In recent times, nanomaterials have been applied in the construction and maintenance of the world´s cultural heritage with the aim of improving the consolidation and protection treatments of damaged stone. These nanomaterials include important advantages that could solve many problems found in the traditional interventions. The present paper aims to carry out a review of the state of art on the application of nanotechnology to the conservation and restoration of the stony cultural heritage. We highlight the different types of nanoparticles currently used to produce conservation treatments with enhanced material properties and novel functionalities.
The basic principle of the patrimony is that the cultural heritage is an incalculable and integral legacy to our future. The obligation to preserve and provide access to it is an absolute necessity. Nowadays, it warns a growing concern for the situation that the degradation of this rich and fragile heritage has reached. In addition, conservation of stone heritage is always a delicate and complex challenge. Multiple variables have to be taken into account to identify the problems, define the necessary conservation actions and to select materials and procedures to be used. The variety of factors to be analyzed includes the intrinsic stone properties, the state of conservation, the degradation mechanisms and the environmental factors. Thus, it is necessary to select the appropriate materials and procedures for a correct conservation treatment. Compatibility, depth of penetration, durability, effect on liquid water and vapor permeability, in addition to biological resistance, application method, or health matters are important factors to take into account.
While the development in material science has generated important nanostructured materials long time ago, conservation of cultural heritage was, until recently, mainly based on the traditional conservation and restoration treatments. These traditional methods, such as the use of synthetic polymers, often lack the vital compatibility with the original substrate and a durable performance. More recently, nanomaterials are being applied in the construction and maintenance of the architectural heritage with the aim of improving the consolidation and protection treatments of damaged building materials. Firstly, Baglioni and co-workers were pioneering in the application of nanoparticles for the cleaning and consolidation of mural paintings (
Consolidation is one of the most important conservation treatments carried out on stone heritage. This treatment is used to improve the cohesion of weathered stone when serious decay patterns and in-depth cohesion loss are present. Consolidation treatments are the most risky conservation actions due to their irreversibility and the likeliness to cause undesired effects, such as the possible loss of the surface that was supposed to be preserved. This risk justifies the numerous studies that have been developed on stone consolidants. The application of nanotechnology in the cultural heritage conservation is marked by the possibility to design consolidant products highly compatible with the original stone substrate. Moreover, when particles have dimensions of about 1–100 nanometers, the materials properties change significantly from those at larger scales. In this sense, nanomaterials have larger surface areas than similar masses of larger-scale materials, which increase their chemical reactivity. In addition, these nanomaterials present the possibility to penetrate deep into the damaged stone materials due to the particle size.
On the other hand, atmospheric pollution is dramatically increasing the external degradation of monuments and buildings due to the deposition of organic matter and other contaminants on the stone substrates (
Degraded stone heritage showing: a) salt weathering of building stone, and b) visible microbial growth and staining.
Since ancient times, these properties are known in different elements (Ag, Ti, Cd, Fe, Pd, Zn, Pt, Co, etc). The nanoparticles must have the following attributes: stability and sustained photoactivity, biologically and chemically inert, nontoxic, low cost, suitability towards visible or near UV light, high conversion efficiency and high quantum yield, could be react with wide range of substrate and high adaptability to various environment and good adsorption in solar spectrum (
The nanoparticles can also be used as additives in construction materials or to modify the synthetic polymers in order to enhance its outdoor performance, and its mechanical and thermal properties (
An important aspect in order to establish the suitability and the effectiveness of the treatments based on nanoparticles is to take into account the synthesis method used for obtaining the nanoparticles. The influence of experimental synthesis parameters that can modify the morphologies, particle sizes, agglomeration level and crystalline structures of the nanoparticles obtained has to be analyzed.
This review focuses on the current state of knowledge in the application of different types of nanoparticles for the improvement of conservation strategies of the cultural built heritage.
One of the most commonly used inorganic consolidants are the products based on hydroxide nanoparticles due to its compatibility with a large part of the built and sculptural heritage. Calcium hydroxide (also known as lime wash putty) is one of the oldest products used in construction, mainly as a binder in mortars (joint mortars, renderings, wall fillings, etc), in mural paintings, as a consolidant product, together with other materials such as rammed-earth.
