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hydrophobic products on sandstones
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Original article
Characterization of TEOS/PDMS/HA nanocomposites for application as consolidant/hydrophobic products on sandstones Yanbing Luo ∗ , Lingzhi Xiao , Xiujuan Zhang School of History and Culture, Center for Archaeological Science, Sichuan University, 610064 Chengdu, China
a r t i c l e
i n f o
Article history: Received 2 June 2014 Accepted 25 August 2014 Available online xxx Keywords: Hydroxyapatite nanocomposite TEOS Hydrophobic Sandstone consolidant
a b s t r a c t Extensive studies have been conducted on the conservation of historical stones. Although many different surface-coating materials have been tested to date, alkoxysilane materials and their composites have shown the most promising results. In this study, composites of nano-hydroxyapatite (n-HA) and tetraethoxysilane (TEOS) were prepared and used for sandstone conservation. The effectiveness of the composites in consolidating and conferring hydrophobic properties to sandstone were evaluated by X-ray diffraction, scanning electron microscopy, static contact angle, mercury intrusion porosimetry, mechanical properties and water capillary absorption. The durability of the materials was examined under different environmental conditions by artificial aging tests. Results showed that the introduction of n-HA and hydroxyl-terminated polydimethylsiloxane into TEOS associated with a neutral catalyst could impart to the stone surface a coarser network for vapor transport and a hydrophobic effect for liquid water at the same time when the TEOS-based nanocomposites were applied as consolidant products on sandstones. Moreover, n-HA played an important role in improving mechanical properties and resistance to artificial aging tests but not in changing the color of sandstone samples. © 2014 Elsevier Masson SAS. All rights reserved.
1. Introduction Since ancient times, sandstones have been widely used as building materials, tombstones, and sculptures throughout China. Owing to their exposure to the effects of natural weathering, acceleration in the rate of decay has been observed in most sandstone samples in recent years. Sandstone degradation mechanisms are controlled by several factors, such as mineral composition and pore/capillary size. One of the most important factors affecting stone degradation is water. Water acts as a vehicle for weathering processes caused by aggressive atmospheric pollutants. Water also causes disintegration, surface erosion, cracking through freezing – thawing or wetting – drying cycles inside the pores, dissolution and transportation of soluble salts that can induce their inter/intraporous crystallization, hydrolysis of silicate rocks and other effects, such as color alteration. Moreover, water favors the growth of microorganisms and crust formation, which are typical signs of stone decay [1,2]. The development of effective consolidants for stones is a key goal in cultural heritage conservation [3]. These materials need to have certain properties, such as good coagulation between
∗ Corresponding author. Tel.: +8 613908062856. E-mail address: [email protected] (Y.B Luo).
grains that comprise the stone relics to restore their strength and resist environmental deterioration. These materials also become hydrophobic to block water penetration. Silicate esters, such as tetraethoxysilane (TEOS), have been widely studied and used for stone consolidation, which is mainly sandstone [3–5]. TEOS-based products polymerize in situ inside the pore structure of the disintegrating stone through a sol–gel process and significantly increase the cohesion of the stone [3]. The advantages of these products are well-known; i.e., the low viscosity of TEOS allows its deep penetration into porous stone; a stable gel with a silicon–oxygen backbone is formed after polymerization with environmental moisture; no deleterious by-products are formed and the resulting ethanol evaporates completely [6,7]. Traditional TEOS-based consolidants are known to leave some open pores to allow air and water vapor to pass back and forth through the stone as the weather changes. However, a well-known drawback of these conservation products is their tendency to form brittle gels that are highly susceptible to forming cracks inside the stone, which is generated by the high capillary pressures produced within the gel pores during its drying phase [8]. To address these problems, several trials have been performed in recent years. Particular attention has been given to composites, which are obtained by introducing various inorganic nanoparticles, such as silica dioxide (SiO2 ) and titanium dioxide (TiO2 ), into hybrid siloxane or silicone polymers. This process enhances surface
http://dx.doi.org/10.1016/j.culher.2014.08.002 1296-2074/© 2014 Elsevier Masson SAS. All rights reserved.
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hydrophobicity and provides an efficient means of preventing the gel from cracking, by increasing gel pore size and reducing capillary pressure while drying inside the stone [6,9–13]. Although the addition of nanoparticles reduces crack formation in the gel, nanoparticles were reported to still be able to act as a barrier to water vapor movement if the nanoparticles were not dispersed correctly [14]. The use of hydroxyl-terminated polydimethylsiloxane (PDMS) as an additive in TEOS sol–gel systems was reported to be another effective and simple way to obtain a hydrophobic product; TEOS/PDMS sol–gel systems were also reported to be a more elastic consolidant to solve the alkosysilane-derived film cracking problem [15–17,11,18]. Usually, the PDMS content was as high as 30% w/w to obtain improved properties [11,17]. Preparation of multifunctional inorganic/polymer nanocomposites presents a challenging task on improving the polymer properties, and opens a brighter perspective for their future application. Compared to conventional hydroxyapatite (HA), nanohydroxyapatite (n-HA) has larger specific surface area and better mechanical properties, and has been increasingly used as functional materials [19–22]. To date, the use of n-HA as consolidant for sandstone has not been investigated. Although the use of hydroxyapatite in consolidating stones has been investigated in recent studies and the in situ formed microsized hydroxyapatite (m-HA) increased significantly the mechanical properties and showed good consolidation effects for carbonate stones [23–25], this process was not easy to control and the unreacted diammonium hydrogen phosphate (DAP) was easy to deposit on sample surface and cause color change. Moreover, in the case of sandstone, which represents an old and wide-spread construction material all over the world, the consolidants with in situ formed m-HA are not effective in improving mechanical properties and resisting artificial aging tests as on carbonate stone. Therefore, in this paper, the possible use of n-HA as a new inorganic consolidant was investigated. The consolidant was obtained by the co-condensation of TEOS and a small amount of PDMS in the presence of n-HA. The composites were applied to sandstone specimens by brushing. Water absorption, hydrophobic property, and artificial aging tests were performed to preliminarily evaluate their efficacy as a sandstone conservation material. For comparison, consolidants with m-HA, without any kinds of HA and commercial product Remmers, were also evaluated. Remmers is a stone strengthener on a silicic acid base and has been widely used for sandstone consolidants [26–28]. The objective of the present study is to investigate the consolidation effects to develop an effective sandstone consolidant suitable for the rainy and humid environment in southwest China.
2. Experimental 2.1. Materials TEOS, which is an analytical grade reagent, was obtained from Kelong Chemical Reagent Corporation. PDMS, which has a weightaverage molecular weight (Mw ) of 3000 g/mol, was supplied by the Shanghai Jiapeng Chemical Industry and used as received. Both of the n-HA, with an average primary particle size of approximately 20 nm and the m-HA, with an average primary particles size of approximately 10 m, were obtained from Beijing DK Nano Technology Co., Ltd. Di-butyltin dilaurate (DBTL), which was obtained from Kelong Chemical Reagent Corp., was used as received. Remmers KSE OH, which is a commercial stone consolidant with polymer silicate of low molecule weight as the main component, was purchased from Remmers Co., Ltd, Germany. Unweathered sandstones were obtained from a quarry located in Yaan, Sichuan
Table 1 Composition of sols under studya . Product
TEOS (% w/w)
PDMS (% w/w)
DBTL (% w/w)
n-HA (% w/w)
m-HA (% w/w)
TEOS/PDMS TEOS/PDMS/HA-0.5 TEOS/PDMS/HA-1 TEOS/PDMS/HA-2 TEOS-PDMS-m-HA-0.5 TEOS-PDMS-m-HA-1 TEOS-PDMS-m-HA-2
100 100 100 100 100 100 100
5 5 5 5 5 5 5
1 1 1 1 1 1 1
0 0.5 1 2 0 0 0
0 0 0 0 0.5 1 2
TEOS: tetraethoxysilane; PDMS: polydimethylsiloxane; m-HA: microsized hydroxyapatite; n-HA: nano-hydroxyapatite; DBTL: di-butyltin dilaurate. a The “w/w” is always referred to the weight of TEOS.
