Abstract
Salt crystallization is one of the harshest deterioration mechanisms affecting heritage materials, causing impressive decay patterns and the loss of a high thickness of original materials. Although salt damage has been widely investigated in the literature from the theoretical and experimental points of view, the solutions to mitigate this problem are still extremely limited. In the present paper, a new biopolymeric treatment based on chitosan was tested on two kinds of porous limestones widely used in historic architecture, aiming at inhibiting the crystallization of sodium sulphate inside the stone and promoting the formation of salt efflorescence over the surface, rather than harmful subflorescence inside the pore network. The treatment was applied to the bare stone and also after an inorganic pre-treatment based on the formation of hydroxyapatite in the stone. Hydroxyapatite was recently proposed for the consolidation and protection of carbonate stones and here it is expected to provide an effective anchoring layer for the chitosan coating on the pores surface, and also to prevent the calcite washout from the stone and hence the removal of chitosan. The effect of hydroxyapatite alone was also tested, for comparison’s sake. Treated and untreated stone specimens were subjected to two different accelerated salt crystallization tests, one based on crystallization cycles (wetting-drying cycles) and the other one based on continuous capillary absorption of a saline solution (“wick effect”), evaluating the results in terms of weight loss, efflorescence formation, and changes in porosity and mechanical properties. The results showed that all the treatments are compatible with the stones, and the combined treatment (hydroxyapatite + chitosan) is extremely promising for the prevention of salt damage.
Highlights
In addition to capillary rise, salts can be autochthonous in the materials or can come from external sources such as deicing salts used in roads, from chemical products used in agriculture, from marine spray in the case of buildings near the coast or from the chemical reactions between building materials and air pollutants (Charola and Bläuer, 2015; Charola and Wendler, 2015)
The weight increases of the samples are almost negligible for all the treatments, due to the low concentration of the solutions applied, which is positive as it suggests that only a thin coating of the two substances formed over the pore surface, avoiding any pore clogging effect
Lecce stone and Globigerina limestone were used, being representative of highly porous lithotypes widely used in heritage buildings and severely affected by salt damage onsite
Summary
One of the major causes of deterioration and damage of cultural heritage is the presence of salts within historic building materials such as stone, brick, mortars and plasters, and in more recent materials such as concrete (Charola, 2000; Sandrolini and Franzoni, 2007; Doehne and Price, 2010; Espinosa-Marzal and Scherer, 2010; Franzoni, 2014; Charola and Bläuer, 2015; Charola and Wendler, 2015; Lubelli et al, 2018; Andreotti et al, 2019). Biopolymeric Treatment Against Salt Deterioration of salts, due to evaporation and/or cooling of saline solutions, and in particular by the solubilization/crystallization cycles, due to the strong sensitivity of salts to the thermo-hygrometric conditions This leads to formation of internal mechanical stress in the material, protracted over time. Salts can enter building materials in different ways, the main one being certainly the capillary absorption of moisture from the soil (Charola, 2000; Doehne and Price, 2010; Franzoni, 2014; Charola and Bläuer, 2015; Charola and Wendler, 2015) In this case, water coming from rain, underground water sources or broken pipes, filters through the ground dissolving the salts contained in it. In addition to capillary rise, salts can be autochthonous in the materials (for example, in Portland cement or in bricks) or can come from external sources such as deicing salts used in roads, from chemical products used in agriculture, from marine spray in the case of buildings near the coast or from the chemical reactions between building materials and air pollutants (Charola and Bläuer, 2015; Charola and Wendler, 2015)
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