It can be used to restore the cohesion loss by filling the porosity of calcareous stones (limestones, marbles, and mortars). When calcium hydroxide is exposed to atmospheric CO2 in wet conditions, the layered network of its hexagonal packing crystal structure favors the incorporation of such CO2 to the structure producing the carbonation process, which consists of reacting and transforming into calcium carbonate (
A schematic illustration of the thin film of dolostone before and after treatment with hydroxide nanoparticles.
Progress in the use of consolidating products based on calcium hydroxide nanoparticles for the consolidation of carbonate stone materials has been increasing in the last years. One of the main objectives has been to improve its properties to ensure the success of the process, including control of morphology and size of the nanoparticles (
One of the main factors that have been proven to be the clue to produce the carbonation has been the effect of the relative humidity. Carbonatation reaction produces different polymorphs of calcium carbonate, which have different stability and morphology. When it comes to be applied in the stony substrate, the behavior is different depending on the relative humidity. Therefore, in dry environments is slower giving rise to low crystallinity calcium carbonate (
Due to the influence of relative humidity on carbonation, not always achieved to form calcite, as it is the case of nanoparticles that are exposed to very low humidity relative (33%) and which still conserve calcium hydroxide although they have been exposed for a long period of time (
In order to accelerate the carbonation process, nowadays are being tested different routines, which include the use simple accelerators as yeast, which contributes to accelerate the carbonation in calcium hydroxide nanoparticles (
The results of treatment with Ca(OH)2 nanoparticles from commercial products obtained by colloidal synthesis (Calosil®) applied on dolomite stones from Redueña (Spain) with different concentrations have demonstrated the development of surface with carbonatic efflorescences, which has been observed in samples treated with the product in high concentration (15 and 25%) and heterogeneous porosity. Changes in the color and brightness, for instance, are reason enough to discourage the application of this treatment in high concentrations on this type of porous dolostones (
It is essential to conduct a previous study of mineralogical and chemical compatibility between consolidating product/stone to avoid developing unaesthetic neomineralizations in the stone. Therefore a mineralogical and textural control can early diagnose possible pathologies developed as a result of no suitable treatments (
ESEM Images and roughness profiles obtained of treated and untreated Laspra dolostone (Spain).
Although portlandite (Nanorestore®) has been applied to dolostones and calcitic dolostones getting an improvement in their hydric and physical properties more effectively in wet than in dry environments, without changes in color, their effect has produced phenomena of dolomite dissolution and calcite recrystallization (
These nanomaterials include significant advantages that could solve many problems found in the traditional interventions of the stone consolidation. However, the physical-chemical compatibility of these consolidants with the stone material is one of the most important requirements for its use in heritage preservation. Therefore, the new goal consists in the development of new consolidant products based on Mg(OH)2 nanoparticles in order to preserve the dolostone used in built heritage (
In the field of the mortars, the commercial products(Calosil®, Nanorestore®, Merck®) have been tested on dry hydrated lime and fine crushed calcareous aggregate (
Dolostone represents one of the materials most used in the history of construction in the world. The damage experienced by these materials generates a significant loss of internal cohesion that makes the application of consolidant products necessary. As mentioned, consolidant products based on calcium hydroxide nanoparticles are one of the most commonly used nanomaterials in stone consolidation. To our knowledge, it is important to take into account that in dolomite crystals, layers of carbonate ions alternate with layers of magnesium and calcium ions (
The dedolomitization is another risk arising as a result of the use of calcium hydroxide nanoparticles for the dolostone consolidation. In alkaline conditions, the dolomite can react with alkali hydroxides causing a fine intergrowth of brucite, calcite and alkali carbonates. This process would regenerate alkalis and thereby permit a continued reaction with the dolomite (
SEM images and XRD patterns of magnesium hydroxide nanoparticles obtained by hydrothermal method, before and after 15 and 31 days at 75%RH.
