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 (Figure 2). However, this approach has resulted in many cases unsatisfactory by the poor penetration of the dissolution inside the stone material and its inability to achieve complete consolidation of the damaged area of the material. New products based on nanoparticles can modify its properties and be used in protection and consolidation of geomaterials including marbles, dolostones, limestones (14), mortars. Significant progress has been achieved using colloidal nanoparticles calcium hydroxide-based, commercially distributed among which the most used are Nanorestore (2), Calosil (15) for obtaining calcium carbonate, which acts on the carbonate rock modifying the pore structure. However, it is important to assess whether their characteristics are compatible or not with petrological aspects and mineralogical or local environmental conditions they are exposed and amend the process and therefore its effectiveness (16). Like all products used in treatments of consolidation, consolidating products based on nanoparticles, different agents are susceptible to extrinsic and intrinsic factors that influence its stability and can, at a given time, alter their specific properties, such as relative humidity (17), exposure time (18), temperature changes (19) or its stability with over time (20). Differences in the carbonation process in nanoparticles are associated with several factors among which are the presence of CO₂ in the environment and the presence of water (liquid or vapour phase) (21) as an accelerator of the process, producing differences in the nucleation and growth of different calcium carbonate polymorphs (22). This behaviour affects the reactivity of minerals, their crystallinity and the porosity and petrophysical properties of the stone surface to which nano lime has been applied (23, 18).

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 (24), rate and the effect of carbonation in crystal growth (25), degree of crystallinity, penetration depth, concentration of solution (23), additives, catalysers (26) or solvents, and solubility (27).

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 (28). However, it is not the same in wet environments where carbonation is faster; however it not only produces calcite, the most stable of all polymorphs, but mainly acicular aragonite or unstable polymorphs as vaterite and even hydrated phases of ikaite or monohydrocalcite that can affect to the substrate in the same way as it would do other salt contributing to the loss of cohesion (22).

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 (28, 29).

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 (26).

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 (23). However, when the same product (Calosil^®) is applied in low concentration (1.5 g/l) can result in a natural method, compatible with the petrological characteristics of the substrate, without secondary damages on the stone, being an effective method to improve the durability of carbonate stones (30).

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 (Figure 3) (23).

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 (18). This behavior is expected being its application more suitable to be applied in limestone.

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 (31).

In the field of the mortars, the commercial products(Calosil^®, Nanorestore^®, Merck^®) have been tested on dry hydrated lime and fine crushed calcareous aggregate (32); powdered lime hydrate and river sand (33) and siliceous and lime mortars (34). The effectiveness depends on the type of mortar and humidity conditions to which they are exposed.

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 (35). Variables such as the possible Mg²⁺/Ca²⁺ ion exchange in the stone-consolidant system, the influence of particle size on carbonation kinetics or the surface tension differences in the medium, have to be taken into account. In this way, the application of calcium hydroxide nanoparticles in dolostone could generate nano-calcite recrystallized, leading the dolomite dissolution due to the calcium ion enrichment (18). This is why the effectiveness of the use of calcium hydroxide nanoparticles is reduced in dolostone substrate.

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 (36). In addition, the expansion and the cracking of dolostone substrates occur when the dedolomitization happens (36–38). Works carried out by Berube et al. (39) showed that extensive dedolomitization occurred when dolomite was immersed in a lime solution. Thus, one of the main goals consists in the development of new consolidant products based on Mg(OH)₂ (brucite) nanoparticles, with morphology, crystalline structures, and compatibility suitable for application in the dolostone used in built heritage (30). The consolidant action of this type of nanoparticles is based on their carbonation reaction in the presence of CO₂ and relative humidity, which result is the precipitation of magnesium carbonate (Mg(CO₃) (Figure 4). The consolidation of dolostone can be achieved through the application of a mixed Ca(OH)₂ and Mg(OH)₂ nanoparticles dispersions. In order to obtain a durable consolidation, mixed calcium and magnesium hydroxides, obtained by thermomechanical treatment of lime and via homogeneous phase reaction, respectively, have been applied in a high porosity dolostone (40).