and cut into blocks of 3 different sizes: 5 cm × 5 cm × 1 cm (for water vapor permeability test), 5 cm × 5 cm × 10 cm (for mechanical test), and 5 cm × 5 cm × 2 cm (for artificial aging tests, change in color, and static contact angle test). First, the sandstone blocks were scraped off using a grinding machine to obtain uniform surfaces and reduce cutting imperfections. Afterwards, the samples were rinsed with deionized water for an hour via ultrasonic agitation, and were dried in an oven at 105 ◦ C up to constant weight for at least 24 h. Each test was performed on samples coming from the same block of sandstone rock, thereby having similar macroscopic features (such as color, shape, grain, and surface roughness). Petrographic analysis using a transmitted light microscope B × 51 (Olympus) indicated that surface porosity and surface pore size of the sandstones were approximately 3.5% and 0.06–0.15 mm, respectively. 2.2. Preparation of TEOS/PDMS/HA nanocomposites TEOS-based nanocomposites were prepared through TEOS hydrolysis in the presence of n-HA and PDMS using DBTL as a catalyst, and the product was labeled as TEOS/PDMS/HA. First, 40 mL TEOS and a known amount of n-HA (0.5, 1, 2% w/w, respectively) were mixed under ultrasonic treatment for 30 min at 50 ◦ C. Next, 5% w/w PDMS was added to the mixture under vigorous stirring at 50 ◦ C by refluxing for 4 h. Lastly, 1% w/w DBTL was added to the mixture under vigorous stirring for 10 min. Ten milliliter of each sol was subsequently poured into a glass Petri dish (d = 10 cm) and allowed to gel and dry for 4 weeks in a sealed container maintained at room conditions (T = 23 ± 2 ◦ C and RH = 50% ± 5%). The gelled samples from each film were detached from the dishes and analyzed by scanning electron microscopy. The TEOS/PDMS/HA nanocomposites with 5% w/w PDMS and 0.5, 1, 2% w/w n-HA were labeled as TEOS/PDMS/HA-0.5, TEOS/PDMS/HA-1 and TEOS/PDMS/HA-2. For comparison, consolidants with m-HA were prepared using the same technique and labeled as TEOS-PDMS-m-HA-0.5, TEOSPDMS-m-HA-1 and TEOS-PDMS-m-HA-2. At the same time, TEOS and PDMS (5% w/w) composites were prepared using the same techniques and were labeled as TEOS/PDMS. The composition of the sols under study is presented in Table 1. 2.3. Consolidation of sandstone samples The TEOS/PDMS/HA-0.5 consolidants and commercial Remmers KSE OH were applied to blocks by brushing thrice with a time interval of no less than 10 min under laboratory conditions (T = 3 ± 2 ◦ C and RH = 50% ± 5%). Before testing, all treated samples were aged and kept at ambient conditions to reach steady weight (m < 0.001 g). The average amount of consolidant applied to the sandstone samples, which was determined by weighting the dry samples before and after the treatment, was approximately 5.0 ± 0.6 mg/cm2 . The data reported is the mean values obtained from three sandstone samples, except when a different number of samples are expressly indicated.
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2.4. Characterization 2.4.1. X-ray diffraction (XRD) Identification of the mineral composition of the samples was recorded on a DX-1000 diffractometer equipped with a graphite ˚ radiation. The generator monochromator and Cu K␣ ( = 1.5418 A) was operated at 40 kV and 25 mA. The samples were scanned at diffraction angles 2◦ to 70◦ at 0.06/s scanning rate. 2.4.2. Microscope examination Scanning electron microscopy (SEM) images were recorded using a Hitachi Fields S-4800 instrument operated at 5 kV. All specimens were sputter-coated with gold prior to examination. 2.4.3. Static contact angle () The static contact angle () was measured at 25 ◦ C using a JY-82A contact angle goniometer (Hebei Chengde Testing Machine Co. Ltd, China). Droplets of distilled water (5 L each, with a microsyringe) were deposited on the surfaces of samples and from a distance remained sufficiently close to the substrate, so that the needle can remain in contact with the applied water droplet after delivery. Afterwards, the needle was withdrawn with minimum perturbation to the droplet, and the image of the droplet was recorded immediately for measurement. The result is an average value obtained from a total of 10 different points on three sample surfaces. 2.4.4. Mercury intrusion porosimetry (MIP) Changes in the pore size distribution and porosity before and after consolidation were carried out on stone specimens with a volume of around 1 cm3 by means of a mercury intrusion porosimetry (MIP, AutoProe IV 9500, Micromeritics, USA). Specimens were cut from parts adjacent to the treated area of the stone under study. 2.4.5. Mechanical properties The compressive strength was evaluated using a programmecontrolled servo concrete and rock mechanics test system (RMTS150, Sichuan University, China) and was conducted at room temperature. All tests were performed according to the Test Method of Engineering Rock Mass (GB/T50266-99). The test was performed under the loading rate of 0.5 MPa/s until failure. The compression strength expressed in MPa was obtained using the equation below: c =
P A
(1)
where P is the failure load (N), and A is the loaded area (mm2 ). The data reported is the mean from five determinations. 2.4.6. Penetration depth It is widely accepted that the penetration depth of consolidants is an important parameter to be taken into account when assessing consolidation effectiveness. Penetration depth of consolidants is often assessed visually [29–31]. In our experiments, the untreated sandstone samples were initially powdered and sieved with a 16 mesh sieve. The powders in graduated glass container with 10 mm diameters were tamped by vibration. The powers had the same mass and height to get the similar porosity in each glass container. Two-milliliters of consolidants were dropped on the surface of the power, and then the glass container was sealed with cotton. The penetration depth of consolidants was measured by recording the position of the consolidant in the compacted sandstone powder after 24 h [31]. 2.4.7. Evaluation of disadvantage effects induced by the treatments Possible disadvantage effects, such as color changes and water vapor permeability reduction, were also evaluated.
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2.4.7.1. Water vapor permeability (WVP). WVP was evaluated by fixing a sample block (5 cm × 5 cm × 1 cm) as a cover on a cylindrical PVC container filled partially with water. Afterwards, the containers were properly sealed with melt paraffin section and placed in a desiccator with a relative humidity of R.H. 35% and at constant temperature of 23 ± 2 ◦ C. The mass of the system of the container, sample block and the water were recorded every 24 h to determine the mass of water vapor passing though the unit surface under controlled conditions. The test of WVP was based on the standard wet cup test (ASTM E96M-2005 standard test methods for water vapor transmission of materials). The WVP expressed in g/(m·s·Pa) was determined by the following equation: WVP =
WVT × l m × l = S × (R1 − R2 ) t × A × S × (R1 − R2 )
(2)
where WVT is rate of water vapor transmission (g/s·m2 ), m is the mass change (g), t is the time when m occurred (s), A is the test area (cup mouth area, m2 ), S is the saturation vapor pressure at test temperature (Pa), l is the thickness of the sample (m), R1 is the relative humidity in the desiccator, and R2 is the relative humidity in the permeation container. 2.4.7.2. Change in color. The change in color was determined using a solid reflection spectrophotometer (Color Reader CD-10 from Minolta Co., Ltd.). The conditions used in the experiment were illuminant D65 and observer 8◦ . CIEL*a*b* color space was used and color variations were evaluated using the parameter total color difference (E*). Each test was performed making four measurements on each treated or untreated sandstone sample; in order to confirm the reproducibility of the results, the test was repeated on three identical sandstone samples, therefore the data reported were the measurements. The average color difference mean values on 12 expressed as E =
2
2
2
(L∗ ) + (a∗ ) + (b∗ ) .
2.4.8. Resistance to artificial aging tests Artificial aging tests aim at simulating the actual polluted, environmental conditions in Sichuan; as well as quantifying the durability of the materials under study [25,26,31–33]. Four types of weathering, which are mentioned below, were conducted sequentially and termed as “artificial aging”. In addition, contact angle, compressive strength, and weight change were determined before and after treatment with the materials under study. In each test, samples treated by TEOS/PDMS/HA, TEOS/PDMS, and commercial Remmers KSE OH were compared against untreated samples. The resistance of artificial aging tests was evaluated after a period of cycles described below. 2.4.8.1. Acid water weathering cycles [31–33]. Acid rain in the outdoor environment has been shown to have a great influence on the durability of the stone relics. We designed a test using H2 SO4 (2%, v/v) to study the acid resistance of the sandstones. The samples were subjected to cycles of immersion in dilute 2% H2 SO4 solution for 16 h followed by absorption of any excess water onto filter paper after taking samples out of water, and another 8 h in an oven at 60 ◦ C. These experimental procedures were repeated until the test samples were completely destroyed. 2.4.8.2. Salt crystallization weathering cycles [31–33]. The samples were subjected to cycles of immersion in a 10% w/v Na2 SO4 solution for 16 h, followed by absorbing any excess water onto filter paper after taking samples out of water, and subsequent drying for 8 h in an oven at 60 ◦ C. These procedures were repeated until the test samples were clearly destroyed.
