Abstract
Salt crystallization can produce severe damage in porous stones, with a dramatic impact on cultural heritage conservation. Such damage is related to the fact that repulsive forces arise between the salt crystals and the pore wall, generating a disjoining pressure that frequently exceeds the tensile strength of stone. In this paper, new treatments are proposed, aimed at preventing salt damage by depositing a thin layer of polymeric coatings over the stone’s pore surfaces. These coating are expected to change the surface chemistry, eliminating the repulsion between the growing crystals and the pore wall and hence the development of the disjoining pressure. Several biopolymers were tested on these substrates: silica glass, calcite, and calcite subjected to a pre-treatment with diammonium hydrogen phosphate (DAP), aimed at preventing calcite dissolution and acting as an anchoring substrate for the polymer coating. Selected polymer treatments were applied to porous Globigerina limestone samples, which were subjected to crystallization tests with sodium sulfate, obtaining promising results (i.e., significant reduction in stone damage), especially when the polymers were applied after the DAP treatment.
Highlights
The stress induced by salt crystallization inside porous materials of historical buildings, such as stone, brick and mortar, is one of the most widespread cause of damage in architectural heritage [1,2,3,4,5]
The degree of supersaturation (r) and its variation (Dr%) with respect to the supersaturation of the reference saline solution calculated for the different solutions on the basis of the phase diagram are reported in the table as well
Based on the screening tests, which are summarized in Sect. 5.1.5, some polymer treatments/concentrations were selected and two types of salt crystallization tests were carried out on Globigerina limestone samples, the first one involving crystallization cycles and the second one involving continuous capillary absorption and evaporation of a saline solution
Summary
The stress induced by salt crystallization inside porous materials of historical buildings, such as stone, brick and mortar, is one of the most widespread cause of damage in architectural heritage [1,2,3,4,5]. This stress may frequently exceed the tensile strength of building materials, which is usually quite low, and cause crack formation and propagation, resulting in severe damage that deteriorates both structural and decorative elements [6]. Salts may come from capillary water absorption from the soil (i.e., rising damp) In this case, the wall surface is wet up to the height at which the rate of water supply falls below the evaporation rate [1]. Salt crystallization can be caused by cooling, due to the strong temperature dependence of the solubility of some salt phases [7]
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