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A polydimethylsiloxane coating to minimize weathering effects on granite
Construction and Building Materials 124 (2016) 1051–1058
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
A polydimethylsiloxane coating to minimize weathering effects on granite Dennis Kronlund a, Mika Lindén b, Jan-Henrik Smått a,⇑ a b
Laboratory of Physical Chemistry and Center of Excellence for Functional Materials, Åbo Akademi University, Porthansgatan 3-5, 20500 Åbo, Finland Inorganic Chemistry II, University of Ulm, Albert-Einstein-Allee 11, 89031 Ulm, Germany
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A sprayable water-repellent PDMS
coating is developed for granite protection. The impact of UV- and water-based weathering mechanisms on granite is studied. The PDMS coatings display great stability in all the studied weathering tests. Degradation effects of pore salt crystallization is eliminated with a PDMS coating.
a r t i c l e
i n f o
Article history: Received 21 April 2016 Received in revised form 2 August 2016 Accepted 29 August 2016
Keywords: Granite Polydimethylsiloxane Coating Hydrophobization Water absorption Degradation Stability Weathering
a b s t r a c t The impact of common weathering mechanisms on granite has been elucidated. A sprayable polydimethylsiloxane (PDMS) solution and a commercial reference coating formulation (Faceal Oleo HD) have been applied on granite to produce durable surface coatings that minimize weathering effects. Sufficiently thick PDMS coatings display superior stability in all the studied weathering tests when compared to the reference coating, as the impact of weathering on the hydrophobic functionality of the PDMS-coated samples is minimal. Furthermore, due to the excellent water and salt blocking behavior of PDMS, the PDMS-coated stones display an overall better stability against weathering compared to untreated stones. Ó 2016 Published by Elsevier Ltd.
1. Introduction Granite is a dense, hard and tough stone material and has therefore gained widespread use throughout human history (from hand tools and projectile weapons to foundation elements and city ⇑ Corresponding author. E-mail address: [email protected] (J.-H. Smått). http://dx.doi.org/10.1016/j.conbuildmat.2016.08.146 0950-0618/Ó 2016 Published by Elsevier Ltd.
walls), and more recently as durable construction materials (used for buildings, bridges, tunnels, etc.) [1]. Furthermore, granite has a relatively good chemical stability due to its high quartz content. Although granite is generally considered to be a very durable stone material, its daily exposure to various weathering phenomena can potentially reduce its strength as a construction material [2–4]. Weathering is a general problem in the degradation of natural stones, especially erosion related to water uptake in the pores of
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the stones [3]. Weathering, which can be divided into physical, chemical and biological degradation routes, creates chemical and structural alterations to the stone over time [5,6]. An important physical degradation route in cold climates is related to the repeated freezing and thawing of water absorbed inside the pores of the stones. As ice has a larger molar volume than liquid water, freezing creates tension throughout the porous stone, which eventually can lead to rock fractures [7,8]. Another form of physical deterioration is linked to salt crystallization in the pores of stone materials (a potential problem in coastal areas), where the formation of salt crystals apply pressure on the porous network [9,10], causing the stone to degrade through a similar mechanism as in ice formation. Furthermore, chemical degradation can occur through several different routes depending on the type of stone material. For instance, granite can be disintegrated into granitic sand, which is a process that occurs through hydrolysis of potassium feldspar into clay minerals [11–13]. There are several sources of H+ in nature that can cause this degradation, including H2CO3 (formed by dissolution of CO2 in water), H2SO4 and HNO3 (in acid rain), as well as a range of weaker organic acids in the soil. Earlier stone conservation studies have focused on reducing water-related weathering effects, by eliminating or at least minimizing the contact of water with the stone surface to improve the durability of the stones [14–17]. Current research approaches utilize polymers [15,18–23], silicones [16,17,24,25] and sol-gelbased hydrophobic coatings [26–28] to achieve these goals. So far, granite has not been widely studied in this field, due to its reputedly high stability against the degradation mechanisms commonly associated with other types of natural stones. Amongst the existing studies on granite, a hydrophobic functionalization of a biomimetic silica nanoparticle-alkoxysilane coating can be mentioned [29]. This coating reduced the water uptake by 20%. In another study, De Rosario et al. applied a novel nanoconsolidant to granite, which improved the stone’s resistance to salt crystallization weathering and slake durability [30]. Furthermore, De Rosario et al. proposed that the observed decrease in waterrelated weathering could be related to the decrease in the pore radii which their consolidation treatment provided, although no capillary absorption data was presented to support this. An interesting stone protection approach that have garnered a lot of attention in recent years is the use of polydimethylsiloxane (PDMS) coatings [16,31–33]. This is due to the high elasticity, toughness and evenness that PDMS coatings can provide. While the presented coatings in references [16,31–33] displayed impressive hydrophobic functionality, so far, no attempts have been made to study the durability of PDMS coatings on natural stones. In this study, we investigate how pure granite and PDMS-coated granite are affected by prolonged exposure to acidic environments, freeze/thaw cycling, salt crystallization, as well as to accelerated aging. Furthermore, the PDMS coatings are also compared to a commercial protective agent (Faceal Oleo HD). The performance of the stone coatings has been evaluated through capillary absorption, water contact angle, color difference and mass loss measurements, both before and after simulated aging, to elucidate the stability of the coatings. We also address a dimension of stone protection that is often overlooked, i.e. how the coating materials themselves are affected by different weathering mechanisms.
2. Materials and methods 2.1. Materials A cross-linked polydimethylsiloxane (PDMS, Dow Corning Sylgard 184) solution was prepared by first mixing a two-part commercial PDMS, consisting of a base component and a
cross-linking curing agent in the ratio 10:1, which was then diluted in toluene (Sigma Aldrich) to form a 10 wt% solution. A commercial water-repellent protective agent (Faceal Oleo HD from Uudenmaan Pintasuojaus, Finland) was used as received as a reference coating to evaluate the effectiveness and performance of the PDMS coatings. The Faceal Oleo HD product consists of waterborne acrylic copolymer with fluorinated groups for hydro- and oleophobicity, non-ionic cross-linking groups and silane groups for bond formation to mineral surfaces. Sodium sulfate (Merck) was used in the salt crystallization tests. Granite from the Fujian province, China, was used throughout all the experiments. X-ray diffraction (XRD) studies of the stones indicated that the granite mainly consists of quartz and feldspar minerals (see the supplementary material for further information, Fig. S1). Stones with the dimensions of 5 5 1 cm3 were used for the UV cabinet aging, the acid degradation tests and the salt crystallization tests, while granite samples with the dimensions 2 2 1 cm3 were used for the freeze-thaw tests (see details in Section 2.4 below). 2.2. Surface treatments The PDMS solution as well as the commercial protective agent was applied to the granite using a custom-made spray coater (Arctic IP Investment Ab, Salo, Finland) with variable spraying pressure, flow rate, and pitch. By varying the sprayhead speed, 20–100 mL/ m2 of the 10 wt% PDMS in toluene solution was applied to the granite, while keeping the flow rate = 0.2 mL/min, the pitch = 2 mm, the pressure = 2 bar and the sprayhead to surface distance = 330 mm. 400 mL/m2 of the Faceal Oleo HD solution was applied using a sprayhead speed of 30 mm/s, a flow rate of 0.5 mL/min, a pitch of 2 mm, a pressure of 1 bar and a sprayhead to surface distance of 330 mm. The used sprayhead was a DAGR airbrush (Devilbiss, Dorset, United Kingdom). After coating, the samples were allowed to react for a week at room temperature (T = 23 ± 2 °C, relative humidity = 45 ± 3%) before any further experiments were performed. 2.3. Initial characterizations In order to evaluate the performance of the produced granite coatings, we have investigated their wettability properties according to the methods described in our earlier studies [34,35]. An initial evaluation of the wettability of the coatings was obtained through water contact angle measurements using a goniometer (KSV CAM 200 Optical goniometer, KSV instruments Ltd., Helsinki, Finland), by depositing 2 lL water droplets on the coated granite surfaces and performing a contact angle fitting with the software supplied by the instrument manufacturer. Note that the measurements were performed in triplicate for each evaluated coating as well as for the pure granite, thus accounting for the reported standard deviations in the results section. Furthermore, the coated stones were also evaluated through water uptake analysis, i.e. capillary absorption, as specified in the standard protocol [36]. In short, pure granite and coated granite samples were placed upon a wet cloth for extended periods of time and their mass gain over time was studied (for further details, see our previous publications [34,35]). The water uptake of the coated samples were further normalized against the absorption of nine pure granite stones, allowing for easy comparison between different samples on a 0–100% scale, where 0% corresponds to no water absorption and 100% corresponds to full water absorption. Also in this test, measurements were performed in triplicate for all the coated samples, thus accounting for the reported standard deviations in the results section. The produced PDMS coatings are intended to block liquid water uptake in natural stones. However, also their water vapor blocking
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properties have to be investigated, because if the coatings were to completely block water vapor from diffusing through the porous network of the granite, the mechanical integrity of the stones could potentially deteriorate over time. We have used a slightly modified method [34] compared to the standard methods [37,38] to evaluate the water vapor permeability rate. The system is configured in such a way that a cylindrical piece of granite with known top area is affixed to the opening of a plastic tube (d = 30 mm) using pure PDMS. Water is introduced into the system with a syringe, after which the system is sealed, and the water vapor permeability through the stone material is studied over time. The measurement setup is available in the supplementary material (Fig. S2). Also in this test, measurements were performed in triplicate for all the coated samples. Another factor to consider when designing protective coatings is their potential effect on the appearance of the investigated building material. This is a paramount architectural consideration and thus, any potential color alteration was evaluated using a Minolta CM3600d spectrophotometer, which provides an absolute color difference, DEab , in the CIELAB color space [39]. Six measurements were taken on each surface in specular component included (SCI) mode, using a spot diameter of 8 mm and illuminant D65 at observer angle 10°. The color change, DEab , was then calculated as [39]:
