E ﬀ ect of Hygrothermal Aging on Hydrophobic Treatments Applied to Building Exterior Claddings

: Hydrophobic materials are among the most commonly used coatings for building exterior cladding. In fact, these products are easily applied to an existing surface, signiﬁcantly reduce water absorption and have a minimal impact on the aesthetic properties. On the other hand, although these products have a proven e ﬀ ectiveness, their long-term durability to weathering has not yet been systematically studied and completely understood. For these reasons, this study aims to correlate the e ﬀ ect of artiﬁcial aging on the moisture transport properties of hydrophobic treatments when applied on building exterior claddings. Three hydrophobic products (an SiO 2 -TiO 2 nanostructured dispersion; a silane / oligomeric siloxane; and a siloxane) were applied on samples of limestone and of a cement-based mortar. The moisture transport properties (water absorption, drying, water vapor permeability) of untreated and treated specimens were characterized. Furthermore, the long-term durability of the specimens was evaluated by artiﬁcial aging, that is, hygrothermal cycles (freeze–thaw and hot–cold). All treatments have signiﬁcant hydrophobic e ﬀ ectiveness and improve the long term-durability of the treated specimens. However, the results showed that the three hydrophobic products have di ﬀ erent e ﬀ ectiveness and durability, with the SiO 2 -TiO 2 nanostructured dispersion being the most durable treatment on limestone, and the siloxane the most suitable for cementitious mortar.


Introduction
The use of coatings on building façades is a fundamental action for the protection of external surfaces from weathering. Protective coatings should be considered for the conservation of historical façades, as well as for the maintenance of modern buildings. In fact, these products can help in the conservation of ancient materials by increasing the durability of the treated surface.
Water is among the most aggressive atmospheric agents, being the main physical-chemical degradation cause of porous building materials [1]. In fact, water can trigger several surface anomalies both on ancient and modern buildings, leading to several physical (e.g., loss of cohesion and material, condensation in the interior of the building, reduction of thermal conductivity, formation of salt efflorescence, hygrothermal aging in buildings) and aesthetic changes (e.g., stains, biofilm formation) on the affected surface [1][2][3].
For these reasons, excessive water penetration and retention should be avoided in order to balance the water content in the façade cladding. In fact, porous building materials can balance the moisture content according to their water transport properties, that is, water vapor adsorption, water capillary Table 1. Average results and standard deviation of capillary water absorption coefficient, open porosity and bulk density of the substrates [19].

. Hydrophobic Products
The selection of hydrophobic products was based on market research. The main aim was the evaluation of products with various chemical compositions and, therefore, that possibly differentiated in terms of their effectiveness and durability.
Silane-modified siloxane polymers are widely adopted as hydrophobic materials; siloxane guarantees improved adhesion properties with the treated substrate, whereas the silane is used as coupling agent and improves the hydrophobic properties of the treatment [20].
Silica nanoparticles, which present hydrophilic properties, can be modified during their synthesis by adding a coupling agent such as silane or sodium sulphate, which confers hydrophobic properties and prevents nanoparticle agglomeration [21,22]. These products are generally composed of hydrophobic hybrid crystalline SiO 2 -TiO 2 nanoparticles with a crystallite size equal to 5-20 nm [23,24]. The use in coatings of high refractive index oxides, such as TiO 2 , has significantly grown in recent years due to the advancements of nanoparticle manufacturing processes and because of their beneficial properties. Nanostructured titanium oxide has photocatalytic properties which can result in multifunctional self-cleaning and biocidal coatings [25].
The physical/chemical characteristics and application protocol of the hydrophobic products is reported in Table 2. It is worth noting that most solvent-based hydrophobic products can be harmful for both the operator and the environment, due to their high content of volatile organic compound (VOC) (e.g., polycyclic aromatic hydrocarbons and alkanes) [9]. Additionally, silane and biocide additives (isothiazol-3-one, 3-iodo-2-propynylbutyl carbamate, among others) are generally toxic for the operator and detrimental for the environment.