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 (41, 42). Literature suggests that when barium ions are in contact with calcium carbonate crystals, along with carbonation, a significant degree of exchange of barium ions for calcium ions is achieved. Therefore, the precipitated barium carbonate is directly bonded to the surface of calcium carbonate crystals and thus, the new mineral phase increases its adherence to the stone material (41, 43). In this way, the consolidant action of the barium hydroxide is based on its carbonation reaction with the carbon dioxide, which results in the formation of barium carbonate (BaCO₃). However, one of the most important disadvantages of using Ba(OH)₂ as a consolidant product is its low penetration capacity into the stone material (44). Several techniques have been proposed in order to introduce the Ba(OH)₂ into the stone matrix. Lewin (45) promoted the carbonation of the barium hydroxide more effectively by adding urea to the solution due to the fact that the urea produces a slow and controlled precipitation of barium carbonate from the barium hydroxide solution. However, Schnabel (46) evaluated the Ba(OH)₂-urea method applied to marble and porous limestones and suggested that the effectiveness of the method is reduced when it is applied by capillarity in situ. In this context, the use of controlled nano-sized barium hydroxide particles allows an increased penetration depth of the nanoparticles into the stone matrix, due to the small average size of the particles (47, 48). Furthermore, barium hydroxide solutions have been widely applied to the consolidation of gypsum materials and wall-paintings through a procedure known as the Ferroni-Dini method or Florentine method (49). The gypsum can be converted into barium sulfate when using the first step of the Ferroni-Dini method (treatment with ammonium carbonate). Later, the application of barium hydroxide solution allows the stone consolidation. Baglioni et al. (48) suggest the use of mixed formulations of Ca(OH)₂ and Ba(OH)₂ nanoparticle dispersions for the pre-consolidation of surfaces extremely affected by sulfates, after the application of the first part of the Ferroni-Dini method.

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. (41). Their results suggested that Ba(OH)₂ produced a homogeneous binding material, consisting of calcite (CaCO₃), witherite (BaCO₃) and barium-calcium carbonate (BaCa(CO₃)₂), without causing any defects. They also found the carbonation of barium hydroxide much faster than that of calcium carbonate. Moreover, barium hydroxide was found to have a positive effect on the sulphate-resistance of lime-based mortars by blocking Ca²⁺ dissolution, when used as an additive material to the lime binder (50). In this way, the formation of barium phases within the mass of lime binder could increase the durability of conservation mortars.

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 (51). These researchers determined that the addition of Ba(OH)₂ could led the development of alkali-activated mortars with a higher strength due to the formation of 2CaO·Al₂O₃·SiO₂·8H₂O hydrates with higher molar Si/Al and Ca/Al ratios (50). More recently the effects of lime and barium water treatment on the consolidation of lime mortars were researched (52). The authors compared the traditional lime water technology and barium hydroxide treatment with the consolidation results obtained using distilled water and lime water with added metakaolin in mortars. They concluded that the barium water built a denser and connected substance in lime mortars, which significantly increased the tensile strength of the tested mortars.

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 (13). The nanoparticles were dispersed in propan-1-ol solvent and were applied for the consolidation of Lecce limestone (Italy) and for wall paintings (the frescoes of the “Grotta del Crociffiso”, Sicily). The authors determined that Sr(OH)₂ nanomaterials can react with gypsum, producing strontium sulphate, an insoluble sulfate, that replace the more soluble gypsum and can reduce the mechanical stress. In addition, the carbonation reaction of Sr(OH)₂ can produce strontium carbonate (SrCO₃), which provides the consolidation. Moreover, Licchelli et al. (53) studied the use of Ca(OH)₂ nanoparticles and Sr(OH)₂ nanoparticles for the consolidation of bioclastic limestones (Lecce stone), and their durability performance, exposing the treated stone specimens to salt crystallization test. They applied the nanoparticles by using capillary absorption and brushing and compared the results obtained with to those observed after the application of tetraethoxysilane (TEOS) on the Lecce stone. They concluded a satisfactory consolidant action of the inorganic nanoparticles compared to the application of TEOS on the stone. Thus, the treatment with Ca(OH)₂ and Sr(OH)₂ nanoparticles was able to increase the resistance of stone to aging. Besides, the performance of the treatment based on Ca(OH)₂ and Sr(OH)₂ nanoparticles by brushing results in a deeper penetration and a more homogeneous distribution of the consolidant nanomaterial into the stone substrate (53).