DEab ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 ðL2 L1 Þ þ ða2 a1 Þ2 þ ðb2 b1 Þ ;
ð1Þ
where L2 is the lightness of the original stone and L1 is the lightness of a coated or weathered stone. The same annotation also applies for a⁄ (the red-green color component), and b⁄ (the blue-yellow color component). Additionally, the differences in individual color coordinates, i.e. DL⁄, Da⁄ and Db⁄, were calculated by subtracting the color coordinate after weathering or coating from the value obtained from the original stone. This data is reported in the supporting data (section D). 2.4. Simulated aging effects 2.4.1. Standardized accelerated aging To test the long-term durability of the coated granite, the samples were exposed to standardized accelerated aging using a QUV Accelerated Weathering Tester (Q-lab, Ohio, USA), which is capable of reproducing the damaging effects caused by sunlight (= photodegradation) and the damaging effects of dew and rain (= chemical weathering). The QUV Accelerated Weathering Tester reproduces the sunlight- and moisture-driven effects by exposing the tested materials to alternating cycles of UV light (through UV lamps) and moisture (through water condensation) at regulated, elevated temperatures. These features allow the QUV Accelerated Weathering Tester to reproduce the damage that corresponds to several months or years of real life outdoors exposure in a few weeks. Granite stones with the dimensions of 5 5 1 cm3 were used to evaluate the effects of the accelerated weathering on pure granite as well as on the PDMS and reference Faceal Oleo HD coatings. In our tests, the samples were exposed to cycles of UV irradiation for 8 h followed by 4 h of water condensation. This test was carried out for 2 months, which according to the European standard [40] would correspond to two years of actual outdoor exposure. The effects of the accelerated UV and moisture driven weathering (UV/M weathering) were evaluated by measuring color difference, contact angles, capillary absorption and weight loss. 2.4.2. Acid degradation Hydrolysis is one of the main processes by which granitic rocks are chemically weathered. To test the acidic resistance of the produced coatings, granite pieces with the dimensions of
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5 5 1 cm3 were used to evaluate the effects of acid degradation on pure granite as well as on the PDMS coatings and the reference Faceal Oleo HD coating. The stones were immersed in an HCl solution with the initial concentration of 0.001 M (pH 3) for 30 days, using 3 mL acid per gram stone. The solution was changed every 7 days, to ensure a continuous hydrolysis. The pH of the reacted solution was measured each week to investigate if H+ was consumed and if the hydrolysis reactions continued throughout the experiment. After acid exposure, the stones were evaluated with contact angle and capillary absorption measurements, and through determination of the mass loss. 2.4.3. Salt weathering – Resistance to salt crystallization The impact of salt crystallization in the pores was evaluated by using the test method defined by De Rosario et al. [30], which was adapted from the RILEM protocol [41]. When investigating the impact of salt crystallization in natural stones, the common way of performing the test is to immerse the stone in a saltcontaining solution [41]. While this is an excellent way of testing the impact of consolidating treatments, it is not suitable for the evaluation of protective coatings only covering one face of the stone. The adapted protocol in [30] circumvents this problem by introducing the salt solutions from one side of the stone through capillary absorption. Thus, only the coated side of the stone will be exposed to the degrading effects. Granite stones with the dimensions of 5 5 1 cm3 were used to evaluate the effects of salt crystallization on pure granite as well as on the coated stones. The experiments were performed in triplicate to provide standard deviations. One ‘‘salt exposure–drying” cycle can be defined as follows; (1) capillary absorption of a 14 wt% sodium sulfate solution for 2 h by placing the treated side of the stones facing the absorbent bed, (2) oven drying at 40 °C for 8 h, and (3) cooling at room temperature (T = 22 ± 3 °C, relative humidity = 40 ± 5%) for 14 h. After every 5 cycles, the tested stones were washed in a 20 °C water bath for 3 days (with continuously renewed water), after which the stones were dried to constant weight. The effects of the salt crystallization were evaluated by measuring the weight loss (every 5 cycles) and by measuring color difference, contact angles and capillary absorption after the last cycle (30 cycles in total). 2.4.4. Frost weathering The freeze-thaw tests attempted to reproduce the stresses, which may arise inside the rock when water turns into ice, generating pressure on the rock material. These effects are generally obtained by varying the temperature under and above 0 °C for samples containing water. The freeze-thaw testing was conducted in a way adopted from the standard ASTM D 5312-04 (see Fig. 1). The water-saturated samples were left to freeze at 18 °C for 16 h and then allowed to thaw at 40 °C for another 8 h. This process is defined as one freeze-thaw cycle and was repeated 30 times in this experiment. Due to size restrictions in the freezer unit used in these experiments, a sample size of 2 2 1 cm3 was used to evaluate the effects of frost weathering on pure granite as well as the PDMS coatings and the reference Faceal Oleo HD coating. The effects of the freeze-thaw cycling were evaluated by measuring the color difference, contact angles, capillary absorption and the weight loss after the last cycle. 3. Results 3.1. Initial characterization of the coatings and the stones The PDMS and Faceal Oleo HD solutions were used to hydrophobize granite samples according to the protocol described
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Fig. 1. Schematics of (a) the freeze-thaw cycling process and (b) the mechanism of degradation.
Table 1 Water contact angles, capillary water absorption values, color differences and water vapor permeability (Q/t) for the granite before and after coating with the different formulations.
*
Fig. 2. Capillary absorption of the granite before and after coating with the different formulations.
in the Materials and methods section. The capillary absorption data is presented in Fig. 2 and summarized in Table 1 together with the corresponding static water contact angles. These results indicate how effective the different water repellent coatings are. The contact angle measurements show that the mean water contact angle on the untreated granite is 41 ± 3°, while all PDMS coatings give rise to water contact angles of 120° or higher, which indicates a successful hydrophobization of the granite surface. Also the Faceal Oleo HD-coated granite displays a contact angle of 122 ± 2°. The relatively large contact angles are probably a result
Sample
WCA [°]
Capillary absorption [%]
Color difference DEab
Q/t [mg/cm2h]
Untreated granite Faceal Oleo HD PDMS – 20 mL/m2 PDMS – 40 mL/m2 PDMS – 60 mL/m2 PDMS – 80 mL/m2 PDMS – 100 mL/m2
41 ± 3 122 ± 2 121 ± 2 124 ± 4 120 ± 1 121 ± 3 125 ± 1
100 ± 10 40 ± 10 30 ± 10 12 ± 6 11 ± 6 6±3 10 ± 6
– 0.34 0.83 2.52 3.11 3.91 5.32
0.0553 ± 0.0007 0.0560 ± 0.0003 0.0512 ± 0.0004 –* 0.0513 ± 0.0005 –* 0.0531 ± 0.0005
Not determined.
of the low polarity of the coating material combined with structural effects of the underlying granite surface. The capillary absorption and standard deviation for nine standard untreated granite stones were measured to be 0.059 ± 0.008 kg/m2 after 48 h on the absorbent bed. The coated (and weathered) samples were normalized against this mean value (0.059 kg/m2) and the normalized values are presented here in the Results Section. The recorded absorption values (in kg/m2 at 48 h) are available in the supplementary material (Table S1). The capillary absorption measurements in Fig. 2 show that the reference coating, Faceal Oleo HD, absorbs 40 ± 10% water compared to pure granite. On the other hand, the PDMS coatings display a decreasing water uptake trend (from 30% to less than 10% compared to pure granite) when the surface coverage is increased from 20 to 100 mL/m2. These
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preliminary results indicate that 40 mL/m2 of the PDMS solution is sufficient to produce a coating that reduces the water absorption to less than 10% compared to the pristine stone material. The measured water vapor permeability rate and color differences for the different coatings are presented in Table 1. The water vapor permeability of stone materials should remain unchanged after a hydrophobization treatment. This is important as water condensation beneath a protective coating as well as accumulation of condensed water behind a treated stone can lead to stone decay [23]. The vapor permeability measurements revealed that the coatings did not affect the vapor permeability of the pure granite significantly, as the PDMS-coated granite samples displayed a permeability of about 95% compared to pure granite. The Faceal Oleo HD-coated stones displayed a permeability that is within the error margins of the pure granite, which is also what the manufacturer advertises. The water vapor penetration through a pure PDMS elastomer layer was found to be negligible, which further proves that the pores are not entirely blocked by PDMS for the coated stones. In order to investigate if any undesired color alterations occurred to the stone surface, colorimetric measurements were conducted on stones treated with different surface coverage of PDMS, as well as the reference coating of Faceal Oleo HD. All hydrophobized samples were compared to the untreated standards and the color differences are summarized in Table 1. The small measured color changes for the PDMS coatings (DEab 0.8–5) revealed that no significant discoloring of the granite surface had occurred even when using a PDMS surface coverage of 100 mL/ m2. It should be noted that the detected differences are slightly above the just noticeable difference level, which is defined as the color change needed to see a difference by the naked eye, i.e. DEab >2.3. By reviewing the color difference data (available in the supplementary material in Tables S2 and S3), we could observe that the lightness parameter (DL⁄) decreased with increasing PDMS coverage, probably as the light reflection decreased slightly with the increased coarseness of the coatings. 3.2. Impact of accelerated aging The effects accelerated aging (using the QUV accelerated weathering tester) had on the water contact angle, the capillary absorption, the mass loss and the color difference are listed in Table 2. The accelerated aging had minimal effects on the untreated granite, as the capillary absorption of the pure granite before and after the aging were within the standard deviations. Similarly, the PDMS coatings were not significantly affected by accelerated weathering, displaying no increase in water absorption, contact angles or color difference. This shows that the PDMS-based coatings are very durable, when UV degradation and moisture effects are concerned. Interestingly, the PDMS coatings displayed mass losses that were slightly lower than observed for the pure granite.