Specimen Preparation
Cylindrical specimens, with 20 cm diameter and 2 cm thickness, were drilled and cut from Moleanos limestone blocks and used for capillary water absorption, drying and water vapor permeability tests. Prismatic specimens (30 cm × 30 cm, 2 cm thickness) were used for the analysis of water absorption under low pressure. Before the application of the hydrophobic product, in order to obtain a constant moisture content in all the specimens, limestone specimens were stored in a conditioned room at T = 23 ± 2 • C and RH = 50 ± 5%.
Concerning the mortar specimens, cylindrical specimens with 20 cm diameter and 2 cm thickness were produced by using dedicated molds; these were used for water vapor permeability tests. Additionally, a 2 cm-thick layer of the mortar was applied on hollow ceramic bricks (29 cm × 17 cm × 4 cm). These latter specimens were used for all the other tests (capillary water absorption, drying kinetics and water absorption under low pressure).
All mortar specimens were cured for 2 days at T = 23 ± 2 • C and RH = 95 ± 5%, and later stored in a conditioned room at T= 23 ± 2 • C and RH= 50 ± 5% for 26 days, before testing.
All specimens (limestone and mortar) were sealed along their side surface with liquid paraffin (applied multiple times by brushing, until obtaining a layer of approximately 1 mm).

Application Protocol
The three hydrophobic products were applied by brushing, following the recommendations of the products technical data sheets (Figure 1a). Two applications were carried (in orthogonal directions) in the case of H SILA/SIL and H NST , whereas one application was performed in the case of H SIL . The interval between applications was around 2 h for H SILA/SIL and 3 h for H NST . The applications were performed under controlled conditions (50 ± 5% RH, T= 20 ± 2 • C) and the treated specimens were stored at the same hygrothermal conditions for 7 days, with the aim of completing the polymerization of the silicon-based products. Untreated samples were stored in the same conditions.

Moisture Transport Properties
The capillarity water absorption coefficient (C) was determined as the initial slope of the absorption curve using a protocol based on EN 1015-18 [26] (Figure 1b). The determination of water absorption by capillary action is achieved through the evolution of the amount of water absorbed by the solid unit surface area (kg/m 2 ), as a function of the square root of time (t 1/2 ).
Water absorption at low pressure was carried out with Karsten tubes applied on the specimens for 60 min, following RILEM recommendations [27]. The water absorption coefficient for 60 min (C 60 , kg·m −2 ·min −1/2 ) was calculated according to the following Equation: where Abp is the water mass absorbed after 60 min (kg), and Ac is the contact area of the pipe with the surface (assumed to be 5.7 cm 2 , i.e., the contact area of the Karsten pipe). Drying tests were carried out sequentially at the end of the capillary water absorption tests in order to allow a direct correlation between the water absorption and drying results. The drying kinetics of the specimens were verified according to RILEM [28] and UNI [29] recommendations, which considers the initial drying (based on the slope of the initial drying curve) and the drying index (DI). The latter is an empirical quantity that expresses the drying curve into a single quantitative parameter reflecting the global drying kinetics. The drying index, obtained from the average drying curve of those of the different specimens (in equivalent conditions), was calculated according to where Mx is the specimen mass weighted during the drying process (g); M1 is the specimen mass in dry state (g), M3 the specimen mass in the saturated state (g), and tf is the final time of the drying process (h). Additionally, with the aim of understanding the influence of the hydrophobic products on the drying kinetics of the treated surface, the different steps of drying and the critical moisture content (i.e., the transition between the 1st and 2nd step of drying) were studied.
The water vapor permeability was determined as specified in EN1015-19 [30]. The water vapor diffusion resistance coefficient (µ) was calculated according to Equations (3)

Moisture Transport Properties
The capillarity water absorption coefficient (C) was determined as the initial slope of the absorption curve using a protocol based on EN 1015-18 [26] (Figure 1b). The determination of water absorption by capillary action is achieved through the evolution of the amount of water absorbed by the solid unit surface area (kg/m 2 ), as a function of the square root of time (t 1/2 ).
Water absorption at low pressure was carried out with Karsten tubes applied on the specimens for 60 min, following RILEM recommendations [27]. The water absorption coefficient for 60 min (C 60 , kg·m −2 ·min −1/2 ) was calculated according to the following Equation: where A bp is the water mass absorbed after 60 min (kg), and A c is the contact area of the pipe with the surface (assumed to be 5.7 cm 2 , i.e., the contact area of the Karsten pipe).
Drying tests were carried out sequentially at the end of the capillary water absorption tests in order to allow a direct correlation between the water absorption and drying results. The drying kinetics of the specimens were verified according to RILEM [28] and UNI [29] recommendations, which considers the initial drying (based on the slope of the initial drying curve) and the drying index (DI). The latter is an empirical quantity that expresses the drying curve into a single quantitative parameter reflecting the global drying kinetics. The drying index, obtained from the average drying curve of those of the different specimens (in equivalent conditions), was calculated according to where M x is the specimen mass weighted during the drying process (g); M 1 is the specimen mass in dry state (g), M 3 the specimen mass in the saturated state (g), and t f is the final time of the drying process (h).
Additionally, with the aim of understanding the influence of the hydrophobic products on the drying kinetics of the treated surface, the different steps of drying and the critical moisture content (i.e., the transition between the 1st and 2nd step of drying) were studied.
The water vapor permeability was determined as specified in EN1015-19 [30]. The water vapor diffusion resistance coefficient (µ) was calculated according to Equations (3) and (4): where Λ is the water vapor permeance (kg/m 2 ·s·Pa); m is the linear relationship slope of time versus mass change (kg/s); A is the specimen area (0.126 m 2 ); ∆ P is the difference between the outdoor and indoor vapor pressure (Pa); µ is the water vapor diffusion resistance coefficient; and e is the specimen thickness (m). In all tests, three untreated specimens and nine treated specimens were analyzed, considering their average values and relative standard deviations.