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2.4.8.3. Wet–dry cycles [31–33]. In order to determine the effect of water on sandstones, simulation of climatic change from sunny to wet/rainy weather was carried out. This test consisted of a total of 60 cycles of immersion and drying as follows: 16 h total immersion in distilled water, absorbing any excess water onto filter paper after taking samples out of water, and subsequent drying for 8 h in an oven at 60 ◦ C for each treatment. The contact angle, compressive strength, and weight change were determined before and after wetdry tests. 2.4.8.4. Freezing–thawing cycles [26,31]. This experiment consisted of a total of 50 cycles. In each cycle, samples were soaked in
distilled water at 23 ± 2 ◦ C for 12 h, followed by freezing at −15 ◦ C for another 6 h after absorbing excess surface water with a filter paper, and subsequent drying for 6 h in an oven at 60 ◦ C, and then soaking again in distilled water at 23 ± 2 ◦ C for 12 h. 3. Results and discussion 3.1. Characterization of gels Fig. 1 shows the gel images obtained after drying under laboratory conditions. TEOS/PDMS materials and the commercial Remmers KSE OH films cracked; and the degree of cracking was
Fig. 1. Images of gel films obtained after drying under laboratory conditions: a: remmers KSE OH; b: TEOS/PDMS; c: TEOS/PDMS/HA-0.5; d: TEOS/PDMS/HA-1; e: TEOS/PDMS/HA-2.; f: TEOS/PDMS/m-HA-0.5.
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Fig. 2. XRD pattern of samples treated by the materials under study.
slightly higher for TEOS/PDMS products (Fig. 1a and b). The addition of n-HA did not change the gelation time, but reduced cracking was observed in the n-HA and TEOS-based materials even when the n-HA concentration was only 0.5% w/w. Aggelakopoulou et al. [14] demonstrated that the presence of nanoparticles physically limits the silicate network from shrinking under capillary pressure. Given this result, the crack-free film might be ascribed to the addition of n-HA in reducing the surface tension of the initial solution, which contributes to reduced capillary pressure. More details are addressed in the following section. Fig. 1 also shows that with a successive increase in n-HA concentration, the gels become whiter at high concentration (> 0.5% w/w, Fig. 1c–e). However, Fig. 1f shows that the TEOS/PDMS/m-HA-0.5 film crack greatly. 3.2. The assessment of the conservation efficiency XRD analysis was used to investigate mineralogical and surfacecoating characteristics of sandstone before and after being treated with consolidants. The XRD patterns of the untreated and treated sandstone samples were compared in Fig. 2. The results clearly showed that quartz was the predominant mineral phase in the untreated sandstone. Small amounts of clay minerals, such as calcite, albite, kaolinite and chlorite, were present when the observed Bragg reflections were compared with those of reference minerals. The peak positions corresponding to the main components did not change after treatment with PDMS and/or n-HA, which implied no change in the crystal structure. However, the XRD patterns of TEOS/PDMS/HA-0.5 revealed the new peaks at 2 = 31.64◦ , 32.12◦ , 32.94◦ when compared with TEOS and TEOS/PDMS. The new signals suggested the presence of n-HA. To further explore surface characteristics, the fracture surfaces of the samples were evaluated by SEM. As shown in Fig. 3b, the TEOS/PDMS/HA-0.5 material creates a crack-free, homogenous, and coarse network. By contrast, the TEOS/PDMS gel forms a dense coating (Fig. 3c). The SEM result showed that the presence of n-HA promoted the coarsening of the gel network. Mosquera et al. [34] demonstrated that TEOS sol prevented cracking during the drying phase by coarsening the pore structure. In the present study, the formation of this coarse gel network can be due to the TEOS oligomer chains, and were integrated on the n-HA surface, which thereby reduced the capillary pressure. Since the main cause of stone weathering is water, surface treatment should prevent water penetration into the bulk of the stone. For this reason, we characterized static contact angle of a
Fig. 3. SEM micrographs of the samples: a: sandstone shows disintegration and erosion between mineral grains; b: with materials of TEOS/PDMS/HA-0.5, shows coarser coating on the stone surface; c: with materials of TEOS/PDMS, shows dense coating on the stone surface.
water droplet on the samples under study. The water droplet was observed to spread out rapidly on the surface of the untreated samples (62◦ ± 3) and soon completely wet the surface of the sample (within 1 min). As shown in Fig. 4, in the case of TEOS/PDMS and TEOS/PDMS/HA-0.5 treated sandstone surface, the contact angle are both above 120◦ , which is a bit higher than that of Remmers KSE OH (112◦ ). In addition, a spherical water droplet can be observed on the sandstone surface, which then fell from the surface in our experiment. This method demonstrates that the addition
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Fig. 4. Water static contact angles of samples after treatment by materials under study. Bars correspond to standard deviations. (photographs of a static water droplet on the sandstone surfaces are shown on the lower position).
of the organic component (PDMS) is a key factor in imparting the hydropobic effect by reducing surface tension. The contact angle of TEOS/PDMS/HA-0.5 treated sandstone surface is slightly higher than TEOS/PDMS-treated surface, as observed in Fig. 4. According to the model of Cassie’s or Wenzel’s theory of the contact angle, one of the effective ways to improve the contact angle is to construct surface roughness [35,36]. The surface roughness of n-HA may be partially increased in its integration into the silica matrix. Thus, a higher increase in the nanocomposite roughness with the integrated hydroxyapatite nanoparticle occurred. Changes in the sandstone porosity after consolidant treatments were evaluated by MIP. The porosity values and pore size distributions are shown in Table 2 and Fig. 5, respectively. The untreated sandstone presented a total porosity value of 14.4% and exhibited a pore size distribution centered at 6–7 m. After application of KSE OH and TEOS/PDMS/HA-0.5, the porosity and pore size distribution were slightly reduced whereas the TEOS/PDMS products caused a significant change in porosity and pore size distribution. The reduction of macropore size should be ascribed to the TEOS/PDMS dense layer reducing the size of the pores and the increase of the porosity and smaller pores should be ascribed to the high shrinkage of the dense gel coating layer after consolidation [6,25].
Fig. 5. Pore size distribution of untreated and treated samples with materials under study.
Fig. 6. Compressive strength versus artificial aging conditions for samples treated with materials under study. Bars correspond to standard deviations. The numbers in brackets correspond to the cycles of aging tests.
The mechanical properties of untreated and treated samples are shown in Fig. 6. Compared with other consolidants, mechanical properties of TEOS/PDMS/HA-0.5 treated stones were improved greatly (19%). The improved mechanical properties could be attributed to the TEOS oligomers and PDMS have a good compatibility with n-HA and could diffuse into the n-HA and establish strong interaction. The strong interfacial interaction and compatibility increase stress transfer to the n-HA, resulting in high mechanical properties [37,38]. The slightly reduced mechanical properties of TEOS-PDMS-m-HA-0.5 treated stones could be ascribed to the weak interfacial interaction and poor dispersion of the m-HA. The ability of the consolidant to penetrate into sandstone is one of the most important factors for a successful treatment. The penetration depth after 24 h is listed in Table 2. Table 2 shows that the samples treated by TEOS/PDMS/HA-0.5 have similar penetration depth when compared with the commercial product Remmers KSE OH. In addition, the depth observed is deeper than that of TEOS/PDMS. Xu et al. [39] reported that the addition of particles to a TEOS solution can increase the viscosity of the solution dramatically when 0.5% w/w was added. Aggelakopoulou et al. [14] also demonstrated that agglomerates were present; agglomerates increase the viscosity and can block pore entrances, which prevent the consolidant from entering the sandstone. The nanosized and well dispersed n-HA in the composite were not a problem for penetration through the sandstone pores. However, Xu et al. [39] reported the water vapor permeability of TEOS-0.2%SiO2 -PDMS materials was reduced largely because of particles that block the stone pores. This finding encouraged us to investigate the effect of water vapor permeability induced by the nanomaterials under study. Table 2 also shows that TEOS/PDMS/HA-0.5 is a material that produces no reduction in the breathability of the sample being evaluated. However, TEOS/PDMS produced a reduction as high as 11%. In this case, n-HA imparted a coarser network for vapor transport to the stone surface. n-HA is suggested to be a possible, suitable consolidation additive. nHA did not alter the water vapor permeability; in addition, the breathability of the original material was retained. A color change caused by the materials under study is contrary to the Code of Ethics for conservation cultural relics. Therefore, in the next part of our study, we studied color changes in the sandstone after treatment. The total color difference values (E*) of the stone after treatment are shown in Table 2. For the materials
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Table 2 Properties of samples with materials under studya . Material
E
Penetration depth (cm)
Water vapor permeability (g/ms Pa)
Porosity (%)
Untreated TEOS/PDMS TEOS/PDMS/HA-0.5 Remmers KSE OH TEOS/PDMS/HA-1 TEOS-PDMS-m-HA-0.5 TEOS-PDMS-m-HA-1
– 1.17 ± 0.25 2.32 ± 0.31 1.16 ± 0.29 5.70 ± 0.41 6.2 ± 0.43 10.9 ± 0.69
– 2.4 ± 0.11 3.1 ± 0.12 3.4 ± 0.14 – 2.1 ± 0.16 –
7.96 ± 0.75 × 10−8 7.12 ± 0.51 × 10−8 7.92 ± 0.54 × 10−8 7.93 ± 0.42 × 10−8 – 6.89 ± 0.32 × 10-8 –
14.4 ± 0.21 16.5 ± 0.75 13.9 ± 0.19 13.8 ± 0.32 – 16.9 ± 0.69 –
TEOS: tetraethoxysilane; PDMS: polydimethylsiloxane; m-HA: microsized hydroxyapatite. a Data correspond to average value ± standard deviation.
