بررسی مکانیسم تخریب ناشی از نمک های محلول در بناهای تاریخی

This paper is based on a research on the problems of salt attack and rising damp in heritage masonry buildings. Amongst common building defects occurrences in heritage buildings, salt attack and rising damp are considered as the most challenging, particularly in building conservation. Rising damp and salt attack, a worldwide phenomenon, are major causes of decay to masonry materials. Moisture and salts in masonry walls can result in damage. The presence of water-soluble salts in porous building materials is one of the principal problems of conservation. Salt attack and rising damp together pose a serious threat to buildings especially with regard to load-bearing walls constructed of brick, stone and mortar. The problem of salt attack is closely associated with rising damp. On its own, rising damp can make buildings unsightly and unpleasant to occupy. The situation is made much worse if there are appreciable quantities of soluble salts present, because the rising damp will carry salts up into the masonry to were the damp evaporates. Moisture from the rising damp makes the salts existing in the building material soluble, or the ground water which contains salts finds its ways through the building wall. Salt damp is caused when moisture from the ground is carried up into the wall of a building. This process involves the movement of water from a high concentration, being the ground, to a low concentration, being the porous wall, which is called capillary action. Rising damp occurs as a result of capillary suction of moisture from the ground into porous masonry building materials such as stone, brick, earth and mortar. They may cause unsightly deterioration of building exteriors and interiors as well as possible building structure failures if left untreated. Their crystalliz tion is in fact the cause of those phenomena of deterioration that so often appear on surfaces: lack of cohesion, scaling, flaking and bulging. When the water transporting the salts through porous materials evaporates, due, for example, to changig climatic conditions, the solution becomes more concentrated. As soon as it becomes supersaturated, the salts crystallize giving rise to both efflorescences on the surface and/or sub-efflorescences below the surface layer. The impact of salt damp is often worse on external wall surfaces, especially those exposed to direct sunlight, where evaporation is higher. This moisture then evaporates on or just below the wall surface leaving the salt residues behind. There the salts are left behind and can often be seen as a white efflorescence on the wall surface. When these salts grow as crystals within the pores of the masonry they can disrupt even the strongest material, leading to fretting and crumbling of the surface. These formations gradually contribute to building dilapidation and reduce the building aesthetic values. The telltale signs of salt attack in a wall are: fretting mortar or stone or brick, bubbling paint, crumbling plaster, and the presence of moisture on the surface of the wall. It is very import nt to know the salt content of deteriorated surfaces both to understand the causes of decay and plan conservation strategies. The nature of the salt provides us with information about its origin and therefore, indicates the possible conservation treatment to adopt. Sources of salts can be the building materials themselves (sand, bricks, mortar), or external sources (soil, materials used for conservation treatment, pollutants in the atmosphere, sea spray, de-icing salts scattered on roads in winter, and products generated by the metabolism of micro- organisms). The results of this research showed that almost all masonry contains soluble salts, principally sulphates, nitrates, chlorides and carbonates of sodium, potassium, calcium, magnesium and ammonium and so, all liquid water present in walls is more or less a diluted salt solution. Sources of these salts may be natural or human induced. But, not all salts generate decay. This depends on their solubility, hygroscopicity, mobility and the hydration level of the r crystalline form.