the decay. In the first phase, salts are introduced in the material and accumulate close to the evaporation surface, while in the second phase damage propagates because of repeated dissolution and crystallization cycles, induced by re-wetting with liquid water and by relative humidity changes. In this paper, the proced...

This recommendation is devoted to testing the resistance of natural stone and fired-clay brick units against salt crystallization. The procedure was developed by the RILEM TC 271-ASC to evaluate the durability of porous building materials against salt crystallization through a laboratory method that allows for accelerated testing without compromising the reliability of the results. The new procedure is designed to replicate salt damage caused by crystallization near the surface of materials as a result of capillary transport and evaporation. A new approach is proposed that considers the presence of two stages in the salt crystallization test. In the first, the accumulation stage, salts gradually accumulate on or near the surface of the material due to evaporation. In the second, the propagation stage, damage initiates and develops due to changes in moisture content and relative humidity that trigger salt dissolution and crystallization cycles. To achieve this, two types of salt were...