Table 2 Recorded mass losses, water contact angles, capillary absorption and color difference on the coated stones after treatment and after two months of accelerated UV/M weathering.
The Faceal Oleo HD-coated stones, on the other hand, display a common problem encountered in the development of protective coatings for natural stones. That is to say, UV radiation seems to be detrimental to these coatings, as the contact angle decreased from hydrophobic (122 ± 2°) to hydrophilic (5 ± 1°), which is significantly lower than the contact angle of pure granite. This further suggests that the hydrophobic functional groups have degraded via UV exposure, making the surface even more hydrophilic. This change in material properties is also reflected in the large increase in water absorption, from the 36 ± 6% absorption recorded after coating to the significantly higher 90 ± 40% after accelerated aging (values normalized to the absorption of the pure granite). The Faceal Oleo HD-coated stones also display a higher mass loss from accelerated aging of 0.060 ± 0.001% compared to the 0.025 ± 0.001% mass loss of the pure granite. As the stones have been coated with a relatively large amount of Faceal Oleo HD (400 mL/m2), a large part of the mass loss is probably due to the degradation of the coating material itself. Obviously, as the pure granite and the PDMS-coated stones also display some mass loss, the degradation of the granite material would also contribute to the mass loss of the Faceal Oleo HD-coated stones. Finally, the color difference measurements revealed no large changes for the PDMS samples, while the pure granite displayed a DEab value of 3.3, which arose from an increase in the lightness (L) parameter. Furthermore, the Faceal Oleo HD-coated stones displayed a DEab value of 1.7, related to an increase in the lightness (DL⁄) parameter and the blue-yellow (Db⁄) component. This increase could be seen on the samples as a yellow discoloration, a known flaw related to UV exposure of certain organic functionalizations [42,43]. The exact color differences are available in the supplementary material (Table S4).
3.3. Impact of acid degradation During the acid degradation experiment, the pH 3 HCl solution was exchanged once a week and it was found that the pH increased to 3.3–3.5 after 1 week for all samples, indicating that the samples were not significantly affected by the acid degradation. After 30 days in a pH 3 HCl solution, the stones were washed and dried until they reached a constant weight and the mass loss from the acid degradation experiment was determined. The capillary absorption, water contact angles and color differences were measured to determine the impact of the acid immersion. The data from these measurements are summarized in Table 3. The effect of acid dissolution on granite can be deduced from the mass losses recorded after the experiment. The pure granite decreased by 0.044 ± 0.002% in mass, while the Faceal Oleo HDcoated sample decreased slightly more, by 0.055 ± 0.003%, and the best PDMS sample decreased by only 0.0315 ± 0.0006%. The positive effect of the PDMS coating is evident in its ability to reduce the direct contact area between the acid and the pure stone
Table 3 Recorded mass losses, water contact angles, capillary absorption and color change after the acid degradation experiment.
Sample
Mass loss [%]
WCA [°]
Capillary absorption [%]
Color difference DEab
Sample
Mass loss [%]
WCA [°]
Capillary absorption [%]
Color difference DEab
Untreated granite Faceal Oleo HD PDMS – 20 mL/m2 PDMS – 40 mL/m2 PDMS – 60 mL/m2 PDMS – 80 mL/m2 PDMS – 100 mL/m2
0.025 ± 0.001 0.060 ± 0.001 0.0221 ± 0.0006 0.024 ± 0.003 0.0209 ± 0.0004 0.023 ± 0.002 0.0215 ± 0.0008
39 ± 1 5±1 115 ± 5 120.5 ± 0.8 126 ± 2 121 ± 1 120 ± 3
112 ± 6 90 ± 40 8±2 7±1 5.9 ± 0.8 6.2 ± 0.8 7.1 ± 0.3
3.32 1.73 1.32 1.34 0.34 0.74 2.67
Untreated granite Faceal Oleo HD PDMS – 20 mL/m2 PDMS – 40 mL/m2 PDMS – 60 mL/m2 PDMS – 80 mL/m2 PDMS – 100 mL/m2
0.044 ± 0.002 0.055 ± 0.003 0.040 ± 0.003 0.0361 ± 0.0004 0.037 ± 0.001 0.0315 ± 0.0006 0.0325 ± 0.0009
44.4 ± 0.6 123 ± 1 119 ± 3 116 ± 2 118 ± 1 120 ± 3 116 ± 2
93 ± 1 36 ± 6 8±2 6.0 ± 0.5 6.6 ± 0.8 7±1 7.1 ± 0.6
1.55 1.8 1.18 4.68 5.04 4.25 4.78
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surface. As only the front side of the stone is fully protected with the coating (accounting for 36% of the stone’s surface area), the rest of the stone surface (64%) is still susceptible to unhindered acid reactions. Using this data and the mass loss of the untreated granite, it follows that an ideal surface coating would have a mass loss of 0.64 0.044% = 0.028%, which is close to the mass loss observed for the best PDMS-coated stones. This means that the face coated with PDMS can efficiently block the transport of hydronium ions in and out of the pore system and thus slow down the degradation rate. It is surprising that the Faceal Oleo HD-coated stones display a mass loss that is higher than the one of pure granite, which could be related to interactions between the coating material and the acid or due to the partial degradation of the Faceal Oleo HD coating mentioned in Section 3.1. The capillary absorption results obtained after the acid degradation show that the investigated coatings are very robust, with differences within the margins of error when compared to the same coated stones before aging. The only exception is observed for the 20 mL/m2 PDMS-coated stones, where the capillary absorption was 30 ± 10% after coating and was actually improved to 8 ± 1% after the acid degradation. This could be due to some restructuring of the PDMS coating through an acid-catalyzed sol-gel reaction. The water contact angles were also largely unchanged after the acid degradation, with some slight decrease for the PDMS coatings, while the Faceal Oleo HD coating was completely unaffected. The color difference measurements after the acid degradation revealed that the appearance of the pure granite, the Faceal Oleo HD, and the 20 mL/m2 PDMS-coated granite were only slightly affected by the acid degradation. While the PDMS coatings with surface coverages of 40–100 mL/m2 displayed color differences in the range of 4–5 units, which from reviewing the measurement data was found to be due to a decrease in the lightness parameter (DL⁄). The exact color differences are available in the supplementary material (Table S5). 3.4. Impact of salt weathering The damaging effects observed from the salt weathering test seem to be the most severe of all the aging mechanism investigated in this study. As seen in Fig. 3 and Table 4, the recorded mass losses during this experiment are greater than in any of the other weathering experiments. The mass of the pure granite decreased by 0.5 ± 0.1% after 30 salt crystallization cycles, while the Faceal
Table 4 Recorded mass losses, water contact angles and capillary absorptions after the salt weathering experiment. Sample
Mass loss [%]
WCA [°]
Capillary absorption [%]
Color difference DEab
Untreated granite Faceal Oleo HD PDMS – 20 mL/m2 PDMS – 40 mL/m2 PDMS – 60 mL/m2 PDMS – 80 mL/m2 PDMS – 100 mL/m2
0.5 ± 0.1 0.6 ± 0.1 0.06 ± 0.01 0.08 ± 0.08 0.07 ± 0.02 0.04 ± 0.01 0.04 ± 0.004
62 ± 5 76 ± 7 127 ± 7 123 ± 3 131 ± 4 127 ± 3 130 ± 6
126 ± 2 117 ± 9 29 ± 8 14 ± 4 9±2 9±1 11 ± 2
1.47 2.21 1.56 2.71 4.18 2.49 3.8
Oleo HD-coated stones displayed a similar mass loss (0.6 ± 0.1%). On the other hand, the PDMS-coated stones displayed mass losses on an order of magnitude lower than for the pure granite and Faceal Oleo HD-coated stones (in the range of 0.04–0.08%), which is probably due to the limited capillary absorption through the coatings that consequently reduces the salt ion diffusion into the pores in these systems. The effect of the salt crystallization cycling is also detectable in the capillary absorption measurements, where the absorption for the pure granite increased by 26 percentage points. A similar effect is seen for the Faceal Oleo HD-coated stones, where the capillary absorption increased from 40 ± 10 to 117 ± 9%, showing that the Faceal Oleo HD coating is ineffective when it comes to preventing salt weathering. In contrast, the capillary absorption for the PDMScoated stones is completely unchanged from the salt weathering, showing that impact of salt weathering can be minimized by using a PDMS coating. The contact angles measured on the stones after the salt crystallization cycles reveal several interesting conclusions. Firstly, the contact angle of the Faceal Oleo HD-coated stones has decreased from 122 ± 2° to 76 ± 7°, indicating a detrimental effect on the surface coating. Secondly, the contact angles for the pure granite and for several of the PDMS-coated stones actually increased from the salt weathering. This could be a result of structural effects due to salt crystallization on the stone surface. The color difference measurements revealed no large changes for the pure granite or for the PDMS-coated granite stones, while the Faceal Oleo HD-coated stones actually increased in DEab from 0.34 after coating to 2.2 after salt weathering, which was due to an increase in the lightness (DL⁄) parameter. The exact color differences are available in the supplementary material (Table S6).
