Mapping the corrosion impact of air pollution on the historical peninsula of Istanbul

This paper presents the most comprehensive study conducted so far for evaluating the corrosion levels related to air quality and the seasonal pollutant (NO₂, SO₂, and O₃) exposure levels over 50 monitoring stations distributed on the historical peninsula of Istanbul. Istanbul has been one of the most important as well as most populated major cities in the world during all the ages. Today, owing to its historical and cultural structures, the historical peninsula of Istanbul has become an attraction point of tourism and is one of the most unique and ancient urban settlement areas in the world. However, the cultural heritage stocks in the peninsula are under the risk of corrosion and critical air pollution level exposure caused by chemical reactions under the multi-pollutant situation of the air pollutants, principally SO₂, NO₂, and O₃, as well as meteorological factors, e.g. humidity and temperature. In the present study, seasonal exposure of NO₂, SO₂, and O₃ pollutants were monitored using passive samplers, and corrosion attack values were calculated using dose-response functions. The geostatistical analyst tool of ArcGIS^® 9.1 was then used for generating GIS-based surface pollution and corrosion distribution maps. Subsequently, two hotspots were clearly identified, the tip of the peninsula and the area around the Ataturk Bridge, which fall under serious corrosion risks for copper, cast bronze, and carbon steel materials. However, no significant risk was identified for cultural heritage materials made of limestone in the peninsula.     

Mapping air pollution effects on atmospheric degradation of cultural heritage

The costs for deterioration and soiling of different materials due to air pollution are huge and the damage to culture targets endangers seriously the rich European cultural heritage. Within the sixth Framework Programme of the EU, the overall aim of the CULT-STRAT project has been to assess and predict the effects of different pollutants on materials and objects of cultural heritage in a multipollutant scenario and to identify indicators and thresholds levels of pollutants. In particular, the present paper reports one of the studies carried out in the CULT-STRAT project at city level and focused on the town of Madrid (Spain). Different maps are shown for the past, present and possible future scenarios: inventory of stock of cultural heritage for each selected material, concentration of selected pollutants (SO₂, NO₂, O₃ and PM₁₀), corrosion (cast bronze) and recession (Portland limestone), exceedance of tolerable degradation thresholds for each material and corrosion-cultural heritage overlapped maps. The model and the methodology developed could be useful if apply it to towns, regions or countries in order to quantify the percentage of Cultural Heritage at risk or to quantify the percentage of the area where corrosion/recession exceeds the established tolerable levels.     

The origin of syngenite in black crusts on the limestone monument King's Gate (Belgrade Fortress,Serbia) – the role of agriculture fertiliser

Black crusts formed on limestone built into the King's Gate represent the most important process in stone deterioration that is occurring in this part of the monumental complex of the Belgrade Fortress (Serbia). Of special importance is the association of salts (namely gypsum and syngenite). Syngenite is a common secondary deposit on granite monuments and on medieval stained glass (i.e., on K-containing materials). However, its appearance over calcareous substrates is not apparent, particularly in cases where cement mortar was not used for bounding. The origins of the potassium and sulphate ions required for syngenite formation are related to meteoritic water, which penetrates the soil above the arch of King's Gate. Water dissolves some soil components and becomes enriched with various ions before coming into contact with the limestone blocks. Enriched water contains two times more K⁺ and SO₄^2? ions than pristine meteoritic water does. The source of the required ions is potassium-sulphate that is present in agricultural fertilisers that are used above the monument. The proposed mechanism for syngenite formation was additionally supported with laboratory experiment. The results of X-ray diffractometry and SEM-EDS analyses of limestone treated with potassium-sulphate solution and sulphuric acid suggest the possibility that the syngenite was formed over calcite:CaCO₃ + K₂SO₄ + H₂SO₄ → K₂Ca(SO₄)₂ H₂O + CO₂↑However, the complex mechanisms of gypsum and syngenite formation under natural conditions (variable concentration of potassium and sulphate ions, intermediates phases, temperature changes, humidity, the amount of disposable water etc.) do not exclude the possibility of syngenite formation over gypsum.     