Accelerated Aging Test
Accelerated aging tests were performed on untreated and treated limestone and mortar specimens to verify the durability of the hydrophobic treatments to weathering cycles [31]. Since temperature shock, rain and solar irradiation are the main degradation agents of porous materials [32], the specimens were subjected to hygrothermal cycles (hot-cold and freeze-thaw).
The test conditions, which represent extreme climate conditions, were adapted from EN 1015:21 [33]. This methodology was also validated in previous research by the authors [34,35]. Hot-cold cycles consist of storing the specimens firstly within a closed apparatus with infrared lamp (Figure 1c), which provides high temperature, and later within a deep-freeze cabinet with low temperature. Freeze-thaw cycles were carried out by exposing the specimens to a sprinkler system (simulating rain), followed again by a storage within a deep-freeze cabinet. Hot-cold and freeze-thaw cycles were carried out sequentially on the same specimens. Eight cycles of each type were performed, improving the indications (four cycles) of the norm previously mentioned (Figure 1c). Further details on the weathering cycles are provided in Table 3. Table 3. Accelerated aging test: Hygrothermal cycles test conditions.

Hot-Cold Cycles Freeze-Thaw Cycles Exposure Time (h/mins)
Infrared lamps (60 ± 2 • C) Sprinkler system, water at T = 20 At the end of the hygrothermal cycles, specimens were stabilized for 48 h at T = 20 ± 2 • C, 50 ± 5% RH. All the tests mentioned in the previous section were repeated on artificially aged untreated and treated specimens.

