Academic literature on the topic 'Subflorescence'

Salt precipitation in porous media is widespread, which has garnered great research attention. However, the mechanisms governing the salt precipitation, water flux, and surface temperature changes in homogeneous and heterogeneous porous media remain unclear. This study investigated the dynamics of salt precipitation, evaporative loss, and surface temperature in homogeneous fine sand (0.1–0.25 mm), coarse sand (1-2 mm), and a heterogeneous column with fine and coarse sand. All sand columns were initially saturated with NaCl solution. The experimental results showed that the salt was precipitated as efflorescence above the surface of the fine sand, whereas it was mainly precipitated as subflorescence below the surface of the coarse sand, causing the unconsolidated sand to form a strong stone-like mass. The evaporated loss was significantly higher in heterogeneous than in homogeneous sand, but this difference in evaporation was insignificant in the stage where vapor diffuses through the precipitated salt to the external air. The salt crust formed not only decreased the surface temperature due to increased albedo by salt precipitation, but also resulted in a more discrete temperature distribution in the porous media. Our research results can promote further understanding of salt precipitation and evaporation in porous media.

The application of non-contact diagnostic methodologies is the current challenge in the frame of the cultural heritage, referred to as preservation, monitoring and restoration. Inspired by the potential shown by infrared thermography in rock mechanics’ non-destructive applications, this paper presents the results achieved by its use for the quick survey of different weathering types affecting natural stones at historical buildings. Infrared thermography allowed recognizing and mapping the different surface temperatures arising from the presence of efflorescence, subflorescence, alveolization, black crusts and bioweathering at limestone and basalt stones. Infrared data were sided by photogrammetric three-dimensional models of surveyed spots, which provided quantitative data on the thickness of rock affected by mechanical weathering, and key correspondence between the two techniques is highlighted. Achieved results show that infrared outcomes are related to different aspects primarily involving the stone face morphology and color, as well as the environmental conditions at the surveying time. Provided interpretations were validated by field visual inspections, which confirmed the good potential of infrared thermography as a quick weathering diagnostic tool. This study can be therefore considered a starting reference for knowledge development in this scientific field.

AbstractCrystallization of salts is a common cause of damage in porous building materials. Understanding of the crystallization mechanism of salts is important in order to prevent or avoid the problem. Subflorescence of salts (i.e., crystallization within the pores of the body) can induce scaling and cracking, while efflorescence (i.e., crystallization in a film of solution on the exterior surface of the body) does not generally affect the coherence and endurance of the building materials.In this paper, we deal with the crystallization behavior of two salts, sodium sulfate and sodium chloride, in two bricks with different capillary porosity. The results reveal quite different crystallization behavior depending on salt and substrate.The supersaturation of the solution is induced in our experiments by evaporation. Indeed, the main reason for the different behavior of these salts is the different ability for supersaturation. Thus, the sodium sulfate solution is prone to be much more supersaturated than sodium chloride. Furthermore, the solution transport, which depends on salt properties, material porosity, pore-clogging and climatic conditions, affects the position of the drying front and, with it, the crystallization front leading to the formation either of efflorescence or of subflorescence. A simulation of the experiments helps us to understand the effect of the influencing factors on the crystallization pattern. Therefore, considering both factors, supersaturation ratio and solution transport, it is possible to predict the different crystallization behavior observed in the experiments.