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. (54). These researchers determined a low penetration depth of the nanosilica dispersions into the stone matrix (2 mm), and the tendency of silica nanoparticles to form a xerogels layer on the surface of the stone. They proposed that a pre-treatment of the stone surface with ethanol could reduce the stone surface tensions improving the penetration depth and thus resulting in a better consolidant action. In addition, works carried out by Okubo et al. (55, 56) showed the influence of the solvent, temperature and humidity in the colloidal crystals of silica spheres (with a diameter of 110 and 103nm) deposited on a cover glass. They determined an important influence of relative humidity and temperature on the macroscopic drying dissipative patterns of colloidal nanosilica (56). Besides, Zornoza-Indart et al. (57) studied the influence of relative humidity in the commercial product Nanoestel^®, based on an aqueous solution of colloidal SiO₂ nanoparticles, and their consolidant action on bioclastic sandstones (calcarenites). The authors have showed that its consolidant action depend strongly on relative humidity. Therefore, compared to the treatment carried out at higher relative humidity (90%RH), the treatment at lower relative humidity (40%) showed an increased efficiency and less adverse changes, marked by less chromatic changes, and no-formation of a cracker layer over the surface stone. Moreover, the effectiveness of consolidation treatments based on a suspension of nanosilica in water and ethyl silicate dispersed in organic solvent applied on a pyroclastic rock were assessed (58). These authors concluded that consolidants increased the resistance of rocks against salt crystallization, although the treatments induced an increase of crystallization pressure.

Nanosilica has been also applied in the field of mortar technology. Zendri et al. (59) studied the reactivity of a colloidal suspension of silica made of particles with an average diameter of 10–15nm, sodium silicate, and ethyl silicate with calcium carbonate and quartz. These consolidant products form a gel of amorphous silica, which after solvent evaporation are transformed itself in xerogels. Firstly, the authors characterized the three silicatic systems and then, they studied its chemical reactivity mixed with calcite and quartz in 1:1 molar ratio. They determined structural differences on the silica produced by the three consolidants. While the xerogel coming from the colloidal silica formed xerogels with systems mainly related to tri-dimensional systems (Q⁴ type), in the xerogels coming from ethyl silicate and sodium silicate prevalence more planar systems (Q³ type), highly available for a chemical interaction with calcium carbonate and quartz. Despite the consolidant based on ethyl silicate proves more reactivity than others, measures of cohesion revealed few differences between the products. The porous values and the distribution of the treated and non-treated mortars evidenced a greater penetration capacity of the colloidal silica due to their small dimensions. Furthermore, the research on the science of cementitious materials at nano atomic level is an important research topic. In this field, the incorporation of nano-silica in cementitious mortars has been shown to improve the durability and sustainability of cement-based materials, due to its enhanced reactivity and specific surface area, which results in a high degree of pozzolanic activity (60, 61).