Capillary Water Absorption and Water Absorption with Karsten Tubes
The results show that, in the case of limestone specimens, the capillary water absorption coefficient (C) of treated specimens considerably reduced after the application of the hydrophobic products (33% in the case of H Sila-Sil , 51% with H sil , 88% with H NST ) (Table 4, Figure 2). Concerning mortar specimens, all treatments show a higher reduction of the capillarity water absorption when compared to treatments on limestone, although the total amount of absorbed water is higher (total saturation of the specimens is not completely achieved at the end of the test). A remarkable reduction of the capillarity water absorption was observed in all treated mortar specimens (>95%), when compared to untreated specimens. This behavior is attributed to the higher open porosity of rendering mortars (Table 1), allowing the easier penetration of hydrophobic products into the pores of the substrate coating [19]. The deeper penetration of the hydrophobic products leads to a reduction of the wettability of the substrate, resulting in an almost hydrophobic surface. All treatments show a similar behavior, almost waterproofing the mortar specimens, and the treatments show a good durability after artificial aging, with a minimal increase of the capillarity water absorption coefficient when compared to unaged specimens. All hydrophobic products penetrated within the porous network of the substrate, reducing the wettability and providing a hydrophobic coating [9]. The higher reduction of the capillary water absorption coefficient of the specimens treated with H NST , which can penetrate deeper within the treated substrate due to its nanosize [1], can be attributed to the chain arrangement (creation of Si-O-Ti bonds) of the TiO 2 -SiO 2 nanoparticles. The copolymerization of the TiO 2 and silane within the silica network can give rise to the formation of a homogeneous organic-inorganic hybrid xerogel, with improved hydrophobic properties [23,24].
After accelerated aging tests, untreated specimens show a significant reduction of the capillary water absorption (around 44%), which can be attributed to the modification (destruction) of capillary pores (typically observed in altered and decayed materials) [36]. In fact, a significant increase of porosity is observed mostly between 5 to 10 freeze-thaw cycles [37]. Thus, artificial aging induces the generation of new pores and the expansion of existing pores in the specimens.
Additionally, the specimens with H NST treatment, which show the highest reduction of capillary water absorption before artificial aging, show an opposite trend after artificial aging, with a decrease of up to 60% when compared to the unaged specimens. On the other hand, aged specimens treated with H Sila/Sil and H Sil show a significant reduction of the capillarity water absorption (80% and 50%, respectively), when compared to the untreated aged specimens. This can be justified by the lower durability of H NST treatment to hygrothermal aging cycles. As a matter of fact, it is reported [38] that weathering can induce a weakening of the film adhesion and reduce the durability of TiO 2 -SiO 2 protective coatings.
After aging, all specimens still maintain a significant reduction (40%, 21% and 52% for H Sila/Sil , H Sil and H NST , respectively) of the capillary water absorption when compared to the untreated specimens.
Concerning mortar specimens, all treatments show a higher reduction of the capillarity water absorption when compared to treatments on limestone, although the total amount of absorbed water is Coatings 2020, 10, 363 8 of 16 higher (total saturation of the specimens is not completely achieved at the end of the test). A remarkable reduction of the capillarity water absorption was observed in all treated mortar specimens (>95%), when compared to untreated specimens. This behavior is attributed to the higher open porosity of rendering mortars (Table 1), allowing the easier penetration of hydrophobic products into the pores of the substrate coating [19]. The deeper penetration of the hydrophobic products leads to a reduction of the wettability of the substrate, resulting in an almost hydrophobic surface. All treatments show a similar behavior, almost waterproofing the mortar specimens, and the treatments show a good durability after artificial aging, with a minimal increase of the capillarity water absorption coefficient when compared to unaged specimens.
Additionally, all aged treated mortar specimens significantly reduce their capillary water absorption coefficient when compared to aged untreated specimens. However, it is worth noting that a slight decrease of the capillarity water absorption coefficient was observed if comparing unaged treated specimens to aged treated ones. This modification can be attributed to the possible alteration of the pore size distribution of the substrates.
A possible cause for the reduction of capillary water absorption after freeze-thaw cycles can be the reduction of capillary suction, resulting from an increase in the amount of bigger pores (above the capillary range) as a consequence of micro-cracking. Additionally, in the case of the cement-based mortar, the exposure to water with these cycles can induce a self-healing effect that promotes hydration reactions, further explaining the reduction of the capillary absorption rate with ageing.
When considering the results of water absorption by Karsten tube in the limestone specimens, a trend similar to that seen in the capillarity water absorption test was observed (75% in the case of H Sila-Sil, 93% with H sil , 92% with H NST ). In the case of the mortar specimens, the reduction was similar to that observed in capillary water absorption tests (96% in the case of H Sila-Sil, 98% with H sil , 99% with H NST ) ( Table 5). Table 5. Average results and relative standard deviation of the water absorption coefficient under pressure (C 60 ) of treated and untreated specimens, before and after artificial aging tests. If comparing the results of capillary water absorption and water absorption under low pressure of the untreated specimens, the opposite trend is seen. In fact, there is a reduction of capillary water absorption after aging; however, an increase of water absorption under low pressure is observed in equivalent conditions [39]. It is generally assumed that pores ranging from 1 to 10 µm act as capillary pores, whereas pores >10 µm contribute to the water permeability through gravity (e.g., percolation) or wind driven water ingress [40,41]. Thus, this confirms that artificial aging cycles possibly contribute to an increase in the amount of pores with dimensions greater than 10-20 µm.
Furthermore, H Sila/Sil e H Sil treatments decrease the water absorption coefficient under pressure after artificial aging, both when applied on mortar or limestone. On the other hand, a decrease of 25% in the C 60 was observed in the aged limestone specimens treated with H NST , if compared to the unaged specimens, whereas an opposite trend is observed when considering treated mortar specimens.
These results point out that H Sila/Sil e H Sil treatments have lower variation of the water absorption after artificial aging, compared to H NST treatments. In fact, the latter shows both an increase of the capillary water absorption and water absorption under low pressure, possibly due to its physical-chemical alteration.