synthesized in our laboratory (composites of 0.5% w/w n-HA and TEOS/PDMS) and for the commercial consolidant Remmers KSE OH, E* was very low, and was below the perceptibility threshold (E* < 3) [15,40]. Only those with m-HA and n-HA content with more than 0.5% w/w created a perceptible change in the sandstone color. Therefore, materials with n-HA content larger than 0.5% w/w and m-HA were excluded in latter experiments, if there is no other special instruction. The images of samples with the artificial aging test time, weight change, color change, contact angle and compressive strength properties of the samples under artificial weathering conditions before and after treatment were determined in our experiments.
As illustrated in Fig. 7, the surface of the samples under acid and salt weathering conditions changes dramatically for untreated sandstone, and with Remmers KSE OH materials even after only six cycles. The sandstone without any protection produced bubbles when the untreated samples were initially placed in the acid solutions. These methods showed big stomata and fine cracking on the surface after only three cycles of acid weathering. This finding was the result of the reaction between the strong proton acid and calcareous cement that produces CO2 gas and water-soluble salts. The untreated samples, as well as samples coated with Remmers KSE OH materials collapsed completely after 6 and 10 cycles of acid weathering, respectively. The surface of samples treated
Fig. 7. Images of samples with materials of Remmer KSE OH (A, A A ), TEOS/PDMS (B, B B ), untreated sandstone (C, C C ), TEOS/PDMS/n-HA-0.5 (D, D D ) under different weathering conditions. A, B, C, D: before aging test; A , B C D : acid aging test (6 cycles); A , B , C , D : salt aging test (6 cycles).
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of a concentration difference between the samples and the salt solution. As for the other three artificial aging tests, the weight changed began directly after disintegration of samples. Thus, a difference in the mechanism was observed between the salt and other weathering conditions. The samples were less affected by the wet–dry and freeze–thawing tests compared to the acid and salt artificial aging tests. However, some changes of mechanical and contact angle after 40 cycles (Figs. 6 and 8) were noted. In summary, in the current study, the TEOS-based composite with n-HA had a better resistance to weathering aging tests because of the effect of nanoparticles. 4. Conclusions
Fig. 8. Contact angle versus artificial aging conditions for samples treated with materials under study. Bars correspond to standard deviations. The numbers in brackets correspond to the cycles of aging tests.
with TEOS/PDMS and TEOS-PDMS-m-HA-0.5 became sandy, while the samples treated with TEOS/PDMS/HA-0.5 remained unchanged even after 15 cycles. However, some decrease in mechanical strength was observed for TEOS/PDMS, TEOS/PDMS/HA-0.5 and TEOS-PDMS-m-HA-0.5 samples at 20%, 12% and 24%, respectively (Fig. 6). Under salt crystallization weathering test, the samples coated with Remmers KSE OH, as well as the untreated sandstone, cracked in cross-section after only 16 and 10 cycles, respectively. The surfaces of samples coated with TEOS-PDMS-m-HA-0.5 and TEOS/PDMS materials were covered by white products after 13 and 18 cycles and flaked slightly after 18 and 22 cycles during salt weathering tests. No obvious change was observed for samples coated with TEOS/PDMS/HA-0.5 after 22 salt crystallization weathering cycles. The contact angle and compressive strength were reduced somewhat for all of the samples, as shown in Figs. 6 and 8. However, the samples coated with TEOS/PDMS/HA-0.5 showed the least decrease in compressive strength. In addition, Fig. 9 shows that the weight of the samples coated with Remmers KSE OH and those without coatings decreased markedly during the salt weathering test. At the same time, it is notable that the weight somewhat increased initially and then decreased during salt weathering cycles (Fig. 9). The increase of weight was ascribed to the salt crystallization that occurred in the samples during salt weathering, because
Fig. 9. Weight change under salt crystallization artificial aging test.
To enhance sandstone consolidation and protection, a new TEOS/PDMS/HA material was synthesized through a simple sol–gel method and was subjected to artificial weathering tests. The addition of PDMS, which has flexible segments, and n-HA, which creates a surface roughness, reduced crack formation in TEOS-based gels during the drying phase. In addition, the hydrophobic effect is ascribed to the organic component (PDMS) by the reduction in the surface tension and the n-HA by increasing surface roughness. No negative effects on the treated sandstone were observed. The artificial aging tests showed that n-HA has an important function in improving resistance to weathering effects. Compared with consolidants with m-HA and others under study, this finding indicates the efficacy of the consolidant with n-HA for protection and conservation process. Acknowledgements This work was supported financially by the Science and Technology Support Programme of Sichuan Province (2013FZ0076), the Advanced Interdisciplinary Innovation Research Project of Sichuan University (skqy201216) and the Younger Fund of the Ministry of Education (10XJCZH005). We also express our gratitude to Donna Strahan from Freer Gallery of Art and Arthur M. Sackler Gallery, Smithsonian Institution for her assistance in improving the quality of our paper. References [1] A. Jain, S. Bhadauria, V. Kumar, R.S. Chauhan, Biodeterioration of sandstone under the influence of different humidity levels in laboratory conditions, Build. Environ. 44 (2009) 1276–1284. [2] P. Martinec, M. Vavro, J. Scucka, M. Maslan, Properties and durability assessment of glauconitic sandstone: a case study on Zamel sandstone from the Bohemian Cretaceous Basin (Czech Republic), Eng. Geol. 115 (2011) 175–181. [3] G. Wheeler, Alkoxysilanes and the consolidation of stone, Getty Publications Book Distribution Center, Los Angeles, 2005. [4] V. Cnudde, M. Dierick, J. Vlassenbroeck, B. Masschaele, E. Lehmann, P. Jacobs, et al., Determination of the impregnation depth of siloxanes and ethylsilicates in porous material by neutron radiography, J. Cult. Herit. 8 (2007) 331–338. [5] A. Tsakalof, P. Manoudis, I. Karapanagiotis, I. Chryssoulakis, C. Panayiotou, Assessment of synthetic polymeric coatings for the protection and preservation of stone monuments, J. Cult. Herit. 8 (2007) 69–72. [6] M.J. Mosquera, D.M. Santos, A. Montes, L. Valdez-Castro, New nanomaterials for consolidating stone, Langmuir 24 (2008) 2772–2778. [7] S. Siegesmund, R. Snethlage, Stone in architecture: properties, durability. Chapter 7: stone conservation, 4th ed., Springer-Verlag Berlin and Heidelberg GmbH & Co. K, Germany, 2011, pp. 411–542. [8] M.J. Mosquera, J. Pozo, L. Esquivias, Stress during drying of two stone consolidants applied in monumental conservation, J. Sol-Gel Sci. Technol. 26 (2003) 1227–1231. [9] 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. [10] V. Daniele, G. Taglieri, Synthesis of Ca(OH)2 nanoparticles with the addition of Triton X-100. Protective treatments on natural stones: Preliminary results, J. Cult. Herit. 13 (2012) 40–46. [11] J.F. Illescas, M.J. Mosquera, Surfactant-synthesized PDMS/silica nanomaterials improve robustness and stain resistance of carbonate stone, J. Phys. Chem. C. 115 (2011) 14624–14634.