Consolidant products based on metal alkoxides (calcium alkoxides) have been recently proposed (62, 63). It is well known that calcium alkoxides react in presence of atmospheric moisture and carbon dioxide to precipitate metal carbonates nano-structured, and alcohols. The limitation of using these alkoxides as consolidant product is their poor solubility and volatility, which results in a low amount of precipitated carbonates in each application. Recently, and with the objective to increase solubility and stability in metal alkoxides, Ossola et al. (64) and Favaro et al. (65) synthesized calcium alkoxides using starting alcohols sterically hindered (2,2-dimetyl-1-propanol) and/or with additional coordinating atoms by using different synthetic pathways. They determined a strong influence of the alkoxide precursors on the resulting vaterite/calcite ratios generated in contact with the atmosphere. Moreover, the potential effectiveness of alkoxides as consolidant product was tested on Carrara marble by ultrasound measurements. These authors also studied the use of commercial magnesium alkoxides for the conservation of built heritage (65). They determined an unsuitable use of this type of alcoxides applied to stone substrates due to the low crystallinity degree of the obtained coatings and the unsuccessfully carbonation of the Mg alcoxides into the required phase magnesite.

Sassoni et al. (66) were pioneers in the use of hydroxyapatite (HAP) as an alternative to calcium oxalate for the consolidation of carbonate stones used in building heritage. Thus, HAP has been applied for the consolidation of limestones (66), marbles (67), and sandstones with different carbonate contents (68, 69). This product is not introduced directly into stone material, but is formed by the reaction between phosphate ions coming from an aqueous solution of diammonium hydrogen phosphate (DAP) applied to the stone and calcium ions coming from the substrate. Among its advantages, HAP has shown a good compatibility with the crystal structure and lattice parameters of calcite (70). Thanks to its low viscosity, this aqueous consolidant product is able to penetrate deeply into the stone, generating a significant improvement in mechanical properties (65) and (66). A comparison with TEOS shows how hydroxyapatite-treated specimens presented an increase in dynamic elastic modulus and a tensile-strength substantially similar to that achieved by TEOS-treated ones. However, differently from samples consolidated with TEOS, hydroxyapatite-treated stones experienced very low variations in porosity and pore size distribution, leading to substantially no variation in the rate of water sorption and a small decrease in water vapor permeability (68). On the other hand, the HAP has been tested as a protective treatment for marble against acid rain corrosion, showing an improvement in the marble resistance to the dissolution (69, 71).

Recently, the role of the application technique on the effectiveness of HAP for limestone has been carried out (72). The authors tested the consolidant action of HAP applied by brushing, poultice and immersion on limestone samples artificially weathered by heating. They determined the application of HAP by brushing as the most suitable method. This methodological application induced a significant mechanical improvement of limestones with limited microstructural, physical and chromatic alterations (69). Furthermore, Sassoni et al. (73) studied the effectiveness and compatibility of hydroxiapatite treatment for limestone, in comparison with ethyl silicate. The researchers concluded that the hydroxiapatite was able to overcome some ethyl silicate limitations (mainly, prolonging curing time and compatibility), being a very promising consolidant product of porous limestones. They also evaluated the durability of the HAP treatment to wetting-drying, freezing-thawing and salt weathering cycles, in comparison with ethyl silicates (74). Their results showed the HAP as a better option for limestone consolidation compared to ethyl silicates. After the accelerated weathering cycles, the samples treated with HAP underwent less deterioration in original properties than the samples treated with ethyl silicates.

Also, the application of hydroxyapatite and strontium-hydroxyapatite nanoparticles as consolidant product in highly porous limestones (chalk stone) was studied by Ion et al. (75). They determined a higher consolidant activity for the strontium-hydroxyapatite nanomaterial than hydroxyapatite. These authors have also tested a consolidant product based on hydroxyapatite jointed with calcium oxalate trihydrate (caoxite) on weathered chalk stone (76), and concluded that the caoxite induced the stabilization of the hydroxyapatite and their mixed composition produced an important reinforcement of chalk stone after accelerated weathering tests.

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 (77). Thus, silicon based-consolidants can be considered as fairly compatible with silicate stones, at least in terms of chemical–mineralogical composition, since TEOS is expected to fully convert into amorphous silica once the hydrolysis/condensation reaction has been completed. Despite these materials have improved physical, chemical and mechanical properties, the alkoxysilanes present important disadvantages, such as their tendency to form brittle gels susceptible to crack during shrinkage and drying and their inefficient chemical bonding to carbonaceous stones (78). As noted by Mosquera el al. (79) cracking is generated by the high capillary pressures supported by the gel network during drying. Smaller pores are subject to higher pressure than larger pores. Thus, dense gels containing micropores, typically obtained from TEOS, are susceptible to cracking.