3.5. Impact of freeze-thaw cycling Based on the recorded mass losses in Table 5, it seems like the effect of frost weathering is more detrimental for the actual protective coating materials than for the granite itself. As seen from Table 5, the mass losses recorded for the PDMS- and the Faceal
Table 5 Recorded mass losses, water contact angles and capillary absorptions after 30 freezethaw cycles.
Fig. 3. Mass loss of pure and coated granite as a function of the number of salt crystallization cycles. The dashed lines are Box Lucas exponential fits, using the model: y ¼ að1 ebx Þ, with a Levenberg Marquardt iteration algorithm, which are included as guidelines for data interpretation.
Sample
Mass loss [%]
WCA [°]
Capillary absorption [%]
Color difference
Untreated granite Faceal Oleo HD PDMS – 20 mL/m2 PDMS – 40 mL/m2 PDMS – 60 mL/m2 PDMS – 80 mL/m2 PDMS – 100 mL/m2
0.001 ± 0.001 0.013 ± 0.002 0.023 ± 0.007 0.0164 ± 0.003 0.0180 ± 0.003 0.0178 ± 0.002 0.0215 ± 0.007
60 ± 2 106 ± 2 112 ± 4 109 ± 4 110 ± 10 109 ± 4 110 ± 5
106 ± 5 50 ± 10 60 ± 20 30 ± 20 8±1 7±2 7±4
4.6 4.77 2.68 2.76 1.4 3.92 1.24
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Oleo HD-coated stones after the frost weathering were actually larger than the mass loss recorded for the pure granite. Furthermore, the contact angle of the pristine granite sample was slightly higher after frost weathering, which might be due to the contact with water/ice in the weathering process, while the percentage of capillary absorption was within the error margins of the pristine sample. The PDMS- and Faceal Oleo HD-coated granite samples display a small decrease in the contact angles after the 30 freezethaw cycles as well as slight deviations in the capillary absorption results, indicating that the coatings were only slightly affected by frost weathering when capillary absorption and surface hydrophobicity are concerned. Finally, while there were noticeable color differences between the samples before and after freeze-thaw aging, it should be noted that the smaller size of the samples used in this test (2 2 cm2 instead of 5 5 cm2) could have an influence on the results due to the phaneritic nature of granite. The measured color differences are included to show that no large, substantial changes occurred from the weathering process. The exact color differences are available in the supplementary material (Table S7).
4. Discussion The outcome of the weathering tests is very informative, as the coating stability and the hydrophobic functionality are found to be closely interlinked. As a sufficiently efficient coating can block water from entering the pores, the degradation effects related to salts and water will be smaller and the coating is less likely to further degrade from weathering effects related to water. The benefit of a good protective coating can be seen in Fig. 4, where the capillary absorption of PDMS coatings with a surface coverage above 60 mL/m2 displayed unchanged capillary absorption after the different weathering tests, showing the superior stability of this coating. Another interesting observation can be made from the two mechanical weathering tests (salt and frost weathering) on the pristine granite and stones coated with Faceal Oleo HD, where 30 salt crystallization cycles seem to have a much larger impact on the tested stones than 30 freeze-thaw cycles. When considering these weathering effects, it is important to note that freezing water crystallizes into an open hexagonal lattice structure [44], while the
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sodium sulfate crystallization proceeds via mirabilite crystal growth [45]. Another difference between the frost and salt weathering is that the freeze-thaw process occurs through frost wedging, i.e. slowly expanding the pores, while salt weathering occurs as a result of internal pressure being applied in the entire salt-filled pore system due to crystal growth. This distinction reveals that frost weathering is more localized, creating far less stress points than in salt weathering, where the effects can be said to be global. From the UV/M weathering tests, it is evident that also UV light is unable to degrade the PDMS network. The reason for this is that PDMS is a very chemically stable hybrid material with a robust silicone backbone that radicals generated by UV radiation simply cannot degrade. The Faceal Oleo HD coating, on the other hand, is completely degraded from UV exposure, due to radical attack on the less stable acrylic polymer network it consists of [46]. Finally, an important factor to contemplate when considering the overall impact of weathering is that none of these processes would occur independently in nature. Rather a combination of different weathering effects can augment the weathering impact on pure granite or coated stones. For instance, sunlight may degrade a protective treatment, such as the Faceal Oleo HD coating investigated in this paper, which allows for a more severe salt weathering due to increased water/salt absorption. 5. Conclusions In this study, PDMS solutions and a commercial reference solution (Faceal Oleo HD) have been spray-coated on granite surfaces with the goal of enhancing the material stability against weathering effects. We have done this by demonstrating the durability of the coatings and by evaluating the impact different weathering effects have on granite. In these tests, salt weathering proved to have the most severe impact on granite, as a relatively large mass loss was observed after 30 cycles of salt crystallization. Furthermore, the pure granite and the Faceal Oleo HD-coated stones displayed much larger mass losses after salt weathering compared to the PDMS-coated stones, which showed mass losses of 10% compared to the pure granite sample. We believe the low water uptake for the PDMS-coated samples also hindered the salt from being introduced into the pores and by this means the salt weathering was reduced. The most dramatic effect on the hydrophobic properties of the coatings could be observed for the Faceal Oleo HD-coated stones after the UV/M weathering, where the contact angle decreased drastically from 122 ± 2° to 5 ± 1°. This demonstrates how some weathering mechanisms severely can affect a surface coating, e.g. in this case the acrylic backbone of the Faceal Oleo HD coating was susceptible to degradation via free radicals created by UV radiation. On the other hand, the PDMS coatings displayed excellent stability when contact angles are concerned, as none of the weathering tests were able to decrease the contact angles below 90°. In general, it can be concluded that the PDMS coatings are very efficient for minimizing weathering effects on granite and additionally the weathering impact on the PDMS coating itself can be considered minimal. Furthermore, as the coatings can be easily applied by spray coating, we believe that the presented coating method will be very useful for hydrophobizing large granite objects and for retrofitting of buildings and bridges. Acknowledgements
Fig. 4. Capillary absorption matrix, showing the capillary water absorption after coating and after the different weathering tests for the investigated coatings.
The Magnus Ehrnrooth Foundation, the Swedish Cultural Foundation and the K.H. Renlund Foundation are greatly acknowledged for financial support (D.K.). Furthermore, the Academy of Finland is also acknowledged for funding through the project 259310 (J.H.S.).