Barium hydroxide (Ba(OH)2) has been widely used as a consolidant product for carbonate stones during decades and represents a potential alternative to calcium hydroxide given its greater solubility in water (
On the other hand, barium hydroxide has also been applied as additive material in conservation mortars. Thus, the effect of barium hydroxide on the setting mechanism of lime-based mortars has been studied by Karatasios et al. (
The effectiveness of barium hydroxide as an additional activator in alkali Ca(OH)2-activated ground granulated blast-furnace slag (GGBS) mortars mixed and cured at air-dried and water conditions has also been reported (
The synthesis of strontium hydroxide (Sr(OH)2) nanoparticles obtained by bottom–up approach, and their use as consolidant product and also as a de-sulphating agent for stone, mortars and wall paintings were proposed (
Recently, studies about the behavior of commercial water-based silica dispersions, with different average radius (from 55 to 9 nm), as consolidant products on limestone (Lecce stone) have been carried out by Falchi et al. (
Nanosilica has been also applied in the field of mortar technology. Zendri et al. (
Consolidant products based on metal alkoxides (calcium alkoxides) have been recently proposed (
Sassoni et al. (
Recently, the role of the application technique on the effectiveness of HAP for limestone has been carried out (
Also, the application of hydroxyapatite and strontium-hydroxyapatite nanoparticles as consolidant product in highly porous limestones (chalk stone) was studied by Ion et al. (
Alkoxysilane-based formulations, and specially methyltrimethoxysilane (MTMOS) and tetraethoxysilane (TEOS), have been the most widely used stone consolidants mainly due to their ability to penetrate easily into porous matrix. These consolidants are absorbed by the stone, hydrolyzed by water to form silanols, which then polymerise in a condensation reaction and form a polymer that increase the cohesion of the stone material (
In order to minimize the tendency to crack of TEOS, novel nanomaterials have been synthesized by a synthesis strategy in which the sol-gel transition is carried out in the presence of a non-ionic surfactant (n-octylamine). The n-octylamine acts as a template to make the pore size of the gel network uniform, and as a basis catalyst of the sol-transition on the stone surface. In this way, the cracking of consolidants is successfully prevented (
Another approach that has been developed in order to optimize the gel phase physical properties is to reduce the cracking of the consolidant products through the addition to different metal colloidal oxides particles to the TEOS-based polymeric resins (
The use of nanoparticle loading in polymeric resins with enhanced photoactivity for self-cleaning and protection applications on building materials will be discussed further below.
Recently, Verganelaki et al. (
More recently, these researchers showed the optimization of the synthesis route by the formation of nano-calcium oxalate simultaneously with the polymerization of TEOS via one-pot sol-gel method (
The hydrophobic character of synthetic polymers, such as hybrid siloxanes or silicone polymers, can be enhanced by incorporation of inorganic oxide nanoparticles, such as silicon dioxide (SiO2), aluminum oxide (Al2O3), tin dioxide (SnO2), and titanium dioxide (TiO2), which present a high water-repellent capacity, generating superhydrophobic coatings (
Thus, during the last years, the use of polymeric materials with hydrophobic properties has been widely studied (
Static contact angle of a water drop of untreated (a) and (b) treated limestone by a water repellent product.
Superhydrophobic surfaces exhibit certain wetting characteristics originating from very high static contact angles (>150°) and very small values of contact angle hysteresis (<10º) (
One important point in the use of nanoparticles polymer composites on the patrimonial stoneworks is to take into account the changes that can generate the presence of nanoparticles in the polymer matrix during the polymerization process (
In this context, relevant superhydrophobic materials have been developed in recent years by the reduction of the surface free energy and roughening the surface. Acrylics, fluorinated acrylic polymers, siloxanes and fluoropolyethers are usually employed as protective coatings. Recently, De Ferri et al. (
Works carried out by B.J Sparks et al. (
On the other hand, it is important to take into account not only the water repellency capacity of the treatment, but also the petrophysical properties of the treated stone substrates and the durability of the treatments in order to prevent damage to the stone heritage (
The TiO2 in the anatase form is widely used for self-cleaning applications, due to their photocatalytic properties for the destruction of organic pollutants (
When photocatalyst TiO2 absorbs Ultraviolet (UV) radiation from sunlight or illuminated light source (fluorescent lamps), it will produce pairs of electrons and holes. The free radicals or electrons that result react with foreign substances on the surface and produce chemical reactions that ultimately decompose the foreign substances through oxidation. Organic matter can be quickly decomposed. The loosened and decomposed material can then be washed away by normal rainfall or other means (
Schematic illustrations of self-cleaning processes on untreated and treated stone surfaces.
TiO2 acts against the onset of degrading processes due to stain, biological attacks and pollution molecules such as NOx, SOx, BTX (
The trend nowadays is to develop hybrid materials which promote simultaneously photocatalytic action and thus serve as self-cleaning, biocidal and for mechanical strengthening. These materials alter the surface and thus the contact angle, so that may protect from the action of water. This is the case of combining powders or suspensions of TiO2 with SiO2, polymers, oligomers or silicon alkoxides.