Drying Rate
When observing the drying curves, two stages can be observed (Figure 3). In the first stage of drying, called the constant drying period or initial drying rate, the drying front is at the surface and the drying rate is constant and controlled by the external conditions [42]. This first phase (initial drying rate) ends after 24 h in the case of all treated and untreated limestone and treated mortar specimens, whereas for untreated mortar specimens it ends after ≥72 h (Figure 3). When compared to the sound untreated specimens, all treated specimens decrease the initial drying rate products (first stage of drying), both in the limestone (3-12%) and mortar specimens (6-36%). More specifically, H Sila/Sil has an almost negligible influence on the initial drying rate of the treated specimens (3% reduction, when compared to untreated specimens), whereas H Sil and H NST induce a slightly higher reduction (up to 12%).
Coatings 2020, 10, x FOR PEER REVIEW 9 of 16 to the sound untreated specimens, all treated specimens decrease the initial drying rate products (first stage of drying), both in the limestone (3%-12%) and mortar specimens (6%-36%). More specifically, HSila/Sil has an almost negligible influence on the initial drying rate of the treated specimens (3% reduction, when compared to untreated specimens), whereas HSil and HNST induce a slightly higher reduction (up to 12%).
In the second stage of drying, identified by the change in the slope of the drying curve, the moisture content can no longer support the demands of the evaporation flux, and, thus, the drying process occurs in the vapour phase. The transition between the first and second step of drying (i.e., the critical moisture content) occurs when the superficial moisture has evaporated. In this phase, the drying front progressively recedes into the material and the properties of the liquid and of the substrate control the rate of drying [42]. When considering the second stage of drying in limestone specimens, although the critical moisture content is identified at 24 h in all cases, it can be observed that (aged and unaged) untreated specimens almost achieve complete drying at 72 h, and a similar trend is observed in the case of the specimens treated with HNST ( Figure 3a). On the other hand, HSil and HSila/Sil slightly delay the drying process (the second step of drying ends at 96 h), when compared to HNST treatment.
It can be concluded that the hydrophobic treatments induce only a slight retarding effect on the drying behavior of the limestone.
When considering the mortar specimens, all the hydrophobic treatments remarkably reduce the total amount of water absorbed; however, the HNST treatment takes a longer time to dry completely when compared to the HSil and HSila/Sil treatments. Conversely, the HNST treatment only slightly influences the initial drying rate (6% reduction) when compared to the HSila/Sil treatment (13%) and, especially, the HSil treatment (36%). Additionally, in accordance with the results observed in the previous section, HNST treatment also increases the drying time (8%) in the mortar specimens, whereas HSil and HSila/Sil show a significant decrease (30%-39%). The difference in the behavior of the hydrophobic products is also observed in the second step of drying, which starts at 24 h in the case of HSila/Sil and HSil treatment, whereas the critical moisture content is identified at 72 h in the case of the HNST treatment (as in the case of untreated specimens). Results obtained with contact angle measurements in a previous work confirm this trend and the drying index (Table 6), that is, a higher reduction of the wettability on both substrates in the case of HSila/Sil treatment [19].
After hygrothermal aging, HSila/Sil treatment show an improvement of the initial drying rate (20% and 27%, for limestone and mortar specimens, respectively), with worse results when compared to HNST treatment (reduction of 5% and 24%, for limestone and mortar specimens, respectively) and HSil treatment (reduction of 9% and 12%, for limestone and mortar specimens, respectively). It can be seen In the second stage of drying, identified by the change in the slope of the drying curve, the moisture content can no longer support the demands of the evaporation flux, and, thus, the drying process occurs in the vapour phase. The transition between the first and second step of drying (i.e., the critical moisture content) occurs when the superficial moisture has evaporated. In this phase, the drying front progressively recedes into the material and the properties of the liquid and of the substrate control the rate of drying [42].
When considering the second stage of drying in limestone specimens, although the critical moisture content is identified at 24 h in all cases, it can be observed that (aged and unaged) untreated specimens almost achieve complete drying at 72 h, and a similar trend is observed in the case of the specimens treated with H NST (Figure 3a). On the other hand, H Sil and H Sila/Sil slightly delay the drying process (the second step of drying ends at 96 h), when compared to H NST treatment.
It can be concluded that the hydrophobic treatments induce only a slight retarding effect on the drying behavior of the limestone.
When considering the mortar specimens, all the hydrophobic treatments remarkably reduce the total amount of water absorbed; however, the H NST treatment takes a longer time to dry completely when compared to the H Sil and H Sila/Sil treatments. Conversely, the H NST treatment only slightly influences the initial drying rate (6% reduction) when compared to the H Sila/Sil treatment (13%) and, especially, the H Sil treatment (36%). Additionally, in accordance with the results observed in the previous section, H NST treatment also increases the drying time (8%) in the mortar specimens, whereas H Sil and H Sila/Sil show a significant decrease (30-39%). The difference in the behavior of the hydrophobic products is also observed in the second step of drying, which starts at 24 h in the case of H Sila/Sil and H Sil treatment, whereas the critical moisture content is identified at 72 h in the case of the H NST treatment (as in the case of untreated specimens). Results obtained with contact angle measurements in a previous work confirm this trend and the drying index (Table 6), that is, a higher reduction of the wettability on both substrates in the case of H Sila/Sil treatment [19]. Table 6. Drying index (I s ) of aged treated and untreated specimens, before and after artificial aging tests. After hygrothermal aging, H Sila/Sil treatment show an improvement of the initial drying rate (20% and 27%, for limestone and mortar specimens, respectively), with worse results when compared to H NST treatment (reduction of 5% and 24%, for limestone and mortar specimens, respectively) and H Sil treatment (reduction of 9% and 12%, for limestone and mortar specimens, respectively). It can be seen that artificial aging slightly speeds up the drying process of the mortar specimens (the critical moisture content is identified at 48 h in the case of untreated aged specimens, and at 72 h with untreated unaged specimens). Additionally, in accordance with previous observations, the second step of drying of aged specimens treated with HSil and HSila/Sil starts at 48 h, and at 96 h in the case of the H NST treatment (Figure 3b).