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[24] C. Esposito Corcione, R. Striani, M. Frigione, UV-cured siloxane-modified methacrylic system containing hydroxyapatite as potential protective coating for carbonate stones, Prog. Org. Coat. 76 (2013) 1236–1242. [25] J. Kapolos, N. Bakaoukas, A. Koliadima, G. Karaiskakis, Evaluation of acrylic polymeric resin and small siloxane molecule for protecting cultural heritage monuments against sulfur dioxide corrosion, Prog. Org. Coat. 59 (2007) 152– 159. [26] Z. Guo, W. Liu, B. Su, Superhydrophobic surfaces: From natural to biomimetic to functional, J. Colloid Interface Sci. 353 (2011) 335–355. [27] C. Kapridaki, L. Pinho, M.J. Mosquera, P. Maravelaki-Kalaitzaki, Producing photoactive, transparent and hydrophobic SiO2-crystalline TiO2 nanocomposites at ambient conditions with application as self-cleaning coatings, Appl. Catal., B 156–157 (2014) 416–427. [28] I. Karapanagiotis, A. Pavlou, P.N. Manoudis, K.E. Aifantis, Water repellent ORMOSIL films for the protection of stone and other materials, Mater. Lett. 131 (2014) 276–279. [29] L. de Ferri, P.P. Lottici, A. Lorenzi, A. Montenero, E. Salvioli-Mariani, Study of silica nanoparticles – polysiloxane hydrophobic treatments for stone-based monument protection, J. Cult. Heritage 12 (2011) 356–363. [30] I. De Rosario, F. Elhaddad, A. Pan, R. Benavides, T. Rivas, M.J. Mosquera, Effectiveness of a novel consolidant on granite: Laboratory and in situ results, Constr. Build. Mater. 76 (2015) 140–149. [31] F. Xu, C. Wang, D. Li, M. Wang, F. Xu, X. Deng, Preparation of modified epoxy– SiO2 hybrid materials and their application in the stone protection, Prog. Org. Coat. 81 (2015) 58–65. [32] Y. Luo, L. Xiao, X. Zhang, Characterization of TEOS/PDMS/HA nanocomposites for application as consolidant/hydrophobic products on sandstones, J. Cult. Heritage 16 (2015) 470–478. [33] D. Li, F. Xu, Z. Liu, J. Zhu, Q. Zhang, L. Shao, The effect of adding PDMS-OH and silica nanoparticles on sol–gel properties and effectiveness in stone protection, Appl. Surf. Sci. 266 (2013) 368–374. [34] D. Kronlund, A. Bergbreiter, A. Meierjohann, L. Kronberg, M. Lindén, D. Grosso, J.-H. Smått, Hydrophobization of marble pore surfaces using a total immersion treatment method – Product selection and optimization of concentration and treatment time, Prog. Org. Coat. 85 (2015) 159–167. [35] D. Kronlund, A. Bergbreiter, M. Lindén, D. Grosso, J.-H. Smått, Hydrophobization of marble pore surfaces using a total immersion treatment method – Influence of co-solvents and temperature on fluorosurfactant vesicle behavior, Colloid Surf., A. 483 (2015) 104–111. [36] SFS-EN 1925:1999 Natural stone test methods – Determination of water absorption coefficient by capillarity. [37] D. Vandevoorde, M. Pamplona, O. Schalm, Y. Vanhellemont, V. Cnudde, E. Verhaeven, Contact sponge method: Performance of a promising tool for measuring the initial water absorption, J. Cult. Heritage 10 (2009) 41–47. [38] P.N. Manoudis, A. Tsakalof, I. Karapanagiotis, I. Zuburtikudis, C. Panayiotou, Fabrication of super-hydrophobic surfaces for enhanced stone protection, Surf. Coat. Tech. 203 (2009) 1322–1328. [39] EN 15886:2010 – Conservation of cultural property – Test methods – Colour measurement of surfaces – European standard, 2010. [40] EOTA technical report TR 010 Exposure procedure for artificial weathering, 2004. [41] Réunion Internationale des Laboratoires d’Essais et de recherche sur les matériaux et les Constructions (RILEM). Crystallization test by total immersion (Test V. 1.) Crystallization test by partial immersion (Test V. 2.). In: Proceed. Intern. Symposium Deterioration and Conservation of Stone monuments. UNESCO-RILEM, Paris; 1978. [42] D. Rosu, L. Rosu, C.N. Cascaval, IR-change and yellowing of polyurethane as a result of UV irradiation, Polym. Degrad. Stab. 94 (2009) 591–596. [43] A. Jentsch, K.-J. Eichhorn, B. Voit, Influence of typical stabilizers on the aging behavior of EVA foils for photovoltaic applications during artificial UVweathering, Polym. Test. 44 (2015) 242–247. [44] M. Akyurt, G. Zaki, B. Habeebullah, Freezing phenomena in ice–water systems, Energy Convers. Manage. 43 (2002) 1773–1789. [45] C. Rodriguez-Navarro, E. Doehne, E. Sebastian, How does sodium sulfate crystallize? Implications for the decay and testing of building materials, Cem. Concr. Res. 30 (2000) 1527–1534. [46] G. Cappelletti, P. Fermo, M. Camiloni, Smart hybrid coatings for natural stones conservation, Prog. Org. Coat. 78 (2015) 511–516.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
A polydimethylsiloxane coating to minimize weathering effects on granite Dennis Kronlund a, Mika Lindén b, Jan-Henrik Smått a,⇑ a b
Laboratory of Physical Chemistry and Center of Excellence for Functional Materials, Åbo Akademi University, Porthansgatan 3-5, 20500 Åbo, Finland Inorganic Chemistry II, University of Ulm, Albert-Einstein-Allee 11, 89031 Ulm, Germany
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A sprayable water-repellent PDMS
coating is developed for granite protection. The impact of UV- and water-based weathering mechanisms on granite is studied. The PDMS coatings display great stability in all the studied weathering tests. Degradation effects of pore salt crystallization is eliminated with a PDMS coating.
a r t i c l e
i n f o
Article history: Received 21 April 2016 Received in revised form 2 August 2016 Accepted 29 August 2016
Keywords: Granite Polydimethylsiloxane Coating Hydrophobization Water absorption Degradation Stability Weathering
a b s t r a c t The impact of common weathering mechanisms on granite has been elucidated. A sprayable polydimethylsiloxane (PDMS) solution and a commercial reference coating formulation (Faceal Oleo HD) have been applied on granite to produce durable surface coatings that minimize weathering effects. Sufficiently thick PDMS coatings display superior stability in all the studied weathering tests when compared to the reference coating, as the impact of weathering on the hydrophobic functionality of the PDMS-coated samples is minimal. Furthermore, due to the excellent water and salt blocking behavior of PDMS, the PDMS-coated stones display an overall better stability against weathering compared to untreated stones. Ó 2016 Published by Elsevier Ltd.
1. Introduction Granite is a dense, hard and tough stone material and has therefore gained widespread use throughout human history (from hand tools and projectile weapons to foundation elements and city ⇑ Corresponding author. E-mail address: [email protected] (J.-H. Smått). http://dx.doi.org/10.1016/j.conbuildmat.2016.08.146 0950-0618/Ó 2016 Published by Elsevier Ltd.
walls), and more recently as durable construction materials (used for buildings, bridges, tunnels, etc.) [1]. Furthermore, granite has a relatively good chemical stability due to its high quartz content. Although granite is generally considered to be a very durable stone material, its daily exposure to various weathering phenomena can potentially reduce its strength as a construction material [2–4]. Weathering is a general problem in the degradation of natural stones, especially erosion related to water uptake in the pores of
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the stones [3]. Weathering, which can be divided into physical, chemical and biological degradation routes, creates chemical and structural alterations to the stone over time [5,6]. An important physical degradation route in cold climates is related to the repeated freezing and thawing of water absorbed inside the pores of the stones. As ice has a larger molar volume than liquid water, freezing creates tension throughout the porous stone, which eventually can lead to rock fractures [7,8]. Another form of physical deterioration is linked to salt crystallization in the pores of stone materials (a potential problem in coastal areas), where the formation of salt crystals apply pressure on the porous network [9,10], causing the stone to degrade through a similar mechanism as in ice formation. Furthermore, chemical degradation can occur through several different routes depending on the type of stone material. For instance, granite can be disintegrated into granitic sand, which is a process that occurs through hydrolysis of potassium feldspar into clay minerals [11–13]. There are several sources of H+ in nature that can cause this degradation, including H2CO3 (formed by dissolution of CO2 in water), H2SO4 and HNO3 (in acid rain), as well as a range of weaker organic acids in the soil. Earlier stone conservation studies have focused on reducing water-related weathering effects, by eliminating or at least minimizing the contact of water with the stone surface to improve the durability of the stones [14–17]. Current research approaches utilize polymers [15,18–23], silicones [16,17,24,25] and sol-gelbased hydrophobic coatings [26–28] to achieve these goals. So far, granite has not been widely studied in this field, due to its reputedly high stability against the degradation mechanisms commonly associated with other types of natural stones. Amongst the existing studies on granite, a hydrophobic functionalization of a biomimetic silica nanoparticle-alkoxysilane coating can be mentioned [29]. This coating reduced the water uptake by 20%. In another study, De Rosario et al. applied a novel nanoconsolidant to granite, which improved the stone’s resistance to salt crystallization weathering and slake durability [30]. Furthermore, De Rosario et al. proposed that the observed decrease in waterrelated weathering could be related to the decrease in the pore radii which their consolidation treatment provided, although no capillary absorption data was presented to support this. An interesting stone protection approach that have garnered a lot of attention in recent years is the use of polydimethylsiloxane (PDMS) coatings [16,31–33]. This is due to the high elasticity, toughness and evenness that PDMS coatings can provide. While the presented coatings in references [16,31–33] displayed impressive hydrophobic functionality, so far, no attempts have been made to study the durability of PDMS coatings on natural stones. In this study, we investigate how pure granite and PDMS-coated granite are affected by prolonged exposure to acidic environments, freeze/thaw cycling, salt crystallization, as well as to accelerated aging. Furthermore, the PDMS coatings are also compared to a commercial protective agent (Faceal Oleo HD). The performance of the stone coatings has been evaluated through capillary absorption, water contact angle, color difference and mass loss measurements, both before and after simulated aging, to elucidate the stability of the coatings. We also address a dimension of stone protection that is often overlooked, i.e. how the coating materials themselves are affected by different weathering mechanisms.