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Original article
Characterization of TEOS/PDMS/HA nanocomposites for application as consolidant/hydrophobic products on sandstones Yanbing Luo ∗ , Lingzhi Xiao , Xiujuan Zhang School of History and Culture, Center for Archaeological Science, Sichuan University, 610064 Chengdu, China
a r t i c l e
i n f o
Article history: Received 2 June 2014 Accepted 25 August 2014 Available online xxx Keywords: Hydroxyapatite nanocomposite TEOS Hydrophobic Sandstone consolidant
a b s t r a c t Extensive studies have been conducted on the conservation of historical stones. Although many different surface-coating materials have been tested to date, alkoxysilane materials and their composites have shown the most promising results. In this study, composites of nano-hydroxyapatite (n-HA) and tetraethoxysilane (TEOS) were prepared and used for sandstone conservation. The effectiveness of the composites in consolidating and conferring hydrophobic properties to sandstone were evaluated by X-ray diffraction, scanning electron microscopy, static contact angle, mercury intrusion porosimetry, mechanical properties and water capillary absorption. The durability of the materials was examined under different environmental conditions by artificial aging tests. Results showed that the introduction of n-HA and hydroxyl-terminated polydimethylsiloxane into TEOS associated with a neutral catalyst could impart to the stone surface a coarser network for vapor transport and a hydrophobic effect for liquid water at the same time when the TEOS-based nanocomposites were applied as consolidant products on sandstones. Moreover, n-HA played an important role in improving mechanical properties and resistance to artificial aging tests but not in changing the color of sandstone samples. © 2014 Elsevier Masson SAS. All rights reserved.
1. Introduction Since ancient times, sandstones have been widely used as building materials, tombstones, and sculptures throughout China. Owing to their exposure to the effects of natural weathering, acceleration in the rate of decay has been observed in most sandstone samples in recent years. Sandstone degradation mechanisms are controlled by several factors, such as mineral composition and pore/capillary size. One of the most important factors affecting stone degradation is water. Water acts as a vehicle for weathering processes caused by aggressive atmospheric pollutants. Water also causes disintegration, surface erosion, cracking through freezing – thawing or wetting – drying cycles inside the pores, dissolution and transportation of soluble salts that can induce their inter/intraporous crystallization, hydrolysis of silicate rocks and other effects, such as color alteration. Moreover, water favors the growth of microorganisms and crust formation, which are typical signs of stone decay [1,2]. The development of effective consolidants for stones is a key goal in cultural heritage conservation [3]. These materials need to have certain properties, such as good coagulation between
∗ Corresponding author. Tel.: +8 613908062856. E-mail address: [email protected] (Y.B Luo).
grains that comprise the stone relics to restore their strength and resist environmental deterioration. These materials also become hydrophobic to block water penetration. Silicate esters, such as tetraethoxysilane (TEOS), have been widely studied and used for stone consolidation, which is mainly sandstone [3–5]. TEOS-based products polymerize in situ inside the pore structure of the disintegrating stone through a sol–gel process and significantly increase the cohesion of the stone [3]. The advantages of these products are well-known; i.e., the low viscosity of TEOS allows its deep penetration into porous stone; a stable gel with a silicon–oxygen backbone is formed after polymerization with environmental moisture; no deleterious by-products are formed and the resulting ethanol evaporates completely [6,7]. Traditional TEOS-based consolidants are known to leave some open pores to allow air and water vapor to pass back and forth through the stone as the weather changes. However, a well-known drawback of these conservation products is their tendency to form brittle gels that are highly susceptible to forming cracks inside the stone, which is generated by the high capillary pressures produced within the gel pores during its drying phase [8]. To address these problems, several trials have been performed in recent years. Particular attention has been given to composites, which are obtained by introducing various inorganic nanoparticles, such as silica dioxide (SiO2 ) and titanium dioxide (TiO2 ), into hybrid siloxane or silicone polymers. This process enhances surface
http://dx.doi.org/10.1016/j.culher.2014.08.002 1296-2074/© 2014 Elsevier Masson SAS. All rights reserved.
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hydrophobicity and provides an efficient means of preventing the gel from cracking, by increasing gel pore size and reducing capillary pressure while drying inside the stone [6,9–13]. Although the addition of nanoparticles reduces crack formation in the gel, nanoparticles were reported to still be able to act as a barrier to water vapor movement if the nanoparticles were not dispersed correctly [14]. The use of hydroxyl-terminated polydimethylsiloxane (PDMS) as an additive in TEOS sol–gel systems was reported to be another effective and simple way to obtain a hydrophobic product; TEOS/PDMS sol–gel systems were also reported to be a more elastic consolidant to solve the alkosysilane-derived film cracking problem [15–17,11,18]. Usually, the PDMS content was as high as 30% w/w to obtain improved properties [11,17]. Preparation of multifunctional inorganic/polymer nanocomposites presents a challenging task on improving the polymer properties, and opens a brighter perspective for their future application. Compared to conventional hydroxyapatite (HA), nanohydroxyapatite (n-HA) has larger specific surface area and better mechanical properties, and has been increasingly used as functional materials [19–22]. To date, the use of n-HA as consolidant for sandstone has not been investigated. Although the use of hydroxyapatite in consolidating stones has been investigated in recent studies and the in situ formed microsized hydroxyapatite (m-HA) increased significantly the mechanical properties and showed good consolidation effects for carbonate stones [23–25], this process was not easy to control and the unreacted diammonium hydrogen phosphate (DAP) was easy to deposit on sample surface and cause color change. Moreover, in the case of sandstone, which represents an old and wide-spread construction material all over the world, the consolidants with in situ formed m-HA are not effective in improving mechanical properties and resisting artificial aging tests as on carbonate stone. Therefore, in this paper, the possible use of n-HA as a new inorganic consolidant was investigated. The consolidant was obtained by the co-condensation of TEOS and a small amount of PDMS in the presence of n-HA. The composites were applied to sandstone specimens by brushing. Water absorption, hydrophobic property, and artificial aging tests were performed to preliminarily evaluate their efficacy as a sandstone conservation material. For comparison, consolidants with m-HA, without any kinds of HA and commercial product Remmers, were also evaluated. Remmers is a stone strengthener on a silicic acid base and has been widely used for sandstone consolidants [26–28]. The objective of the present study is to investigate the consolidation effects to develop an effective sandstone consolidant suitable for the rainy and humid environment in southwest China.
2. Experimental 2.1. Materials TEOS, which is an analytical grade reagent, was obtained from Kelong Chemical Reagent Corporation. PDMS, which has a weightaverage molecular weight (Mw ) of 3000 g/mol, was supplied by the Shanghai Jiapeng Chemical Industry and used as received. Both of the n-HA, with an average primary particle size of approximately 20 nm and the m-HA, with an average primary particles size of approximately 10 m, were obtained from Beijing DK Nano Technology Co., Ltd. Di-butyltin dilaurate (DBTL), which was obtained from Kelong Chemical Reagent Corp., was used as received. Remmers KSE OH, which is a commercial stone consolidant with polymer silicate of low molecule weight as the main component, was purchased from Remmers Co., Ltd, Germany. Unweathered sandstones were obtained from a quarry located in Yaan, Sichuan
Table 1 Composition of sols under studya . Product
TEOS (% w/w)
PDMS (% w/w)
DBTL (% w/w)
n-HA (% w/w)
m-HA (% w/w)
TEOS/PDMS TEOS/PDMS/HA-0.5 TEOS/PDMS/HA-1 TEOS/PDMS/HA-2 TEOS-PDMS-m-HA-0.5 TEOS-PDMS-m-HA-1 TEOS-PDMS-m-HA-2
100 100 100 100 100 100 100
5 5 5 5 5 5 5
1 1 1 1 1 1 1
0 0.5 1 2 0 0 0
0 0 0 0 0.5 1 2
TEOS: tetraethoxysilane; PDMS: polydimethylsiloxane; m-HA: microsized hydroxyapatite; n-HA: nano-hydroxyapatite; DBTL: di-butyltin dilaurate. a The “w/w” is always referred to the weight of TEOS.