Substrates
After artificial aging, H Sila/Sil treatment shows the highest variation of drying behavior, with an increase (26%) of the DI for limestone and decrease of 21% for mortar specimens (the slower the drying, the higher the DI). In accordance with previous observations, H Sil treatment also induces an increase (15%) of the DI for limestone, and, conversely, a significant decrease (26%) for mortar specimens. On the other hand, H NST treatment shows the best performance on limestone specimens, with only a slight DI increase (6%), and, however, a significant decrease for mortar specimen (28%).

Water Vapor Permeability
The results of the water vapor permeability test, as expressed by the water vapor diffusion resistance coefficient (µ), show a µ decrease for all hydrophobic treatments on limestone and mortar specimens, reducing the breathability of the substrates ( Table 7). The reduction in water vapor permeability is an inevitable consequence of the water repellence properties of polymer film; however, the lowest possible decrease is pursued [43]. Table 7. Average results and relative standard deviation of the water vapor diffusion resistance coefficient (µ) of treated and untreated specimens, before and after artificial aging tests. This reduction is more significant in limestone specimens, whereas it is almost negligible in mortar specimens. In fact, an increase of µ of 227% (H Sila/Sil ), 129% *(H Sil ) and 74% (H NST ) is observed on treated limestone specimens, when compared to untreated ones, whereas a minimal µ increase (<4%) is observed on the treated mortar specimens.

Substrates
After artificial aging, only the H NST treatment applied on limestone maintains reasonably higher µ values (increase of 7%) when compared to untreated specimens, whereas H Sil and H Sila/Sil treatments still induce a drastic µ increase (46% and 188%, respectively). In the case of mortar specimens, the higher increase of µ was observed with H Sila/Sil treatment (9%), and only a slight µ decrease in the case of H Sil and H NST treatments (<2%). In general, the hydrophobic treatment that illustrates the most suitable behavior to water vapor permeability was H NST , with a moderate reduction of the µ on both substrates, even after artificial aging.