Nano TiO2 is commercialized for its application in marbles, limestones and dolostones. However, its efficiency is different depending on the synthesis procedure used (
The incorporation of polymeric matrices for dispersing the TiO2 represents a coating technology with hydrophobic, consolidating and biocidal properties (
The combination of oligomers and TiO2 nanoparticles for obtaining mesoporous titania-silica composites with photocatalytic properties, (ethoxysilane oligomers mixed with TiO2 in the presence of a nonionic surfactant) have resulted to be an effective adhesive with crack-free coatings for stone. These coatings give self-cleaning properties to stone, improving its mechanical resistance and giving hydrophobic properties (
TiO2 mixed with a silica sol interlayer (TEOS as precursor) is used for limestone protection. Promising results are reported for application in calcarenites (
In clay brick surfaces, the inhibitory effect of TiO2 nanocoatings against microalgal growth under weak UV exposure conditions is not effective. But, under optimal UV exposure conditions prevents the adhesion of microorganisms on the treated substrates (
Very few studies with TiO2 nanoparticles have been conducted so far to remedy the problems of pollution in mortars. One of them shows how the efficacy varies depending on the way of application, either as suspension, as powder or mixed. However adhesion problems in the surface layer have been observed. Furthermore, high content of anatase would be necessary for having a good performance, which would result very expensive (
Biodeterioration is one of the main degradation processes of outdoor stone heritage. The microorganisms are involved in the deterioration of stone due to the fact that they secrete enzymes and organic acids during their metabolic processes, highly harmful for the stone heritage. Thus, in recent years important studies have been centered on innovative strategies consisting in the application of the combination of consolidants, water repellents, and nanoparticles with antimicrobial activity. In this context, nanostructured metal oxides might be an effective tool for controlling the biodeterioration because present great advantages (high surface to volume ratio, small particle size, and longer lifetimes (
The possibility of embedding copper (Cu) nanoparticles in polymer matrices in order to obtain preservative treatments against biological growth for stone heritage has been reported by different authors (
On the other hand, the use of silver (Ag) nanoparticles as antimicrobial coatings for stone heritage has been studied in recent times. Silver is widely used as a strong antimicrobial for a long time. In the field of cultural heritage, the capacity of silver nanoparticles grafted to Italian Serena sandstone surfaces to inhibit bacterial viability has been studied by (
Also, nanostructured zinc oxide (ZnO) has shown to be bio-safe and bio-compatible and possesses antibacterial and antifungal activity due to their surface-chemical activity (
The use of nanotechnology applied to the stone heritage conservation field creates possibilities to produce conservation treatments with enhancing material properties and novel functionalities. These nanomaterials include important advantages that could solve many problems found in the traditional interventions through the development of new nanomaterials or the improvement of the traditional treatments with the incorporation of nanoparticles. In this way, as has been described in the work, innovative applications have reached by using nanoparticles in building materials such as stone deterioration, environmental pollution remediation and self-cleaning and anti-microbial effect. Thus, nanotechnology has been shown to have an important impact in the cultural heritage and construction sector, improving the durability and enhanced performance of construction materials, energy efficiency and safety of the monuments.
It is essential to examine the behaviour and the stability of the nanoparticles obtained when exposed to environmental factors as the relative humidity, time of exposition and CO2 concentration. Moreover, an exhaustive knowledge of the petrophysical properties of the stone materials and their durability before and after the treatments by nanoparticles are crucial factors to take into account in order to assess the effectiveness treatments. This knowledge is crucial when designing and implementing the interventions and materials for the safeguard of cultural heritage.
Finally, it can be pointed out that the study of the effectiveness, compatibility and durability of these new nanomaterials are necessary in order to avoid the use of inadequate treatments, which modify the aesthetic, physical and chemical properties of stony materials, causing new pathologies. Also, the knowledge of the industrial production, the capacity utilization, and the price of raw materials are important aspects need to be considered. The human health risks and environmental implications resulting from the use of the new nanomaterials should be taken into account when designing treatments based on nanoparticles.
This study has been supported by the Autonomous Region Program of Madrid the Geomateriales 2 (s2013/mit_2914) and Multimat-Challenge (S2013/MIT-2862), the Innovation and Education Ministry (Climortec, BIA2014-53911-R and MAT2013-47460-C5-5-P) the Complutense University of Madrid’s Research Group: “Applied Petrology for Heritage Conservation Research Group (921349) and the Carlos III University of Madrid´s Research Group: “Powder Technology-UC3M”.