and cut into blocks of 3 different sizes: 5 cm × 5 cm × 1 cm (for water vapor permeability test), 5 cm × 5 cm × 10 cm (for mechanical test), and 5 cm × 5 cm × 2 cm (for artificial aging tests, change in color, and static contact angle test). First, the sandstone blocks were scraped off using a grinding machine to obtain uniform surfaces and reduce cutting imperfections. Afterwards, the samples were rinsed with deionized water for an hour via ultrasonic agitation, and were dried in an oven at 105 ◦ C up to constant weight for at least 24 h. Each test was performed on samples coming from the same block of sandstone rock, thereby having similar macroscopic features (such as color, shape, grain, and surface roughness). Petrographic analysis using a transmitted light microscope B × 51 (Olympus) indicated that surface porosity and surface pore size of the sandstones were approximately 3.5% and 0.06–0.15 mm, respectively. 2.2. Preparation of TEOS/PDMS/HA nanocomposites TEOS-based nanocomposites were prepared through TEOS hydrolysis in the presence of n-HA and PDMS using DBTL as a catalyst, and the product was labeled as TEOS/PDMS/HA. First, 40 mL TEOS and a known amount of n-HA (0.5, 1, 2% w/w, respectively) were mixed under ultrasonic treatment for 30 min at 50 ◦ C. Next, 5% w/w PDMS was added to the mixture under vigorous stirring at 50 ◦ C by refluxing for 4 h. Lastly, 1% w/w DBTL was added to the mixture under vigorous stirring for 10 min. Ten milliliter of each sol was subsequently poured into a glass Petri dish (d = 10 cm) and allowed to gel and dry for 4 weeks in a sealed container maintained at room conditions (T = 23 ± 2 ◦ C and RH = 50% ± 5%). The gelled samples from each film were detached from the dishes and analyzed by scanning electron microscopy. The TEOS/PDMS/HA nanocomposites with 5% w/w PDMS and 0.5, 1, 2% w/w n-HA were labeled as TEOS/PDMS/HA-0.5, TEOS/PDMS/HA-1 and TEOS/PDMS/HA-2. For comparison, consolidants with m-HA were prepared using the same technique and labeled as TEOS-PDMS-m-HA-0.5, TEOSPDMS-m-HA-1 and TEOS-PDMS-m-HA-2. At the same time, TEOS and PDMS (5% w/w) composites were prepared using the same techniques and were labeled as TEOS/PDMS. The composition of the sols under study is presented in Table 1. 2.3. Consolidation of sandstone samples The TEOS/PDMS/HA-0.5 consolidants and commercial Remmers KSE OH were applied to blocks by brushing thrice with a time interval of no less than 10 min under laboratory conditions (T = 3 ± 2 ◦ C and RH = 50% ± 5%). Before testing, all treated samples were aged and kept at ambient conditions to reach steady weight (m < 0.001 g). The average amount of consolidant applied to the sandstone samples, which was determined by weighting the dry samples before and after the treatment, was approximately 5.0 ± 0.6 mg/cm2 . The data reported is the mean values obtained from three sandstone samples, except when a different number of samples are expressly indicated.
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2.4. Characterization 2.4.1. X-ray diffraction (XRD) Identification of the mineral composition of the samples was recorded on a DX-1000 diffractometer equipped with a graphite ˚ radiation. The generator monochromator and Cu K␣ ( = 1.5418 A) was operated at 40 kV and 25 mA. The samples were scanned at diffraction angles 2◦ to 70◦ at 0.06/s scanning rate. 2.4.2. Microscope examination Scanning electron microscopy (SEM) images were recorded using a Hitachi Fields S-4800 instrument operated at 5 kV. All specimens were sputter-coated with gold prior to examination. 2.4.3. Static contact angle () The static contact angle () was measured at 25 ◦ C using a JY-82A contact angle goniometer (Hebei Chengde Testing Machine Co. Ltd, China). Droplets of distilled water (5 L each, with a microsyringe) were deposited on the surfaces of samples and from a distance remained sufficiently close to the substrate, so that the needle can remain in contact with the applied water droplet after delivery. Afterwards, the needle was withdrawn with minimum perturbation to the droplet, and the image of the droplet was recorded immediately for measurement. The result is an average value obtained from a total of 10 different points on three sample surfaces. 2.4.4. Mercury intrusion porosimetry (MIP) Changes in the pore size distribution and porosity before and after consolidation were carried out on stone specimens with a volume of around 1 cm3 by means of a mercury intrusion porosimetry (MIP, AutoProe IV 9500, Micromeritics, USA). Specimens were cut from parts adjacent to the treated area of the stone under study. 2.4.5. Mechanical properties The compressive strength was evaluated using a programmecontrolled servo concrete and rock mechanics test system (RMTS150, Sichuan University, China) and was conducted at room temperature. All tests were performed according to the Test Method of Engineering Rock Mass (GB/T50266-99). The test was performed under the loading rate of 0.5 MPa/s until failure. The compression strength expressed in MPa was obtained using the equation below: c =
P A
(1)
where P is the failure load (N), and A is the loaded area (mm2 ). The data reported is the mean from five determinations. 2.4.6. Penetration depth It is widely accepted that the penetration depth of consolidants is an important parameter to be taken into account when assessing consolidation effectiveness. Penetration depth of consolidants is often assessed visually [29–31]. In our experiments, the untreated sandstone samples were initially powdered and sieved with a 16 mesh sieve. The powders in graduated glass container with 10 mm diameters were tamped by vibration. The powers had the same mass and height to get the similar porosity in each glass container. Two-milliliters of consolidants were dropped on the surface of the power, and then the glass container was sealed with cotton. The penetration depth of consolidants was measured by recording the position of the consolidant in the compacted sandstone powder after 24 h [31]. 2.4.7. Evaluation of disadvantage effects induced by the treatments Possible disadvantage effects, such as color changes and water vapor permeability reduction, were also evaluated.
3
2.4.7.1. Water vapor permeability (WVP). WVP was evaluated by fixing a sample block (5 cm × 5 cm × 1 cm) as a cover on a cylindrical PVC container filled partially with water. Afterwards, the containers were properly sealed with melt paraffin section and placed in a desiccator with a relative humidity of R.H. 35% and at constant temperature of 23 ± 2 ◦ C. The mass of the system of the container, sample block and the water were recorded every 24 h to determine the mass of water vapor passing though the unit surface under controlled conditions. The test of WVP was based on the standard wet cup test (ASTM E96M-2005 standard test methods for water vapor transmission of materials). The WVP expressed in g/(m·s·Pa) was determined by the following equation: WVP =
WVT × l m × l = S × (R1 − R2 ) t × A × S × (R1 − R2 )
(2)
where WVT is rate of water vapor transmission (g/s·m2 ), m is the mass change (g), t is the time when m occurred (s), A is the test area (cup mouth area, m2 ), S is the saturation vapor pressure at test temperature (Pa), l is the thickness of the sample (m), R1 is the relative humidity in the desiccator, and R2 is the relative humidity in the permeation container. 2.4.7.2. Change in color. The change in color was determined using a solid reflection spectrophotometer (Color Reader CD-10 from Minolta Co., Ltd.). The conditions used in the experiment were illuminant D65 and observer 8◦ . CIEL*a*b* color space was used and color variations were evaluated using the parameter total color difference (E*). Each test was performed making four measurements on each treated or untreated sandstone sample; in order to confirm the reproducibility of the results, the test was repeated on three identical sandstone samples, therefore the data reported were the measurements. The average color difference mean values on 12 expressed as E =
2
2
2
(L∗ ) + (a∗ ) + (b∗ ) .
2.4.8. Resistance to artificial aging tests Artificial aging tests aim at simulating the actual polluted, environmental conditions in Sichuan; as well as quantifying the durability of the materials under study [25,26,31–33]. Four types of weathering, which are mentioned below, were conducted sequentially and termed as “artificial aging”. In addition, contact angle, compressive strength, and weight change were determined before and after treatment with the materials under study. In each test, samples treated by TEOS/PDMS/HA, TEOS/PDMS, and commercial Remmers KSE OH were compared against untreated samples. The resistance of artificial aging tests was evaluated after a period of cycles described below. 2.4.8.1. Acid water weathering cycles [31–33]. Acid rain in the outdoor environment has been shown to have a great influence on the durability of the stone relics. We designed a test using H2 SO4 (2%, v/v) to study the acid resistance of the sandstones. The samples were subjected to cycles of immersion in dilute 2% H2 SO4 solution for 16 h followed by absorption of any excess water onto filter paper after taking samples out of water, and another 8 h in an oven at 60 ◦ C. These experimental procedures were repeated until the test samples were completely destroyed. 2.4.8.2. Salt crystallization weathering cycles [31–33]. The samples were subjected to cycles of immersion in a 10% w/v Na2 SO4 solution for 16 h, followed by absorbing any excess water onto filter paper after taking samples out of water, and subsequent drying for 8 h in an oven at 60 ◦ C. These procedures were repeated until the test samples were clearly destroyed.