2. Materials and methods 2.1. Materials A cross-linked polydimethylsiloxane (PDMS, Dow Corning Sylgard 184) solution was prepared by first mixing a two-part commercial PDMS, consisting of a base component and a
cross-linking curing agent in the ratio 10:1, which was then diluted in toluene (Sigma Aldrich) to form a 10 wt% solution. A commercial water-repellent protective agent (Faceal Oleo HD from Uudenmaan Pintasuojaus, Finland) was used as received as a reference coating to evaluate the effectiveness and performance of the PDMS coatings. The Faceal Oleo HD product consists of waterborne acrylic copolymer with fluorinated groups for hydro- and oleophobicity, non-ionic cross-linking groups and silane groups for bond formation to mineral surfaces. Sodium sulfate (Merck) was used in the salt crystallization tests. Granite from the Fujian province, China, was used throughout all the experiments. X-ray diffraction (XRD) studies of the stones indicated that the granite mainly consists of quartz and feldspar minerals (see the supplementary material for further information, Fig. S1). Stones with the dimensions of 5 5 1 cm3 were used for the UV cabinet aging, the acid degradation tests and the salt crystallization tests, while granite samples with the dimensions 2 2 1 cm3 were used for the freeze-thaw tests (see details in Section 2.4 below). 2.2. Surface treatments The PDMS solution as well as the commercial protective agent was applied to the granite using a custom-made spray coater (Arctic IP Investment Ab, Salo, Finland) with variable spraying pressure, flow rate, and pitch. By varying the sprayhead speed, 20–100 mL/ m2 of the 10 wt% PDMS in toluene solution was applied to the granite, while keeping the flow rate = 0.2 mL/min, the pitch = 2 mm, the pressure = 2 bar and the sprayhead to surface distance = 330 mm. 400 mL/m2 of the Faceal Oleo HD solution was applied using a sprayhead speed of 30 mm/s, a flow rate of 0.5 mL/min, a pitch of 2 mm, a pressure of 1 bar and a sprayhead to surface distance of 330 mm. The used sprayhead was a DAGR airbrush (Devilbiss, Dorset, United Kingdom). After coating, the samples were allowed to react for a week at room temperature (T = 23 ± 2 °C, relative humidity = 45 ± 3%) before any further experiments were performed. 2.3. Initial characterizations In order to evaluate the performance of the produced granite coatings, we have investigated their wettability properties according to the methods described in our earlier studies [34,35]. An initial evaluation of the wettability of the coatings was obtained through water contact angle measurements using a goniometer (KSV CAM 200 Optical goniometer, KSV instruments Ltd., Helsinki, Finland), by depositing 2 lL water droplets on the coated granite surfaces and performing a contact angle fitting with the software supplied by the instrument manufacturer. Note that the measurements were performed in triplicate for each evaluated coating as well as for the pure granite, thus accounting for the reported standard deviations in the results section. Furthermore, the coated stones were also evaluated through water uptake analysis, i.e. capillary absorption, as specified in the standard protocol [36]. In short, pure granite and coated granite samples were placed upon a wet cloth for extended periods of time and their mass gain over time was studied (for further details, see our previous publications [34,35]). The water uptake of the coated samples were further normalized against the absorption of nine pure granite stones, allowing for easy comparison between different samples on a 0–100% scale, where 0% corresponds to no water absorption and 100% corresponds to full water absorption. Also in this test, measurements were performed in triplicate for all the coated samples, thus accounting for the reported standard deviations in the results section. The produced PDMS coatings are intended to block liquid water uptake in natural stones. However, also their water vapor blocking
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properties have to be investigated, because if the coatings were to completely block water vapor from diffusing through the porous network of the granite, the mechanical integrity of the stones could potentially deteriorate over time. We have used a slightly modified method [34] compared to the standard methods [37,38] to evaluate the water vapor permeability rate. The system is configured in such a way that a cylindrical piece of granite with known top area is affixed to the opening of a plastic tube (d = 30 mm) using pure PDMS. Water is introduced into the system with a syringe, after which the system is sealed, and the water vapor permeability through the stone material is studied over time. The measurement setup is available in the supplementary material (Fig. S2). Also in this test, measurements were performed in triplicate for all the coated samples. Another factor to consider when designing protective coatings is their potential effect on the appearance of the investigated building material. This is a paramount architectural consideration and thus, any potential color alteration was evaluated using a Minolta CM3600d spectrophotometer, which provides an absolute color difference, DEab , in the CIELAB color space [39]. Six measurements were taken on each surface in specular component included (SCI) mode, using a spot diameter of 8 mm and illuminant D65 at observer angle 10°. The color change, DEab , was then calculated as [39]:
DEab ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 ðL2 L1 Þ þ ða2 a1 Þ2 þ ðb2 b1 Þ ;
ð1Þ
where L2 is the lightness of the original stone and L1 is the lightness of a coated or weathered stone. The same annotation also applies for a⁄ (the red-green color component), and b⁄ (the blue-yellow color component). Additionally, the differences in individual color coordinates, i.e. DL⁄, Da⁄ and Db⁄, were calculated by subtracting the color coordinate after weathering or coating from the value obtained from the original stone. This data is reported in the supporting data (section D). 2.4. Simulated aging effects 2.4.1. Standardized accelerated aging To test the long-term durability of the coated granite, the samples were exposed to standardized accelerated aging using a QUV Accelerated Weathering Tester (Q-lab, Ohio, USA), which is capable of reproducing the damaging effects caused by sunlight (= photodegradation) and the damaging effects of dew and rain (= chemical weathering). The QUV Accelerated Weathering Tester reproduces the sunlight- and moisture-driven effects by exposing the tested materials to alternating cycles of UV light (through UV lamps) and moisture (through water condensation) at regulated, elevated temperatures. These features allow the QUV Accelerated Weathering Tester to reproduce the damage that corresponds to several months or years of real life outdoors exposure in a few weeks. Granite stones with the dimensions of 5 5 1 cm3 were used to evaluate the effects of the accelerated weathering on pure granite as well as on the PDMS and reference Faceal Oleo HD coatings. In our tests, the samples were exposed to cycles of UV irradiation for 8 h followed by 4 h of water condensation. This test was carried out for 2 months, which according to the European standard [40] would correspond to two years of actual outdoor exposure. The effects of the accelerated UV and moisture driven weathering (UV/M weathering) were evaluated by measuring color difference, contact angles, capillary absorption and weight loss. 2.4.2. Acid degradation Hydrolysis is one of the main processes by which granitic rocks are chemically weathered. To test the acidic resistance of the produced coatings, granite pieces with the dimensions of
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5 5 1 cm3 were used to evaluate the effects of acid degradation on pure granite as well as on the PDMS coatings and the reference Faceal Oleo HD coating. The stones were immersed in an HCl solution with the initial concentration of 0.001 M (pH 3) for 30 days, using 3 mL acid per gram stone. The solution was changed every 7 days, to ensure a continuous hydrolysis. The pH of the reacted solution was measured each week to investigate if H+ was consumed and if the hydrolysis reactions continued throughout the experiment. After acid exposure, the stones were evaluated with contact angle and capillary absorption measurements, and through determination of the mass loss. 2.4.3. Salt weathering – Resistance to salt crystallization The impact of salt crystallization in the pores was evaluated by using the test method defined by De Rosario et al. [30], which was adapted from the RILEM protocol [41]. When investigating the impact of salt crystallization in natural stones, the common way of performing the test is to immerse the stone in a saltcontaining solution [41]. While this is an excellent way of testing the impact of consolidating treatments, it is not suitable for the evaluation of protective coatings only covering one face of the stone. The adapted protocol in [30] circumvents this problem by introducing the salt solutions from one side of the stone through capillary absorption. Thus, only the coated side of the stone will be exposed to the degrading effects. Granite stones with the dimensions of 5 5 1 cm3 were used to evaluate the effects of salt crystallization on pure granite as well as on the coated stones. The experiments were performed in triplicate to provide standard deviations. One ‘‘salt exposure–drying” cycle can be defined as follows; (1) capillary absorption of a 14 wt% sodium sulfate solution for 2 h by placing the treated side of the stones facing the absorbent bed, (2) oven drying at 40 °C for 8 h, and (3) cooling at room temperature (T = 22 ± 3 °C, relative humidity = 40 ± 5%) for 14 h. After every 5 cycles, the tested stones were washed in a 20 °C water bath for 3 days (with continuously renewed water), after which the stones were dried to constant weight. The effects of the salt crystallization were evaluated by measuring the weight loss (every 5 cycles) and by measuring color difference, contact angles and capillary absorption after the last cycle (30 cycles in total). 2.4.4. Frost weathering The freeze-thaw tests attempted to reproduce the stresses, which may arise inside the rock when water turns into ice, generating pressure on the rock material. These effects are generally obtained by varying the temperature under and above 0 °C for samples containing water. The freeze-thaw testing was conducted in a way adopted from the standard ASTM D 5312-04 (see Fig. 1). The water-saturated samples were left to freeze at 18 °C for 16 h and then allowed to thaw at 40 °C for another 8 h. This process is defined as one freeze-thaw cycle and was repeated 30 times in this experiment. Due to size restrictions in the freezer unit used in these experiments, a sample size of 2 2 1 cm3 was used to evaluate the effects of frost weathering on pure granite as well as the PDMS coatings and the reference Faceal Oleo HD coating. The effects of the freeze-thaw cycling were evaluated by measuring the color difference, contact angles, capillary absorption and the weight loss after the last cycle. 3. Results 3.1. Initial characterization of the coatings and the stones The PDMS and Faceal Oleo HD solutions were used to hydrophobize granite samples according to the protocol described
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Fig. 1. Schematics of (a) the freeze-thaw cycling process and (b) the mechanism of degradation.
Table 1 Water contact angles, capillary water absorption values, color differences and water vapor permeability (Q/t) for the granite before and after coating with the different formulations.