Degradation of lead-based pigments by salt solutions

Interactions of lead-based pigments with a number of inorganic salts, one of the most dangerous degradation agents of wall paintings, were studied under laboratory conditions. The results were used to interpret colour changes observed in 11th century frescoes from the church of Saint George in Kostoľany pod Tríbečom, the oldest preserved wall paintings in Slovakia. Interactions of selected pigments (lead white, massicot, red lead) with different salt solutions were performed within long-term laboratory experiments. We used a selection of naturally occurring salts (Na₂SO₄, MgSO₄, CaSO₄, NaCl, NaNO₃, Ca(NO₃)₂, Na₂CO₃, K₂CO₃ and urea) and, additionally, a range of synthetic salts that are often applied to the wall paintings during their cleaning and conservation (NaHCO₃, KHCO₃, (NH₄)₂CO₃, NH₄HCO₃). The reaction products were identified by X-ray powder diffraction. Red lead (Pb₃O₄) has a tendency to darken in all salt solutions containing dissolved atmospheric CO₂ due to disproportionation to plattnerite (PbO₂) and cerussite (PbCO₃). Massicot (PbO) in a wet state reacts with atmospheric CO₂ to form hydrocerussite and finally cerussite. Lead white (PbCO₃ and Pb₂CO₃(OH)₂) reacts with sulphates, carbonates and chlorides to form their respective salts in high yield and, sometimes, without any apparent colour change. Samples taken from the dark brown parts of the wall paintings in Saint George's church were analysed using laboratory X-ray powder microdiffraction. The presence of hydrocerussite, cerussite, plattnerite and lead magnesium carbonate was revealed. According to the results of laboratory experiments, the original lead-based pigment of the now darkened parts was red lead.     

Efflorescence on thin sections of calcareous stones

Limestone and marble, still frequently used as building materials are especially vulnerable to the destructive effects of efflorescence. The effect of interaction between five different calcareous stones and corrosive atmospheres has been investigated. A novel technique of stone degradation analysis has been used where thin sections of fresh stone materials were exposed in a corrosion chamber under controlled conditions (temperature, relative humidity (RH), SO₂ and NO₂ concentration). Following 1-week’s exposure; observations of the initial crystallisation were studied by light microscopy, scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD). The results obtained from the surface analysis clearly showed sulphation of the samples and formation of gypsum. Observations of the initial corrosion indicated differences in the location of efflorescence and its shape among and within the samples. The mineralogy, grain shape and size, mineral defects and existence of cracks and pores, all influenced the substrate’s reactivity. The most vulnerable areas and the places where the corrosion started on the calcitic stones were the triple grain junctions followed by grain boundaries, and on the dolomitic marble cracks and pores.     

LIBS-spectroscopy for monitoring and control of the laser cleaning process of stone and medieval glass

On-line monitoring or even closed-loop control is necessary to avoid over-cleaning in case the ablation process is not self-limiting. Therefore, the laser-induced breakdown spectroscopy (LIBS) was used. Basic investigations were carried out on original sandstone samples (Elbsandstein) with strong encrustations as well as medieval stained glass samples (13th century from Cologne Cathedral). The spectroscopic study has shown that the plasma emission can be used for determination of the elemental composition of the ablated material. The plasma was initiated by 248-nm pulses of an KrF-excimer laser (30 ns FWHM). For the spectroscopic analysis, a grating spectrograph in combination with an optical multichannel analyser was used. For the glass and stone samples we obtained a continual alteration of the LIBS spectrum (vanishing of peaks and generating of new element peaks) during the removal process. Thus, certain element peaks can be used to distinguish between encrustation layer and valuable underlying material. To show the potential of LIBS we designed an experimental laser cleaning set-up including closed-loop LIBS control and demonstrated successful automatic cleaning of an original glass fragment.     

Mineralogical and chemical characterisation of the Medicean glass mosaic tesserae and mortars of the Grotta del Buontalenti,Giardino di Boboli,Florence, Italy

The Grotta of Buontalenti (GB) was built at the end of the 1500s during the Medicean period. One of the rooms that constitutes the Grotta is decorated with mosaics composed of tesserae of stained glass. The GB underwent several restorations, which took place between the end of the 1700s and 1980s, which are poorly described and documented, leading to some uncertainties about the extent and phase(s) of restorations of the mosaics. The original tesserae and mortars of the GB were analysed for their mineralogical and chemical composition. Results indicate that glass tesserae have a common sodic base composition. Cluster analyses, however, performed considering the glass base composition (i.e. SiO₂, K₂O, Na₂O, MgO, CaO, and Al₂O₃), identify three groups of samples: A, B, and C. A and B are characterised by chromophores such as Se and Cr suggesting a recent age (50–150 years) of the tesserae. On the contrary, chromophores (Fe, Mn, Cu, Co) and opacifiers (mainly SnO₂, Pb₂Sb₂O₇, Ca₂Sb₂O₇) of the third group (C) indicate a preparation procedure known since ancient times. Based on their mineralogy and petrography, four different types of mortars were recognised. The mortars cannot be precisely dated. However, by matching their composition with that of the glass tesserae, it can be concluded that one type of mortar may be of the Medicean period, whereas the others are relatively recent, probably of this century. This evidence, in accordance with historical data, points to a Medicean age of group C tesserae. Restorations of the GB involved partial substitution of the mortars whereas the glass tesserae were largely recycled with limited addition of new materials.     