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2.4.8.3. Wet–dry cycles [31–33]. In order to determine the effect of water on sandstones, simulation of climatic change from sunny to wet/rainy weather was carried out. This test consisted of a total of 60 cycles of immersion and drying as follows: 16 h total immersion in distilled water, absorbing any excess water onto filter paper after taking samples out of water, and subsequent drying for 8 h in an oven at 60 ◦ C for each treatment. The contact angle, compressive strength, and weight change were determined before and after wetdry tests. 2.4.8.4. Freezing–thawing cycles [26,31]. This experiment consisted of a total of 50 cycles. In each cycle, samples were soaked in
distilled water at 23 ± 2 ◦ C for 12 h, followed by freezing at −15 ◦ C for another 6 h after absorbing excess surface water with a filter paper, and subsequent drying for 6 h in an oven at 60 ◦ C, and then soaking again in distilled water at 23 ± 2 ◦ C for 12 h. 3. Results and discussion 3.1. Characterization of gels Fig. 1 shows the gel images obtained after drying under laboratory conditions. TEOS/PDMS materials and the commercial Remmers KSE OH films cracked; and the degree of cracking was
Fig. 1. Images of gel films obtained after drying under laboratory conditions: a: remmers KSE OH; b: TEOS/PDMS; c: TEOS/PDMS/HA-0.5; d: TEOS/PDMS/HA-1; e: TEOS/PDMS/HA-2.; f: TEOS/PDMS/m-HA-0.5.
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Fig. 2. XRD pattern of samples treated by the materials under study.
slightly higher for TEOS/PDMS products (Fig. 1a and b). The addition of n-HA did not change the gelation time, but reduced cracking was observed in the n-HA and TEOS-based materials even when the n-HA concentration was only 0.5% w/w. Aggelakopoulou et al. [14] demonstrated that the presence of nanoparticles physically limits the silicate network from shrinking under capillary pressure. Given this result, the crack-free film might be ascribed to the addition of n-HA in reducing the surface tension of the initial solution, which contributes to reduced capillary pressure. More details are addressed in the following section. Fig. 1 also shows that with a successive increase in n-HA concentration, the gels become whiter at high concentration (> 0.5% w/w, Fig. 1c–e). However, Fig. 1f shows that the TEOS/PDMS/m-HA-0.5 film crack greatly. 3.2. The assessment of the conservation efficiency XRD analysis was used to investigate mineralogical and surfacecoating characteristics of sandstone before and after being treated with consolidants. The XRD patterns of the untreated and treated sandstone samples were compared in Fig. 2. The results clearly showed that quartz was the predominant mineral phase in the untreated sandstone. Small amounts of clay minerals, such as calcite, albite, kaolinite and chlorite, were present when the observed Bragg reflections were compared with those of reference minerals. The peak positions corresponding to the main components did not change after treatment with PDMS and/or n-HA, which implied no change in the crystal structure. However, the XRD patterns of TEOS/PDMS/HA-0.5 revealed the new peaks at 2 = 31.64◦ , 32.12◦ , 32.94◦ when compared with TEOS and TEOS/PDMS. The new signals suggested the presence of n-HA. To further explore surface characteristics, the fracture surfaces of the samples were evaluated by SEM. As shown in Fig. 3b, the TEOS/PDMS/HA-0.5 material creates a crack-free, homogenous, and coarse network. By contrast, the TEOS/PDMS gel forms a dense coating (Fig. 3c). The SEM result showed that the presence of n-HA promoted the coarsening of the gel network. Mosquera et al. [34] demonstrated that TEOS sol prevented cracking during the drying phase by coarsening the pore structure. In the present study, the formation of this coarse gel network can be due to the TEOS oligomer chains, and were integrated on the n-HA surface, which thereby reduced the capillary pressure. Since the main cause of stone weathering is water, surface treatment should prevent water penetration into the bulk of the stone. For this reason, we characterized static contact angle of a
Fig. 3. SEM micrographs of the samples: a: sandstone shows disintegration and erosion between mineral grains; b: with materials of TEOS/PDMS/HA-0.5, shows coarser coating on the stone surface; c: with materials of TEOS/PDMS, shows dense coating on the stone surface.
water droplet on the samples under study. The water droplet was observed to spread out rapidly on the surface of the untreated samples (62◦ ± 3) and soon completely wet the surface of the sample (within 1 min). As shown in Fig. 4, in the case of TEOS/PDMS and TEOS/PDMS/HA-0.5 treated sandstone surface, the contact angle are both above 120◦ , which is a bit higher than that of Remmers KSE OH (112◦ ). In addition, a spherical water droplet can be observed on the sandstone surface, which then fell from the surface in our experiment. This method demonstrates that the addition
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Fig. 4. Water static contact angles of samples after treatment by materials under study. Bars correspond to standard deviations. (photographs of a static water droplet on the sandstone surfaces are shown on the lower position).
of the organic component (PDMS) is a key factor in imparting the hydropobic effect by reducing surface tension. The contact angle of TEOS/PDMS/HA-0.5 treated sandstone surface is slightly higher than TEOS/PDMS-treated surface, as observed in Fig. 4. According to the model of Cassie’s or Wenzel’s theory of the contact angle, one of the effective ways to improve the contact angle is to construct surface roughness [35,36]. The surface roughness of n-HA may be partially increased in its integration into the silica matrix. Thus, a higher increase in the nanocomposite roughness with the integrated hydroxyapatite nanoparticle occurred. Changes in the sandstone porosity after consolidant treatments were evaluated by MIP. The porosity values and pore size distributions are shown in Table 2 and Fig. 5, respectively. The untreated sandstone presented a total porosity value of 14.4% and exhibited a pore size distribution centered at 6–7 m. After application of KSE OH and TEOS/PDMS/HA-0.5, the porosity and pore size distribution were slightly reduced whereas the TEOS/PDMS products caused a significant change in porosity and pore size distribution. The reduction of macropore size should be ascribed to the TEOS/PDMS dense layer reducing the size of the pores and the increase of the porosity and smaller pores should be ascribed to the high shrinkage of the dense gel coating layer after consolidation [6,25].
Fig. 5. Pore size distribution of untreated and treated samples with materials under study.
Fig. 6. Compressive strength versus artificial aging conditions for samples treated with materials under study. Bars correspond to standard deviations. The numbers in brackets correspond to the cycles of aging tests.
The mechanical properties of untreated and treated samples are shown in Fig. 6. Compared with other consolidants, mechanical properties of TEOS/PDMS/HA-0.5 treated stones were improved greatly (19%). The improved mechanical properties could be attributed to the TEOS oligomers and PDMS have a good compatibility with n-HA and could diffuse into the n-HA and establish strong interaction. The strong interfacial interaction and compatibility increase stress transfer to the n-HA, resulting in high mechanical properties [37,38]. The slightly reduced mechanical properties of TEOS-PDMS-m-HA-0.5 treated stones could be ascribed to the weak interfacial interaction and poor dispersion of the m-HA. The ability of the consolidant to penetrate into sandstone is one of the most important factors for a successful treatment. The penetration depth after 24 h is listed in Table 2. Table 2 shows that the samples treated by TEOS/PDMS/HA-0.5 have similar penetration depth when compared with the commercial product Remmers KSE OH. In addition, the depth observed is deeper than that of TEOS/PDMS. Xu et al. [39] reported that the addition of particles to a TEOS solution can increase the viscosity of the solution dramatically when 0.5% w/w was added. Aggelakopoulou et al. [14] also demonstrated that agglomerates were present; agglomerates increase the viscosity and can block pore entrances, which prevent the consolidant from entering the sandstone. The nanosized and well dispersed n-HA in the composite were not a problem for penetration through the sandstone pores. However, Xu et al. [39] reported the water vapor permeability of TEOS-0.2%SiO2 -PDMS materials was reduced largely because of particles that block the stone pores. This finding encouraged us to investigate the effect of water vapor permeability induced by the nanomaterials under study. Table 2 also shows that TEOS/PDMS/HA-0.5 is a material that produces no reduction in the breathability of the sample being evaluated. However, TEOS/PDMS produced a reduction as high as 11%. In this case, n-HA imparted a coarser network for vapor transport to the stone surface. n-HA is suggested to be a possible, suitable consolidation additive. nHA did not alter the water vapor permeability; in addition, the breathability of the original material was retained. A color change caused by the materials under study is contrary to the Code of Ethics for conservation cultural relics. Therefore, in the next part of our study, we studied color changes in the sandstone after treatment. The total color difference values (E*) of the stone after treatment are shown in Table 2. For the materials
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Table 2 Properties of samples with materials under studya . Material
E
Penetration depth (cm)
Water vapor permeability (g/ms Pa)
Porosity (%)
Untreated TEOS/PDMS TEOS/PDMS/HA-0.5 Remmers KSE OH TEOS/PDMS/HA-1 TEOS-PDMS-m-HA-0.5 TEOS-PDMS-m-HA-1
– 1.17 ± 0.25 2.32 ± 0.31 1.16 ± 0.29 5.70 ± 0.41 6.2 ± 0.43 10.9 ± 0.69
– 2.4 ± 0.11 3.1 ± 0.12 3.4 ± 0.14 – 2.1 ± 0.16 –
7.96 ± 0.75 × 10−8 7.12 ± 0.51 × 10−8 7.92 ± 0.54 × 10−8 7.93 ± 0.42 × 10−8 – 6.89 ± 0.32 × 10-8 –
14.4 ± 0.21 16.5 ± 0.75 13.9 ± 0.19 13.8 ± 0.32 – 16.9 ± 0.69 –
TEOS: tetraethoxysilane; PDMS: polydimethylsiloxane; m-HA: microsized hydroxyapatite. a Data correspond to average value ± standard deviation.