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 (80). These new consolidant products have been tested in sedimentary and metamorphic rocks and more recently, in granite (81). In this last work, the authors defined the application protocol and commercial requirements for their application to exterior conditions. The addition of a small amount of surfactant during the preparation of the nanocomposites in order to obtain crack-free consolidants for stone conservation has also been employed by other researchers (82). Furthermore, Zornoza-Indart et al. (83) evaluated the consolidant action of a nanostructured consolidant developed by Illescas et al. (78) composed by a silica oligomer precursor Wacker TES 40 WN^® with the addition of n-octylamine as a surfactant, under different environmental conditions (dry, RH = 40 ± 2%, and very humid conditions, RH = 84 ± 2% and 95 ± 2%). They determined an increased consolidant effectiveness in the stone specimens treated in the very humid consolidation environment. They also detected negative results concerning to the chromatic changes obtained in the treated substrates, marked by a visually change by the shift to yellow color.

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 (84, 85). The loading of colloidal oxide particles to TEOS has shown to enlarge the pore size within the network and increases the modulus, resulting in a lower capillary pressure (86). Also, the excessive agglomeration can be prevented through the modification of the final gel by absorbing nanoparticles onto the oxide particles (87). In this way, the development of nano particle–modified silica consolidants, allow reducing the cracking seen in conventional treatments and improve the consolidation. In order to avoid color problems that usually leave a whitish surface on the material Miliani et al. (85) studied the incorporation of oxide nanoparticles (~200–300 nm) to the initial silicate network (TEOS) for the consolidation of Ohio Massilian sandstones. These consolidant products consisted of a silicate binder loaded with alumina (Al₂O₃), titania (TiO₂), and silica (SiO₂) nanoparticles. PMCs provided a remarkable protection against salt damage. However, the TiO₂-PMC application caused an unacceptable color change on Ohio sandstone. Nevertheless, alumina and silica provided composites giving color changes near the threshold. Anyway, particle-modified consolidants offered substantially improved properties compared with the unfilled ethyl silicate consolidant, particularly with respect to protection from salt crystallization damage (85).

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. (88), proposed a novel crack free calcium oxalate-silica nanocomposite for protecting both historic and modern building materials. The nanocomposite was obtained by using two-step sol gel method consisting in the reaction of calcium hydroxide with oxalic acid, followed by its loading into TEOS. Therefore, nanoparticles of calcium oxalate were mixed with TEOS to produce a crack-free xerogel with an average pore radius of approximately 15 nm (88). Among the advantages of these new nanocomposites is the use of calcium oxalate, compound identified in historical patina layers on monuments (89). In addition, the formation of a stable component composed of calcium oxalate and silica is a well-established process in plant biomineralization (88). The authors concluded that the proposed hybrid nanocalcium oxalate-silica reached a penetration depth of more than 1 cm from the surface of the treated stone without altering its microstructure, and improving its tensile strength. Results indicated also that the changes in water permeability and total color produced by the treatment were within acceptable limits.

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 (90). The new nanocomposite incorporated particles of synthesized amorphous nano-calcium oxalate monohydrate in a silica matrix, obtaining a microporous hybrid material with an average pore diameter of 2.73 nm. The consolidation effectiveness of the method was evaluated in calcareous building stones and cement mortars. They conclude the optimal penetration depth of the nanocomposite into the substrates without occluding the pores with crack-free microstructure, and improving the mechanical properties of the treated limestones and cement mortars. The incorporation of amorphous nanocalcium oxalate into the silica matrix provided a good interfacial compatibility between the nanocomposite and the carbonaceous stones. Furthermore, the nanocomposites showed good stability under conditions of high relative humidity (90%) and UV irradiation.