Revival of traditional European dyeing techniques yellow and red colorants

Based on both a historic and documentary and a physicochemical approach, the study is attempting to codify well-adapted antique, medieval and traditional textile dyeing recipes; to reconstruct them through a detailed sequence of simple and reproducible operations; and to optimize the methodology at all stages, i.e. extraction of the colouring principles, as well as mordanting and dyeing; the latter being evaluated through standard light and wash fastness assessments, and on the basis of systematically obtained colorimetric data. As colouring agents are considered saffron, turmeric, weld, Persian berries, henna, safflower, dyer's bugloss, madder, scale insects, sappan and Brazil wood. A variety of mordants, namely FeCl₃, SnCl₂, K₂Cr₂O₇, ZnCl₂, CuSO₄, and KAl(SO₄)₂·12H₂O, is anticipated to meet both early and rather recent options, while being easily available. Based on a large collection of ancient proposals and optimized by means of methodical assessments, the standardized dyeing processes proposed are simple, reproducible, and perfectly reliable for use in the preservation of the relevant tangible heritage, and the revival of traditional arts and crafts; and are consequently beneficial to artistic weavers, as well as conservators and restorers of textile products.     

Dimensional studies of specific microscopic fibre structures in deteriorated parchment before and during shrinkage

This paper reports the first systematic study of the dimensions of morphological changed microscopic collagen fibres in historical and new reference parchments with the aim to improve the knowledge on deterioration and the diagnosis of their damage in connection with conservation activities. The dimensions of fibres from the parchments were measured before and during shrinkage, with special emphasis on fibres with degradation characteristics designated as “pearls on a string” and “butterflies”. In addition, measurements of the total shrinkage of the length of fibres and pieces of parchment were also carried out. The observations support the assumption that in vivo transformation of the fibre structure by natural ageing is similar to that taking place when heating parchment and fibres in water. Based on statistical cluster analysis, four subpopulations of pearls representing different stages of deterioration are found. Moreover, the dimensional changes in the specific structures observed at room temperature can be related to specific temperatures in the interval of shrinkage suggesting that the hydrothermal stability of the fibres may be predicted on the basis of the ratio between length and width of the so-called pearls (P_l/P_w). The total shrinkage measured is drastic, in average around 56% for fibres and 43 to 48% for pieces of parchment depending on the direction of the skin. The total shrinkage of fibres corresponds to a mean change in the ratio P_l/P_w from around 10 to around 2.3. Finally, relations between the measured dimensions at the microscopic level with those of collagen at nanoscopic and molecular levels including known and potential chemical splitting points strongly indicate that the formation of the butterfly-like fragments formed by cleavage of two adjacent pearls in the fibre can be ascribed to chemical modification of specific tripeptides in the collagen molecule.     

Silica glass interaction with calcium hydroxide: a surface chemistry approach

The study of the interaction between silica glass and saturated Ca(OH)₂ solutions can be a useful approach to resolve the problem of the adhesion between lime-sand mortar and clay bricks. Since it is reasonable that the silica–calcium hydroxide system well simulates a brick–mortar system, experimental observations concerning the interaction of silica glass and this strong basic solution should be of value for the comprehension of the chemical reactions that could take place at the mortar–brick interface, maybe affecting the adhesion between the two building materials. We demonstrated the effects of saturated Ca(OH)₂ solutions on commercial pure silica glass (fused silica) and on silica films obtained via a sol–gel process by means of dip-coating. Silica samples were dipped in the solutions at different temperatures (room temperature, 60 and 80 °C) and at different time intervals (1 and 21 h) and then they were analysed by means of surface techniques: X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and atomic force microscopy (AFM). It has been shown that Ca(OH)₂ reacts with the silica glass network. The experimental results clearly show a very different behaviour of sol–gel silica with respect to fused silica, probably because of their different nanostructure. Many problems concerning the interaction of silica and Ca(OH)₂ are still to be solved, but the results of this research strengthen the idea that adhesion between lime-sand mortar and clay bricks is caused not only by carbonation of calcium hydroxide contained in the mortar, but even by some chemical reactions involving the brick constituents and calcium hydroxide itself. The final products, calcium silicates, may induce a chemical continuity between lime-sand mortar and clay bricks.     