*
Fig. 2. Capillary absorption of the granite before and after coating with the different formulations.
in the Materials and methods section. The capillary absorption data is presented in Fig. 2 and summarized in Table 1 together with the corresponding static water contact angles. These results indicate how effective the different water repellent coatings are. The contact angle measurements show that the mean water contact angle on the untreated granite is 41 ± 3°, while all PDMS coatings give rise to water contact angles of 120° or higher, which indicates a successful hydrophobization of the granite surface. Also the Faceal Oleo HD-coated granite displays a contact angle of 122 ± 2°. The relatively large contact angles are probably a result
Sample
WCA [°]
Capillary absorption [%]
Color difference DEab
Q/t [mg/cm2h]
Untreated granite Faceal Oleo HD PDMS – 20 mL/m2 PDMS – 40 mL/m2 PDMS – 60 mL/m2 PDMS – 80 mL/m2 PDMS – 100 mL/m2
41 ± 3 122 ± 2 121 ± 2 124 ± 4 120 ± 1 121 ± 3 125 ± 1
100 ± 10 40 ± 10 30 ± 10 12 ± 6 11 ± 6 6±3 10 ± 6
– 0.34 0.83 2.52 3.11 3.91 5.32
0.0553 ± 0.0007 0.0560 ± 0.0003 0.0512 ± 0.0004 –* 0.0513 ± 0.0005 –* 0.0531 ± 0.0005
Not determined.
of the low polarity of the coating material combined with structural effects of the underlying granite surface. The capillary absorption and standard deviation for nine standard untreated granite stones were measured to be 0.059 ± 0.008 kg/m2 after 48 h on the absorbent bed. The coated (and weathered) samples were normalized against this mean value (0.059 kg/m2) and the normalized values are presented here in the Results Section. The recorded absorption values (in kg/m2 at 48 h) are available in the supplementary material (Table S1). The capillary absorption measurements in Fig. 2 show that the reference coating, Faceal Oleo HD, absorbs 40 ± 10% water compared to pure granite. On the other hand, the PDMS coatings display a decreasing water uptake trend (from 30% to less than 10% compared to pure granite) when the surface coverage is increased from 20 to 100 mL/m2. These
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preliminary results indicate that 40 mL/m2 of the PDMS solution is sufficient to produce a coating that reduces the water absorption to less than 10% compared to the pristine stone material. The measured water vapor permeability rate and color differences for the different coatings are presented in Table 1. The water vapor permeability of stone materials should remain unchanged after a hydrophobization treatment. This is important as water condensation beneath a protective coating as well as accumulation of condensed water behind a treated stone can lead to stone decay [23]. The vapor permeability measurements revealed that the coatings did not affect the vapor permeability of the pure granite significantly, as the PDMS-coated granite samples displayed a permeability of about 95% compared to pure granite. The Faceal Oleo HD-coated stones displayed a permeability that is within the error margins of the pure granite, which is also what the manufacturer advertises. The water vapor penetration through a pure PDMS elastomer layer was found to be negligible, which further proves that the pores are not entirely blocked by PDMS for the coated stones. In order to investigate if any undesired color alterations occurred to the stone surface, colorimetric measurements were conducted on stones treated with different surface coverage of PDMS, as well as the reference coating of Faceal Oleo HD. All hydrophobized samples were compared to the untreated standards and the color differences are summarized in Table 1. The small measured color changes for the PDMS coatings (DEab 0.8–5) revealed that no significant discoloring of the granite surface had occurred even when using a PDMS surface coverage of 100 mL/ m2. It should be noted that the detected differences are slightly above the just noticeable difference level, which is defined as the color change needed to see a difference by the naked eye, i.e. DEab >2.3. By reviewing the color difference data (available in the supplementary material in Tables S2 and S3), we could observe that the lightness parameter (DL⁄) decreased with increasing PDMS coverage, probably as the light reflection decreased slightly with the increased coarseness of the coatings. 3.2. Impact of accelerated aging The effects accelerated aging (using the QUV accelerated weathering tester) had on the water contact angle, the capillary absorption, the mass loss and the color difference are listed in Table 2. The accelerated aging had minimal effects on the untreated granite, as the capillary absorption of the pure granite before and after the aging were within the standard deviations. Similarly, the PDMS coatings were not significantly affected by accelerated weathering, displaying no increase in water absorption, contact angles or color difference. This shows that the PDMS-based coatings are very durable, when UV degradation and moisture effects are concerned. Interestingly, the PDMS coatings displayed mass losses that were slightly lower than observed for the pure granite.
Table 2 Recorded mass losses, water contact angles, capillary absorption and color difference on the coated stones after treatment and after two months of accelerated UV/M weathering.
The Faceal Oleo HD-coated stones, on the other hand, display a common problem encountered in the development of protective coatings for natural stones. That is to say, UV radiation seems to be detrimental to these coatings, as the contact angle decreased from hydrophobic (122 ± 2°) to hydrophilic (5 ± 1°), which is significantly lower than the contact angle of pure granite. This further suggests that the hydrophobic functional groups have degraded via UV exposure, making the surface even more hydrophilic. This change in material properties is also reflected in the large increase in water absorption, from the 36 ± 6% absorption recorded after coating to the significantly higher 90 ± 40% after accelerated aging (values normalized to the absorption of the pure granite). The Faceal Oleo HD-coated stones also display a higher mass loss from accelerated aging of 0.060 ± 0.001% compared to the 0.025 ± 0.001% mass loss of the pure granite. As the stones have been coated with a relatively large amount of Faceal Oleo HD (400 mL/m2), a large part of the mass loss is probably due to the degradation of the coating material itself. Obviously, as the pure granite and the PDMS-coated stones also display some mass loss, the degradation of the granite material would also contribute to the mass loss of the Faceal Oleo HD-coated stones. Finally, the color difference measurements revealed no large changes for the PDMS samples, while the pure granite displayed a DEab value of 3.3, which arose from an increase in the lightness (L) parameter. Furthermore, the Faceal Oleo HD-coated stones displayed a DEab value of 1.7, related to an increase in the lightness (DL⁄) parameter and the blue-yellow (Db⁄) component. This increase could be seen on the samples as a yellow discoloration, a known flaw related to UV exposure of certain organic functionalizations [42,43]. The exact color differences are available in the supplementary material (Table S4).
3.3. Impact of acid degradation During the acid degradation experiment, the pH 3 HCl solution was exchanged once a week and it was found that the pH increased to 3.3–3.5 after 1 week for all samples, indicating that the samples were not significantly affected by the acid degradation. After 30 days in a pH 3 HCl solution, the stones were washed and dried until they reached a constant weight and the mass loss from the acid degradation experiment was determined. The capillary absorption, water contact angles and color differences were measured to determine the impact of the acid immersion. The data from these measurements are summarized in Table 3. The effect of acid dissolution on granite can be deduced from the mass losses recorded after the experiment. The pure granite decreased by 0.044 ± 0.002% in mass, while the Faceal Oleo HDcoated sample decreased slightly more, by 0.055 ± 0.003%, and the best PDMS sample decreased by only 0.0315 ± 0.0006%. The positive effect of the PDMS coating is evident in its ability to reduce the direct contact area between the acid and the pure stone
Table 3 Recorded mass losses, water contact angles, capillary absorption and color change after the acid degradation experiment.
Sample
Mass loss [%]
WCA [°]
Capillary absorption [%]
Color difference DEab
Sample
Mass loss [%]
WCA [°]
Capillary absorption [%]
Color difference DEab
Untreated granite Faceal Oleo HD PDMS – 20 mL/m2 PDMS – 40 mL/m2 PDMS – 60 mL/m2 PDMS – 80 mL/m2 PDMS – 100 mL/m2
0.025 ± 0.001 0.060 ± 0.001 0.0221 ± 0.0006 0.024 ± 0.003 0.0209 ± 0.0004 0.023 ± 0.002 0.0215 ± 0.0008
39 ± 1 5±1 115 ± 5 120.5 ± 0.8 126 ± 2 121 ± 1 120 ± 3
112 ± 6 90 ± 40 8±2 7±1 5.9 ± 0.8 6.2 ± 0.8 7.1 ± 0.3
3.32 1.73 1.32 1.34 0.34 0.74 2.67
Untreated granite Faceal Oleo HD PDMS – 20 mL/m2 PDMS – 40 mL/m2 PDMS – 60 mL/m2 PDMS – 80 mL/m2 PDMS – 100 mL/m2
0.044 ± 0.002 0.055 ± 0.003 0.040 ± 0.003 0.0361 ± 0.0004 0.037 ± 0.001 0.0315 ± 0.0006 0.0325 ± 0.0009
44.4 ± 0.6 123 ± 1 119 ± 3 116 ± 2 118 ± 1 120 ± 3 116 ± 2
93 ± 1 36 ± 6 8±2 6.0 ± 0.5 6.6 ± 0.8 7±1 7.1 ± 0.6
1.55 1.8 1.18 4.68 5.04 4.25 4.78
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surface. As only the front side of the stone is fully protected with the coating (accounting for 36% of the stone’s surface area), the rest of the stone surface (64%) is still susceptible to unhindered acid reactions. Using this data and the mass loss of the untreated granite, it follows that an ideal surface coating would have a mass loss of 0.64 0.044% = 0.028%, which is close to the mass loss observed for the best PDMS-coated stones. This means that the face coated with PDMS can efficiently block the transport of hydronium ions in and out of the pore system and thus slow down the degradation rate. It is surprising that the Faceal Oleo HD-coated stones display a mass loss that is higher than the one of pure granite, which could be related to interactions between the coating material and the acid or due to the partial degradation of the Faceal Oleo HD coating mentioned in Section 3.1. The capillary absorption results obtained after the acid degradation show that the investigated coatings are very robust, with differences within the margins of error when compared to the same coated stones before aging. The only exception is observed for the 20 mL/m2 PDMS-coated stones, where the capillary absorption was 30 ± 10% after coating and was actually improved to 8 ± 1% after the acid degradation. This could be due to some restructuring of the PDMS coating through an acid-catalyzed sol-gel reaction. The water contact angles were also largely unchanged after the acid degradation, with some slight decrease for the PDMS coatings, while the Faceal Oleo HD coating was completely unaffected. The color difference measurements after the acid degradation revealed that the appearance of the pure granite, the Faceal Oleo HD, and the 20 mL/m2 PDMS-coated granite were only slightly affected by the acid degradation. While the PDMS coatings with surface coverages of 40–100 mL/m2 displayed color differences in the range of 4–5 units, which from reviewing the measurement data was found to be due to a decrease in the lightness parameter (DL⁄). The exact color differences are available in the supplementary material (Table S5). 3.4. Impact of salt weathering The damaging effects observed from the salt weathering test seem to be the most severe of all the aging mechanism investigated in this study. As seen in Fig. 3 and Table 4, the recorded mass losses during this experiment are greater than in any of the other weathering experiments. The mass of the pure granite decreased by 0.5 ± 0.1% after 30 salt crystallization cycles, while the Faceal