synthesized in our laboratory (composites of 0.5% w/w n-HA and TEOS/PDMS) and for the commercial consolidant Remmers KSE OH, E* was very low, and was below the perceptibility threshold (E* < 3) [15,40]. Only those with m-HA and n-HA content with more than 0.5% w/w created a perceptible change in the sandstone color. Therefore, materials with n-HA content larger than 0.5% w/w and m-HA were excluded in latter experiments, if there is no other special instruction. The images of samples with the artificial aging test time, weight change, color change, contact angle and compressive strength properties of the samples under artificial weathering conditions before and after treatment were determined in our experiments.
As illustrated in Fig. 7, the surface of the samples under acid and salt weathering conditions changes dramatically for untreated sandstone, and with Remmers KSE OH materials even after only six cycles. The sandstone without any protection produced bubbles when the untreated samples were initially placed in the acid solutions. These methods showed big stomata and fine cracking on the surface after only three cycles of acid weathering. This finding was the result of the reaction between the strong proton acid and calcareous cement that produces CO2 gas and water-soluble salts. The untreated samples, as well as samples coated with Remmers KSE OH materials collapsed completely after 6 and 10 cycles of acid weathering, respectively. The surface of samples treated
Fig. 7. Images of samples with materials of Remmer KSE OH (A, A A ), TEOS/PDMS (B, B B ), untreated sandstone (C, C C ), TEOS/PDMS/n-HA-0.5 (D, D D ) under different weathering conditions. A, B, C, D: before aging test; A , B C D : acid aging test (6 cycles); A , B , C , D : salt aging test (6 cycles).
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of a concentration difference between the samples and the salt solution. As for the other three artificial aging tests, the weight changed began directly after disintegration of samples. Thus, a difference in the mechanism was observed between the salt and other weathering conditions. The samples were less affected by the wet–dry and freeze–thawing tests compared to the acid and salt artificial aging tests. However, some changes of mechanical and contact angle after 40 cycles (Figs. 6 and 8) were noted. In summary, in the current study, the TEOS-based composite with n-HA had a better resistance to weathering aging tests because of the effect of nanoparticles. 4. Conclusions
Fig. 8. Contact angle versus artificial aging conditions for samples treated with materials under study. Bars correspond to standard deviations. The numbers in brackets correspond to the cycles of aging tests.
with TEOS/PDMS and TEOS-PDMS-m-HA-0.5 became sandy, while the samples treated with TEOS/PDMS/HA-0.5 remained unchanged even after 15 cycles. However, some decrease in mechanical strength was observed for TEOS/PDMS, TEOS/PDMS/HA-0.5 and TEOS-PDMS-m-HA-0.5 samples at 20%, 12% and 24%, respectively (Fig. 6). Under salt crystallization weathering test, the samples coated with Remmers KSE OH, as well as the untreated sandstone, cracked in cross-section after only 16 and 10 cycles, respectively. The surfaces of samples coated with TEOS-PDMS-m-HA-0.5 and TEOS/PDMS materials were covered by white products after 13 and 18 cycles and flaked slightly after 18 and 22 cycles during salt weathering tests. No obvious change was observed for samples coated with TEOS/PDMS/HA-0.5 after 22 salt crystallization weathering cycles. The contact angle and compressive strength were reduced somewhat for all of the samples, as shown in Figs. 6 and 8. However, the samples coated with TEOS/PDMS/HA-0.5 showed the least decrease in compressive strength. In addition, Fig. 9 shows that the weight of the samples coated with Remmers KSE OH and those without coatings decreased markedly during the salt weathering test. At the same time, it is notable that the weight somewhat increased initially and then decreased during salt weathering cycles (Fig. 9). The increase of weight was ascribed to the salt crystallization that occurred in the samples during salt weathering, because
Fig. 9. Weight change under salt crystallization artificial aging test.
To enhance sandstone consolidation and protection, a new TEOS/PDMS/HA material was synthesized through a simple sol–gel method and was subjected to artificial weathering tests. The addition of PDMS, which has flexible segments, and n-HA, which creates a surface roughness, reduced crack formation in TEOS-based gels during the drying phase. In addition, the hydrophobic effect is ascribed to the organic component (PDMS) by the reduction in the surface tension and the n-HA by increasing surface roughness. No negative effects on the treated sandstone were observed. The artificial aging tests showed that n-HA has an important function in improving resistance to weathering effects. Compared with consolidants with m-HA and others under study, this finding indicates the efficacy of the consolidant with n-HA for protection and conservation process. Acknowledgements This work was supported financially by the Science and Technology Support Programme of Sichuan Province (2013FZ0076), the Advanced Interdisciplinary Innovation Research Project of Sichuan University (skqy201216) and the Younger Fund of the Ministry of Education (10XJCZH005). We also express our gratitude to Donna Strahan from Freer Gallery of Art and Arthur M. Sackler Gallery, Smithsonian Institution for her assistance in improving the quality of our paper. References [1] A. Jain, S. Bhadauria, V. Kumar, R.S. Chauhan, Biodeterioration of sandstone under the influence of different humidity levels in laboratory conditions, Build. Environ. 44 (2009) 1276–1284. [2] P. Martinec, M. Vavro, J. Scucka, M. Maslan, Properties and durability assessment of glauconitic sandstone: a case study on Zamel sandstone from the Bohemian Cretaceous Basin (Czech Republic), Eng. Geol. 115 (2011) 175–181. [3] G. Wheeler, Alkoxysilanes and the consolidation of stone, Getty Publications Book Distribution Center, Los Angeles, 2005. [4] V. Cnudde, M. Dierick, J. Vlassenbroeck, B. Masschaele, E. Lehmann, P. Jacobs, et al., Determination of the impregnation depth of siloxanes and ethylsilicates in porous material by neutron radiography, J. Cult. Herit. 8 (2007) 331–338. [5] A. Tsakalof, P. Manoudis, I. Karapanagiotis, I. Chryssoulakis, C. Panayiotou, Assessment of synthetic polymeric coatings for the protection and preservation of stone monuments, J. Cult. Herit. 8 (2007) 69–72. [6] M.J. Mosquera, D.M. Santos, A. Montes, L. Valdez-Castro, New nanomaterials for consolidating stone, Langmuir 24 (2008) 2772–2778. [7] S. Siegesmund, R. Snethlage, Stone in architecture: properties, durability. Chapter 7: stone conservation, 4th ed., Springer-Verlag Berlin and Heidelberg GmbH & Co. K, Germany, 2011, pp. 411–542. [8] M.J. Mosquera, J. Pozo, L. Esquivias, Stress during drying of two stone consolidants applied in monumental conservation, J. Sol-Gel Sci. Technol. 26 (2003) 1227–1231. [9] 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. [10] V. Daniele, G. Taglieri, Synthesis of Ca(OH)2 nanoparticles with the addition of Triton X-100. Protective treatments on natural stones: Preliminary results, J. Cult. Herit. 13 (2012) 40–46. [11] J.F. Illescas, M.J. Mosquera, Surfactant-synthesized PDMS/silica nanomaterials improve robustness and stain resistance of carbonate stone, J. Phys. Chem. C. 115 (2011) 14624–14634.
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