Analysis of calcium acetate efflorescences formed on ceramic tiles in a museum environment

Salt crystallization is a major cause of damage in porous materials such as stone, brick and ceramics. The paper reports results of an analytical study on tiles of glazed ceramics that are seriously damaged by acetate salt crystallization. Measurements of the ionic composition of the salt mixture in the tiles and in the efflorescences were carried out. Based on the available information on the phase equilibria in the system comprising of the main constituents chloride, nitrate, acetate, calcium and sodium, the crystallization pathways of the various solid phases are traced. It is shown that a combination of qualitative XRD analysis of the phases present in the efflorescences together with a quantitative determination of the ionic composition is sufficient for the quantitative analysis of major crystalline species in the efflorescences, i.e. Ca₃(CH₃COO)₃Cl(NO₃)₂ · 7H₂O, thecotrichite, and NaCl, halite. The concentrations of these salts are obtained from a solution of the mass balance by least squares analysis. Their formation from the salt mixtures present in the tiles that are subject to acetic acid vapor attack is consistent with the solubility diagram of the above mentioned quinary system. Finally, the possibilities of preventive conservation of salt contaminated ceramics in typical museum environments are discussed.     

Synthesis of Ca(OH)2 nanoparticles with the addition of Triton X-100. Protective treatments on natural stones: Preliminary results

Calcium hydroxide is typically used in Cultural Heritage conservation for superficial protective treatments thanks to its conversion into calcium carbonate. Calcium carbonate is, in fact, compatible with many carbonate-based architectonic surfaces, because its characteristics are similar to those of the restored materials. In order to improve calcium hydroxide treatments, Ca(OH)₂ particles with sub-micrometric dimensions (nanolimes), are synthesised by a chemical precipitation process: a sodium hydroxide solution, used as precipitator, is added, drop by drop, to a calcium chloride one. In this paper, a nanometric calcium hydroxide, to be used in stones treatment, is produced adding in the initial solutions a surfactant agent (Triton X-100); the solutions are then mixed together simultaneously, drastically reducing the time needful for preparation. Different contents of surfactant are employed, and the influence on particles dimension and carbonatation process is analysed too. The obtained Ca(OH)₂ nanoparticles are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron diffraction (ED). The Ca(OH)₂ nanoparticles are regularly shaped, hexagonally plated and with side dimensions less than 200 nm: in particular, increasing the surfactant content, the particle dimension reaches values until 20 nm. Comparing nanolime suspensions obtained without and with the addition of the surfactant agent, the last ones have comparable, if not better, performances in terms of average particle size and morphology, crystallinity and reactivity. Afterwards, the alcoholic nanolime suspension obtained by using Triton X-100 is applied on some natural stones; in fact, the aim of this section is to compare the obtained results with those achieved using a nanolime synthesised without the surfactant agent. To evaluate the treatment effectiveness in a preliminary way, standard tests are performed and compared with the same tests previously obtained by the nanolime synthesised without the surfactant agent: “Scotch Tape Test” and capillarity test.     

Smart surfaces for architectural heritage: Preliminary results about the application of TiO2-based coatings on travertine

The development and application of self-cleaning treatments on historical and architectural stone surfaces could be a significant improvement in conservation, protection and maintenance of Cultural Heritage. In this paper, a TiO₂-based coating has been investigated in order to evaluate its possible use as a self-cleaning treatment. This coating was obtained by a sol-gel and a hydrothermal (134 °C) processes and then it was applied on travertine (a limestone often used in historical and monumental buildings) in two ways, obtaining a single-layer and a three-layer treatment, respectively. In order to verify its potential use in the field of Cultural Heritage, the maintenance of appearance properties of the treated travertine surfaces was monitored by colour and gloss analyses. Besides, de-pollution and soiling removal tests were carried out under ultraviolet-light exposure to evaluate photo-induced effects and self-cleaning efficiency. Results seem to allow the use of TiO₂-based treatments on historical and architectural surfaces made up by travertine, where de-pollution and self-cleaning photo-induced effects are well evident, maintaining their original visual appearance. Anyway, before applying TiO₂-based coatings as conservative treatments, further tests are needed especially on their durability, that is mandatory for Cultural Heritage applications. On-site test in an urban environment and accelerated test by weatherometer are currently under way.     