Discussion
The variation of the moisture transport properties of the substrates treated with the hydrophobic products is presented in Figure 4. In general, it can be observed that the hydrophobic products induce a decrease of the capillary water absorption coefficient (C), this decrease being more relevant with H NST treatment on limestone specimens. However, after artificial aging, H Sila/Sil and H Sil treatments show a higher durability, with a higher decrease of the C, if compared to specimens treated with H NST . Concerning mortar specimens, all treatments maintain a similar water absorption coefficient (considerably lower than untreated specimens), even after artificial aging.
The different moisture transport properties of mortar and limestone can be attributed to the higher open porosity (30%) of the mortar when compared to the studied limestone (11%). Additionally, mortar specimens generally have a higher volume of coarse pores (> 100 µm) when compared to this type of compact Moleanos limestone [40].
Regarding the difference in the effectiveness and durability of the hydrophobic products, it is worth noting that (monomer) silane molecules are considerably smaller (10 to 15 Å) when compared to (oligomeric) siloxane molecules (25 to 75 Å) [44]. Thus, the longer Si-O chains of the siloxanes compared to the silane ones have a lower penetration depth in the compact limestone. Additionally, organo-modified siloxanes have an extremely high reactivity, which can hinder their in-depth penetration [2]. The silane has a potentially deeper penetration in the treated surface, with a higher reduction of the hydrophilicity and, thus, improved hydrophobic effectiveness. Furthermore, concerning the silane, it is generally assumed that the larger the molecule of the alkyl group linked to the silicon atom (which constitutes the structure of this compound), the higher the water repellency of the silane [8,45]. On the other hand, the long siloxane chains are more affected by environmental agents and weathering, undergoing degradation processes which can reduce their effectiveness as hydrophobic products [14].
The optimal performance obtained with the nanostructured product based on SiO2 and TiO2 (HNST) on the limestone can probably be attributed to the chain arrangement of the TiO2-SiO2 nanoparticles, due to the creation of the Si-O-Ti bond [23]. In fact, the copolymerization of the TiO2 and silane within the silica network can give rise to the formation of homogeneous organic-inorganic hybrid xerogel [24]. Additionally, the nanosize (<0.1 µm) of the silicon titanium oxide particles can match the dimension of pore network of the limestone. On the other hand, the low durability of HNST treatment can be attribute to the photocatalytic oxidation (of the organic radicals) and thermal degradation of the SiO2-TiO2 composite, which can weaken the adhesion and thus, durability of the coating [38].  Concerning the drying index, the hydrophobic products induced an increase of the DI on limestone specimens, with higher variation in the case of the HSil and HSila/Sil treatments. After aging, HSila/Sil significantly increase the DI of the treated limestone specimens, whereas HSil and HNST maintain values similar to unaged specimens. Greater variations are observed in mortar specimens; in fact, the HNST treatment induces an increase of the DI, and a drastic decrease is observed after artificial aging, indicating the probable degradation of the HNST treatment. On the other hand, the HSila/Sil and HSil treatments decrease the DI on mortar specimens, with a lower decrease of the latter after artificial aging compared with the HNST treatment. As reported in other works [46], the silane/siloxanes products can modify the pore netwok of the treated material by increasing the volume of capillary pores, probably also due to an air entraining effect of liquid siloxane. This feature can justify the improved resistance to freeze-thaw cycles, and thus, durability of HSila/Sil and HSil The different moisture transport properties of mortar and limestone can be attributed to the higher open porosity (30%) of the mortar when compared to the studied limestone (11%). Additionally, mortar specimens generally have a higher volume of coarse pores (>100 µm) when compared to this type of compact Moleanos limestone [40].
Regarding the difference in the effectiveness and durability of the hydrophobic products, it is worth noting that (monomer) silane molecules are considerably smaller (10 to 15 Å) when compared to (oligomeric) siloxane molecules (25 to 75 Å) [44]. Thus, the longer Si-O chains of the siloxanes compared to the silane ones have a lower penetration depth in the compact limestone. Additionally, organo-modified siloxanes have an extremely high reactivity, which can hinder their in-depth penetration [2]. The silane has a potentially deeper penetration in the treated surface, with a higher reduction of the hydrophilicity and, thus, improved hydrophobic effectiveness. Furthermore, concerning the silane, it is generally assumed that the larger the molecule of the alkyl group linked to the silicon atom (which constitutes the structure of this compound), the higher the water repellency of the silane [8,45]. On the other hand, the long siloxane chains are more affected by environmental agents and weathering, undergoing degradation processes which can reduce their effectiveness as hydrophobic products [14].
The optimal performance obtained with the nanostructured product based on SiO 2 and TiO 2 (H NST ) on the limestone can probably be attributed to the chain arrangement of the TiO 2 -SiO 2 nanoparticles, due to the creation of the Si-O-Ti bond [23]. In fact, the copolymerization of the TiO 2 and silane within the silica network can give rise to the formation of homogeneous organic-inorganic hybrid xerogel [24]. Additionally, the nanosize (<0.1 µm) of the silicon titanium oxide particles can match the dimension of pore network of the limestone. On the other hand, the low durability of H NST treatment can be attribute to the photocatalytic oxidation (of the organic radicals) and thermal degradation of the SiO 2 -TiO 2 composite, which can weaken the adhesion and thus, durability of the coating [38].
Concerning the drying index, the hydrophobic products induced an increase of the DI on limestone specimens, with higher variation in the case of the H Sil and H Sila/Sil treatments. After aging, H Sila/Sil significantly increase the DI of the treated limestone specimens, whereas H Sil and H NST maintain values similar to unaged specimens. Greater variations are observed in mortar specimens; in fact, the H NST treatment induces an increase of the DI, and a drastic decrease is observed after artificial aging, indicating the probable degradation of the H NST treatment. On the other hand, the H Sila/Sil and H Sil treatments decrease the DI on mortar specimens, with a lower decrease of the latter after artificial aging compared with the H NST treatment. As reported in other works [46], the silane/siloxanes products can modify the pore netwok of the treated material by increasing the volume of capillary pores, probably also due to an air entraining effect of liquid siloxane. This feature can justify the improved resistance to freeze-thaw cycles, and thus, durability of H Sila/Sil and H Sil treatments compared to H NST . Furthermore, the difference in the moisture transport properties of treated limestone and mortar can be attributed also to the higher roughness of the mortar, compared to flatter surface of the limestone, which accounts for better adhesion of both fissured and crack free SiO 2 -TiO 2 films to the substrate [37].
All treatments induce an increase of the µ in limestone specimens. More specifically, specimens treated with H Sila/Sil and H Sil show the highest µ increase, which significantly decreases after artificial aging in the case of the H NST treatment. The H NST treatment shows a lower µ after artificial aging compared with unaged specimens. Considering the mortar specimens, it can be concluded that µ variation is extremely low with all treatments, both before and after artificial aging. The H Sila/Sil is the only treatment which induced a slight increase of the µ after artificial aging.
Ultimately, it would be expectable that a substrate with higher DI would have also a higher µ (i.e., lower WVP). For treated limestone specimens this trend is confirmed; however, specimens treated with H Sila/Sil have a higher µ/DI ratio (even after artificial aging) compared to the other treatments. A different behavior is observed with treated mortar specimens: Aged and unaged specimens with H NST treatment show the trend mentioned above, whereas unaged H Sil treatment and aged H Sila/Sil treatment show a DI decrease and a µ increase.