Table 4 Recorded mass losses, water contact angles and capillary absorptions after the salt weathering experiment. Sample
Mass loss [%]
WCA [°]
Capillary absorption [%]
Color difference DEab
Untreated granite Faceal Oleo HD PDMS – 20 mL/m2 PDMS – 40 mL/m2 PDMS – 60 mL/m2 PDMS – 80 mL/m2 PDMS – 100 mL/m2
0.5 ± 0.1 0.6 ± 0.1 0.06 ± 0.01 0.08 ± 0.08 0.07 ± 0.02 0.04 ± 0.01 0.04 ± 0.004
62 ± 5 76 ± 7 127 ± 7 123 ± 3 131 ± 4 127 ± 3 130 ± 6
126 ± 2 117 ± 9 29 ± 8 14 ± 4 9±2 9±1 11 ± 2
1.47 2.21 1.56 2.71 4.18 2.49 3.8
Oleo HD-coated stones displayed a similar mass loss (0.6 ± 0.1%). On the other hand, the PDMS-coated stones displayed mass losses on an order of magnitude lower than for the pure granite and Faceal Oleo HD-coated stones (in the range of 0.04–0.08%), which is probably due to the limited capillary absorption through the coatings that consequently reduces the salt ion diffusion into the pores in these systems. The effect of the salt crystallization cycling is also detectable in the capillary absorption measurements, where the absorption for the pure granite increased by 26 percentage points. A similar effect is seen for the Faceal Oleo HD-coated stones, where the capillary absorption increased from 40 ± 10 to 117 ± 9%, showing that the Faceal Oleo HD coating is ineffective when it comes to preventing salt weathering. In contrast, the capillary absorption for the PDMScoated stones is completely unchanged from the salt weathering, showing that impact of salt weathering can be minimized by using a PDMS coating. The contact angles measured on the stones after the salt crystallization cycles reveal several interesting conclusions. Firstly, the contact angle of the Faceal Oleo HD-coated stones has decreased from 122 ± 2° to 76 ± 7°, indicating a detrimental effect on the surface coating. Secondly, the contact angles for the pure granite and for several of the PDMS-coated stones actually increased from the salt weathering. This could be a result of structural effects due to salt crystallization on the stone surface. The color difference measurements revealed no large changes for the pure granite or for the PDMS-coated granite stones, while the Faceal Oleo HD-coated stones actually increased in DEab from 0.34 after coating to 2.2 after salt weathering, which was due to an increase in the lightness (DL⁄) parameter. The exact color differences are available in the supplementary material (Table S6).
3.5. Impact of freeze-thaw cycling Based on the recorded mass losses in Table 5, it seems like the effect of frost weathering is more detrimental for the actual protective coating materials than for the granite itself. As seen from Table 5, the mass losses recorded for the PDMS- and the Faceal
Table 5 Recorded mass losses, water contact angles and capillary absorptions after 30 freezethaw cycles.
Fig. 3. Mass loss of pure and coated granite as a function of the number of salt crystallization cycles. The dashed lines are Box Lucas exponential fits, using the model: y ¼ að1 ebx Þ, with a Levenberg Marquardt iteration algorithm, which are included as guidelines for data interpretation.
Sample
Mass loss [%]
WCA [°]
Capillary absorption [%]
Color difference
Untreated granite Faceal Oleo HD PDMS – 20 mL/m2 PDMS – 40 mL/m2 PDMS – 60 mL/m2 PDMS – 80 mL/m2 PDMS – 100 mL/m2
0.001 ± 0.001 0.013 ± 0.002 0.023 ± 0.007 0.0164 ± 0.003 0.0180 ± 0.003 0.0178 ± 0.002 0.0215 ± 0.007
60 ± 2 106 ± 2 112 ± 4 109 ± 4 110 ± 10 109 ± 4 110 ± 5
106 ± 5 50 ± 10 60 ± 20 30 ± 20 8±1 7±2 7±4
4.6 4.77 2.68 2.76 1.4 3.92 1.24
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Oleo HD-coated stones after the frost weathering were actually larger than the mass loss recorded for the pure granite. Furthermore, the contact angle of the pristine granite sample was slightly higher after frost weathering, which might be due to the contact with water/ice in the weathering process, while the percentage of capillary absorption was within the error margins of the pristine sample. The PDMS- and Faceal Oleo HD-coated granite samples display a small decrease in the contact angles after the 30 freezethaw cycles as well as slight deviations in the capillary absorption results, indicating that the coatings were only slightly affected by frost weathering when capillary absorption and surface hydrophobicity are concerned. Finally, while there were noticeable color differences between the samples before and after freeze-thaw aging, it should be noted that the smaller size of the samples used in this test (2 2 cm2 instead of 5 5 cm2) could have an influence on the results due to the phaneritic nature of granite. The measured color differences are included to show that no large, substantial changes occurred from the weathering process. The exact color differences are available in the supplementary material (Table S7).
4. Discussion The outcome of the weathering tests is very informative, as the coating stability and the hydrophobic functionality are found to be closely interlinked. As a sufficiently efficient coating can block water from entering the pores, the degradation effects related to salts and water will be smaller and the coating is less likely to further degrade from weathering effects related to water. The benefit of a good protective coating can be seen in Fig. 4, where the capillary absorption of PDMS coatings with a surface coverage above 60 mL/m2 displayed unchanged capillary absorption after the different weathering tests, showing the superior stability of this coating. Another interesting observation can be made from the two mechanical weathering tests (salt and frost weathering) on the pristine granite and stones coated with Faceal Oleo HD, where 30 salt crystallization cycles seem to have a much larger impact on the tested stones than 30 freeze-thaw cycles. When considering these weathering effects, it is important to note that freezing water crystallizes into an open hexagonal lattice structure [44], while the
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sodium sulfate crystallization proceeds via mirabilite crystal growth [45]. Another difference between the frost and salt weathering is that the freeze-thaw process occurs through frost wedging, i.e. slowly expanding the pores, while salt weathering occurs as a result of internal pressure being applied in the entire salt-filled pore system due to crystal growth. This distinction reveals that frost weathering is more localized, creating far less stress points than in salt weathering, where the effects can be said to be global. From the UV/M weathering tests, it is evident that also UV light is unable to degrade the PDMS network. The reason for this is that PDMS is a very chemically stable hybrid material with a robust silicone backbone that radicals generated by UV radiation simply cannot degrade. The Faceal Oleo HD coating, on the other hand, is completely degraded from UV exposure, due to radical attack on the less stable acrylic polymer network it consists of [46]. Finally, an important factor to contemplate when considering the overall impact of weathering is that none of these processes would occur independently in nature. Rather a combination of different weathering effects can augment the weathering impact on pure granite or coated stones. For instance, sunlight may degrade a protective treatment, such as the Faceal Oleo HD coating investigated in this paper, which allows for a more severe salt weathering due to increased water/salt absorption. 5. Conclusions In this study, PDMS solutions and a commercial reference solution (Faceal Oleo HD) have been spray-coated on granite surfaces with the goal of enhancing the material stability against weathering effects. We have done this by demonstrating the durability of the coatings and by evaluating the impact different weathering effects have on granite. In these tests, salt weathering proved to have the most severe impact on granite, as a relatively large mass loss was observed after 30 cycles of salt crystallization. Furthermore, the pure granite and the Faceal Oleo HD-coated stones displayed much larger mass losses after salt weathering compared to the PDMS-coated stones, which showed mass losses of 10% compared to the pure granite sample. We believe the low water uptake for the PDMS-coated samples also hindered the salt from being introduced into the pores and by this means the salt weathering was reduced. The most dramatic effect on the hydrophobic properties of the coatings could be observed for the Faceal Oleo HD-coated stones after the UV/M weathering, where the contact angle decreased drastically from 122 ± 2° to 5 ± 1°. This demonstrates how some weathering mechanisms severely can affect a surface coating, e.g. in this case the acrylic backbone of the Faceal Oleo HD coating was susceptible to degradation via free radicals created by UV radiation. On the other hand, the PDMS coatings displayed excellent stability when contact angles are concerned, as none of the weathering tests were able to decrease the contact angles below 90°. In general, it can be concluded that the PDMS coatings are very efficient for minimizing weathering effects on granite and additionally the weathering impact on the PDMS coating itself can be considered minimal. Furthermore, as the coatings can be easily applied by spray coating, we believe that the presented coating method will be very useful for hydrophobizing large granite objects and for retrofitting of buildings and bridges. Acknowledgements
Fig. 4. Capillary absorption matrix, showing the capillary water absorption after coating and after the different weathering tests for the investigated coatings.
The Magnus Ehrnrooth Foundation, the Swedish Cultural Foundation and the K.H. Renlund Foundation are greatly acknowledged for financial support (D.K.). Furthermore, the Academy of Finland is also acknowledged for funding through the project 259310 (J.H.S.).
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