The role of copper on colour of palaeo-Christian glass mosaic tesserae: An XAS study

This work reports mainly the results of an X-ray Absorption Spectroscopy (XAS) study carried out on coloured glass tesserae from the palaeo-Christian mosaic which decorated the votive chapel of St. Prosdocimus (Padova) until its replacement with the current frescoes of Renaissance age, and which is one of the only two known mosaics in the Veneto region (Italy). The study aims at clarifying how the different local structure, oxidation state and quantity of copper influenced colour. Analysis of high-resolution Cu-K edge X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectra showed that copper is present as cuprite (Cu₂O) in orange samples and as metallic copper in red and brown ones. These phases are responsible for both the colour and opacity of the samples. In addition, Cu¹⁺ ions linked to the oxygen atoms of the glass framework were identified in ratios of about 60% and 30% of total copper in orange and red/brown samples, respectively. In blue and green samples, copper is dispersed in the glass matrix as a mixture of Cu¹⁺ and Cu²⁺ ions, and no crystalline phases are visible. In this context, the Cu¹⁺ and Cu²⁺ contents in glass were also quantified thanks to suitable standards, demonstrating that, when Cu²⁺ is the main chromophorous ion, colour intensity is directly correlated to its content in the glass. In particular, in green and blue samples, coloured by copper, Cu²⁺ content varies from 26% to 56% of total copper, and the higher contents of Cu²⁺ are shown by more intensely coloured samples. It should be stressed here that the green colour of the analysed tesserae is given by the physical interaction of blue colour, due to Cu²⁺ ions, and yellow colour, due to Pb antimonates used as opacifiers.     

Analysis of a varnish applied by Nicéphore Niépce to make a print used for photographic purpose transparent

Nicéphore Niépce carried out a lot of meticulous experiments that led him to the invention of photography. In particular, in the 1820s, he obtained heliographs by coating a substrate with a light-sensitive substance, which was then exposed to light under a paper print made translucent with the help of varnish. The objective of the work described here is to determine how Niépce made these paper prints transparent. Did he use his experimental knowledge on photosensitive resins, or did he apply commonly used recipes of his time to make paper transparent? To date, no studies have been carried out on the varnish used by Nicéphore Niépce and no previous research has undertaken analytical investigations on Nicéphore's prints, using Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) and Gas Chromatography/Mass Spectrometry (GC/MS). In this paper, we identify the varnishes used by Nicéphore Niépce on four transparent prints, now belonging to the Nicéphore Niépce museum in Chalon-sur-Saône, France. The varnish he used was based on a diterpenic Pinaceae sp. resin (most probably colophony). The finding is supported by historical knowledge about varnishes.     

Self-cleaning materials on Architectural Heritage: Compatibility of photo-induced hydrophilicity of TiO2 coatings on stone surfaces

Titanium dioxide (TiO₂) can be used to realize transparent self-cleaning coatings on stone surfaces as an active and preventive protection system, limiting cleaning and maintenance actions, reducing their costs in Architectural Heritage. This self-cleaning ability is due to photo-induced hydrophilicity on treated surfaces. The aim of this investigation is to analyze this effect, since it could bring to a greater water absorption, a potential source of damage for stone surfaces. Titania sol, obtained by sol-gel and hydrothermal processes, was deposited on travertine by spray coating, in two different ways. Water absorption by capillarity, static contact angle and a specific surface water absorption analysis were assessed before and after the TiO₂ treatments. The effects of deposited amount of titania on the characteristics of treated surfaces were evaluated. It was shown that there were no evident changes in the substrate reactivity without ultraviolet (UV) light exposure, while it seems that hydrophilicity due to UV light does not lead to higher water absorption, thus encouraging the use of TiO₂ coatings in the field of Architectural Heritage. However, before widely applying this conservative treatment, some further researches are recommended in order to better assess its durability and sustainability.     

Preservation of aged paper using borax in alcohols and the supercritical carbon dioxide system

Selecting an appropriate paper deacidification agent is very important for the deacidification of paper. The use of three deacidification agents (i.e., iso-butylamine, calcium propionate, and borax) is studied for the deacidification of paper using the immersion treatment by investigating the paper surface pH, alkaline residue, paper whiteness, strength, and other performance indicators. Results show the deacidification by borax solution not only results in the promotion of a proper pH range, high level of alkali reserves, and ignorable influence to paper appearance, but also to the enhancement of the mechanical intensities of paper even after artificial aging. Supercritical carbon dioxide (CO₂SCF), as a solvent system, is used in the deacidification of acidic papers using the borax solution of water and alcohol. CO₂SCF improved the deacidification process by significantly improving the pH value and the base residual value. The borax in supercritical fluids can be better combined with cellulose hydroxyl to improve the mechanical properties of paper substantially. The treatment of borax in CO₂SCF could be an alternative for acidic papers. Aside from improving the pH and depositing a sufficient alkaline residual, CO₂SCF also strengthens the mechanical properties of treated papers.     