Conclusions
In this paper, the effectiveness and durability of three commercially available hydrophobic products (a silicon and titanium dioxides-based nanostructured dispersion-H NST ; a silane/oligomeric siloxane-H Sila/Sil ; and a siloxane-H Sil ) when applied to a Moleanos limestone and on a cement-based mortar, were analyzed. The alteration of the moisture transport properties (water absorption by capillarity and under low pressure, drying kinetics and water vapor permeability) of the treated substrates, prior to and after artificial aging tests, was evaluated.
Results show that the effectiveness and durability of the water-repellent treatment is influenced both by the type of hydrophobic product and by the treated substrate.
Although the products were applied at different concentrations, following the recommendations of the producers, all treatments induce a significant decrease of the values of the capillary water absorption and water absorption under low pressure on the mortar specimens. A lower decrease was observed in the limestone specimens. This difference is attributed to the higher open porosity of the mortar specimens compared to limestone specimens, thus allowing a deeper penetration of the hydrophobic products, which increases the water-repellency of the treated mortar. After artificial aging, all hydrophobic treatments show a significant durability to the type and duration of the artificial aging cycles considered in this work, maintaining a reasonably low water absorption in both mortar and limestone specimens, when compared to untreated specimens. The H NST treatment shows a slightly greater loss of efficacy in terms of water capillary absorption and water vapor permeability after artificial aging, mostly when applied on limestone. Therefore, it can be considered less durable.
The treatments induce also a variation of the drying index, which increases with all treatments on limestone specimens (even after artificial drying) and a general decrease on mortar specimens, except for the H NST treatment before aging, which slightly increases the DI. On the other hand, the H NST treatment shows a lower drying index after artificial aging.
H Sila/Sil and H Sil treatments significantly reduce the water vapor permeability of limestone specimens, whereas the H NST treatment induces a smaller decrease, with values similar to those of untreated specimens after artificial aging. The WVP of the treated mortar specimens was not significantly affected by the hydrophobic treatments, even after aging tests.
These observations confirm that the hydrophobic products are generally more effective and durable on mortar specimens, rather than on low-porosity limestone, as in the case of the studied Moleanos limestone.
When pondering the variation of all the moisture transport properties, the hydrophobic product based on siloxane (H Sil ) has the best performance on cement-based mortar; in fact, the molecular structure of siloxanes matches to the higher porosity of this substrate. On the other hand, although it has a lower durability compared to the other treatments, H NST has the best performance when applied on Moleanos limestone, with a significant decrease of the capillary water absorption and a low variation of the drying index and of the WVP. This behavior can result from the combination of the low porosity and micro-sized pores of the stone with the surface deposition of a nanostructured layer. Additionally, the presence of TiO 2 confers antibacterial activity against the microorganism growth and pollutant absorption.
The H Sila/Sil treatment significantly decreases its water-repellent properties. However, it hinders the drying process and the breathability of the substrates, even after artificial aging. Thus, based on the results of this study, its use is not recommended in either limestone or mortar.
Further tests (e.g., optimization of the protocol; artificial aging cycles with UV light, pollutants and/or biological colonization; FTIR analysis of the hydrophobic products; morphological analysis by SEM-EDS; contact angle measurements of the treated substrates, among others) are ongoing to correlate the effect of the physical-chemical aging on the effectiveness of hydrophobic products, and ultimately, to verify the durability to more prolonged weathering action (from lab to real natural scale tests) of the hydrophobic products.