La cristallisation d'un sel initialement dissous dans l'eau contenue dans des matériaux poreux tels que les ciments, béton, mortiers, briques, ..., est une cause majeure de dégradations intervenant dans les constructions et dans certains éléments de notre patrimoine culturel (monuments, murs, sculptures, fresques, ...). Les sels cristallisés à la surface du milieu poreux (efflorescence) n'affectent pas généralement la durabilité de l'ouvrage mais pose un réel problème esthétique pour les façades des bâtiments. Dans le cas d'une cristallisation interne au milieu, les contraintes exercées sur la matrice solide peuvent endommager l'ouvrage allant jusqu'à l'apparition de fissures. Dans ce contexte, le présent travail porte sur l'évaporation avec cristallisation de sel en milieu granulaire et plus précisément sur l'étude de la localisation des cristaux et des déplacements mécaniques induits. La première partie de cette thèse porte sur l'étude des facteurs contrôlant la localisation des cristaux à la surface d'un milieu hétérogène et s'appuie sur l'identification de deux situations de base. Dans la première situation dite de mèche, le milieu poreux est en contact permanent avec une solution saline qui l'alimente à sa base. Cette situation peut correspondre à des parties basses de murs ou d'ouvrages, lorsque le matériau est proche de l'eau des sols. La deuxième situation dite de séchage peut correspondre à des matériaux situés suffisamment en hauteur dans un mur ou un ouvrage. Contrairement au cas de la situation de mèche, le milieu poreux n'est pas alimenté en solution quand l'évaporation se produit. Ces deux situations conduisent à des localisations exactement opposées: à la surface du milieu grossier en séchage, à la surface du milieu fin pour la situation de mèche. Dans le cas de la situation de mèche, la modélisation proposée montre que la structure de l'écoulement joue un rôle clé dans la localisation de la cristallisation alors que pour la situation de séchage, la désaturation préférentielle du milieu grossier est un effet dominant. Concernant la colonisation de la totalité de la surface du milieu poreux par l'efflorescence, l'étude suggère qu'il existe une vitesse d'évaporation critique au regard de laquelle ce phénomène apparait. La deuxième partie de la thèse est dédiée au phénomène de surrection de surface (" surface heave ") à l'aide d'expériences au sein de cellules quasi bidimensionnelles de type Hele Shaw complétés par des expériences en milieu granulaire confiné au sein de tubes cylindriques. Un modèle de croissance de la subflorescence est proposé dans lequel la loi de croissance de la subflorescence est contrôlée par l'évaporation. Ce modèle prédit une très légère sursaturation en haut de la subflorescence compatible avec l'existence d'une pression de cristallisation. Ce modèle permet d'identifier deux régimes principaux de déplacement: un régime dit de désaturation de la subflorescence et un régime de colmatage. Des expressions simples sont proposées pour estimer la croissance de la subflorescence et donc le déplacement induit. Ces relations permettent de déterminer la sursaturation au sommet de la subflorescence et donc la pression de cristallisation
The crystallization of salt, initially dissolved in water, from porous materials (like cements, concrete, mortar, bricks) is a major cause of degradation occurring in buildings and in certain elements of our cultural landscape (monuments, walls, sculptures, frescoes,). Salt crystallization at the surface of a porous medium (efflorescence) don't affect structure durability but create an aesthetic problem for building construction. However, when salt crystallization occurs inside the porous media (subflorescence), pressure on the solid matrix can damage structures by cracking. In this context, the present work focuses on evaporation with salt crystallization in a granular medium and in particular on the locus of crystals and induced mechanical displacements. The first part of this thesis focuses on the study of the factors controlling the localization of crystals atthe surface of an heterogeneous medium. This study is based on the identification of two basic situations of evaporation. In the first so-called evaporation wicking situation, the porous medium is in contact at its bottom with an aqueous solution and the medium remains fully saturated by the solution during evaporation. This situation may correspond to lower parts of walls or structures, when the material is close to the ground water. The second so-called drying situation can correspond to materials located sufficiently high in a wall or a structure. Contrary to the case of wicking situation, the porous medium isn't supplied with a saline solution when evaporation occurs. These two situations lead to a markedly different locus of the efflorescence formation: on the surface of the coarse medium in drying situation and on the surface of the fine medium for the wicking situation. The study emphasizes the key-role of the velocity field induced in the porous domain in the case of the evaporation-wicking situation. In the case of the drying situation, a key aspect lies in the local increase in the ion mass fraction due to the local desaturation, i.e. the local shrinking of the liquid volume containing the ions. Regarding the colonization of the entire surface of the porous medium by efflorescence, the study suggests that there is a critical evaporation rate against which this phenomenon appears. The second part of the thesis deals with the study of surface heave phenomena using experiments in quasi-two-dimensional Hele-Shaw cell supplemented by experiments in a confined granular medium of cylindrical tubes. A growth model of subflorescence is proposed in which the growing subflorescence is controlled by evaporation and not by the precipitation of salt. This model predicts a very slight supersaturation on the top of the subflorescence compatible with the existence of a crystallization pressure. This model makes it possible to identify two main regimes of displacement: a regime known as desaturation of the subflorescence and a clogging regime. Simple expressions are proposed to estimate the growth of subflorescence and thus the induced displacement. These relationships make it possible to determine the supersaturation at the top of the subflorescence and therefore the crystallization pressure