Deacidification of paper using dispersions of Ca(OH)2 nanoparticles in isopropanol. Study of efficiency

The efficiency of a recently described non-aqueous method for paper deacidification using Ca(OH)₂ nanoparticles in isopropanol was evaluated by pH and colorimetric measurements and by the analysis of the degree of polymerization (DP). Samples of plain paper and paper with iron gall ink were tested. The results were compared with non treated samples and samples submitted to the traditional treatment with saturated aqueous Ca(OH)₂ solution. By comparing the two conservation methods, the aqueous one shows higher neutralization reaction kinetics than the non-aqueous one. The iron gall ink samples maintain their coloration closer to the original after the non-aqueous Ca(OH)₂ nanoparticles treatment, in contrary to the aqueous treatment that changes the ink aesthetics considerably. Artificial aging tests revealed a general increase in the aging stability of deacidified paper samples. The Ca(OH)₂ nanoparticles treatment can be an alternative for papers who can not be treated by the classical aqueous treatment, e.g. papers with water soluble components.     

Non-invasive spatial tissue discrimination in ancient mummies and bones in situ by portable nuclear magnetic resonance

Historic mummies and skeletons have been investigated extensively by modern diagnostic imaging using computed tomography. But magnetic resonance imaging (MRI) has never been applied successfully to mummies in a non-invasive way without tissue rehydration. The aim of this study is to show the feasibility and diagnostic impact of mobile MR technology to historic human tissues.The natural glacier mummy Iceman, a mummified recent human cadaver, historic mummified body parts, historic bones, and living volunteers have been analysed by non-invasive, single sided NMR with the NMR-MOUSE^®. We acquired high-resolution depth profiles and T₂ relaxation curves of the head region of the Iceman mummy in situ in the storage room at the Museum and of the cadaver in the hospital. A spatial differentiation of surface ice layer, cutis, and skull bone up to a depth of 5 mm was possible. In ancient Egyptian mummified specimens, the thickness of a fingernail and a differentiation of a single bandage layer versus the skin underneath were possible. A comparison of depth profiles through different foreheads of mummies, skulls, and living people gives strong evidence, that single-sided NMR with the NMR-MOUSE is a non-invasive technique to determine bone density. Our results demonstrate for the first time the feasibility of non-clinical MRI to visualize historic human tissues in a non-invasive approach.     

Effectiveness of crystallization inhibitors in preventing salt damage in building materials

Salt crystallization in porous materials constitutes one of the most frequent causes of decay of buildings in a wide range of environments. Up to now no definitive solution exists to limit salt crystallization damage, unless of removing either the salt or the moisture. The possibility of making the process of salt crystallization less harmful by means of crystallization inhibitors has only recently been considered. Crystallization inhibitors are known to delay nucleation and to modify the growth rate of the crystals in bulk solution, but their possible application for the prevention of salt decay in porous material is still controversial.The present paper reports a series of systematic investigations performed on three different materials (a limestone, a sandstone and a brick) contaminated with two types of salt (sodium chloride and sodium sulphate) and two types of inhibitors (sodium-ferrocyanide and diethylenetriaminepentakis methylphosphonic acid). Drying experiments have been performed to study the effect of the inhibitors on the salt solution transport. Accelerated crystallization experiments have investigated the possible consequences of the application of the inhibitor on salt crystallization damage (quantified as material loss).Environmental Scanning Electron Microscope (ESEM) has been used to study how the presence of inhibitors affects the crystallization morphology of salt in the material.The results show that the effect of the inhibitor strongly depends on the type of substrate. NaFeC, when applied on NaCl contaminated limestone and brick was able to enhance the drying and to favour the appearance of efflorescences, whereas the effect on the sandstone was very limited. DTPMP enhanced the drying of Na₂SO₄ laden sandstone, but had no influence on the drying of both brick and limestone. These different have been attributed to the properties of the materials, in particular pore size distribution and composition.The effects of the inhibitor on salt crystallization damage were less evident: no differences in surface damage, quantified as material loss, were observed between specimens with and without inhibitor.