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Applications of Polymer, Composite, and Coating Materials

5S multifunctional intelligent coating with super-durable, superhydrophobic, self-monitoring, self-heating, and selfhealing properties for existing construction application Bo Pang, Jiajia Qian, Yunsheng Zhang, Yantao Jia, Henmei Ni, Sze Dai Pang, Guojian Liu, Rusheng Qian, Wei She, Lin Yang, and Zhiyong Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08303 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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ACS Applied Materials & Interfaces

5S Multifunctional Intelligent Coating with Super-durable, Superhydrophobic, Selfmonitoring, Self-heating, and Self-healing Properties for Existing Construction Application Bo Panga,b, Jiajia Qiana,b, Yunsheng Zhanga,b*,Yantao Jiac*, Henmei Nid, Sze Dai Pangf, Guojian Liua,b, Rusheng Qiana,b, Wei Shea,b, Lin Yange, Zhiyong Liub a School of Materials Science and Engineering, Southeast University, Nanjing 211189, China b Jiangsu Key Laboratory for Construction Materials, Southeast University, Nanjing 211189, China c College of Mechanics and Materials, Hohai University, Nanjing 211189, China d School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China e School of Water Conservancy and Environment, Zhengzhou University, Zhengzhou 450001, China f Department of Civil and Environmental Engineering, National University of Singapore, E1 Engineering Drive 2, Singapore 117 576, Singapore

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Abstract There is a constant drive to develop ultrahigh performance multifunctional coatings for existing construction used in modern engineering technologies. For these materials to be used in unsound infrastructure protections, they are required to present enhanced robustness while bearing functionalities to meet multiple uses. Singlefunction coating is not smart enough to provide satisfactory protection, and the preparation process of multifunctional materials is complex, costly, and provides poor durability. Thus, existing coatings are not suitable to generate intelligent closed-loop protection system. Herein, we report an innovative 5S multifunctional intelligent coating (5SC) for existing construction materials with super-durable, superhydrophobic, self-monitoring, self-heating, and self-healing properties. The 5SC material showed highly durable superhydrophobic properties as revealed by the main failure tests of building materials including physical friction (abrasion, scratching), 100% tensile strain, photoaging (3000 h of ultraviolet (UV) aging), acid corrosion (concentrated hydrochloric acid and sulfuric acid), freeze-thaw aging (salty solution). The coated surface was highly sensible to pressure, with monitoring thresholds from 1 to 30000 N per 0.01 m2. It showed an early heating rate as high as 6 ºC/min while maintaining very good self-monitoring and ice-melting drainage performance to protect the existing structures. This novel composite material is suitable for constructions in extreme areas where corrosion and freeze-thaw damage can occur. This multifunctional material presents a very broad range of applications and

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development potential in the construction field. Key words: superhydrophobic; self-heating; self-healing; durable; coatings 1. Introduction Due to the inherent hydrophilic and porous properties of concrete, corrosive solutions are readily adsorbed and infiltrated into the substrate via capillary effect through communicating pores. This process results in surface erosion, internal steel corrosion, and aggravated freeze-thaw damage, which ultimately decrease the service life of concrete structures.1-3 Frost heaving stress causes the expansion of matrix cracks, which accelerates concrete deterioration.4 This issue is particularly serious in alpine regions or zones with monsoon climate. Even though high-performance and high-durability concrete has been widely researched and developed, when concrete is not conveniently protected, it undergoes performance degradation and service failure.5-7 The abuse of deicing salt leads to aggravation of freeze-thaw damage in concrete, which increases the economic burden, contaminates groundwater, and destroys the vegetation. In the case of inaccessible areas (e.g, protection of construction structures such as roofs, towers, concrete pipes, and bearing columns face more difficulties. A variety of multifunctional cement-based materials and protective coatings has been developed recently. Although the incorporation of various nanomaterials such as nano-silica (NS)8-9, graphene oxide (GO)10, carbon nanotubes (CNTs)11 resulted in improved durability of concrete and superior resistance to melting snow and ice, this

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approach is costly and not suitable for existing buildings. The main anticorrosion/icing coating technologies involve physical protection with a resin coating (e.g., epoxy coating12, polyurethane coating13, silicone coating14, etc.) and hydrophobic-superhydrophobic chemical protection15-17 (e.g., silane impregnation) or a combination of both. However, superhydrophobic coatings suffer from two major drawbacks. First, as a conventional method, superhydrophobic chemical protection cannot be applied at large scale by using immersion methods, and its robustness does not match the lifespan of the building structure.18-23 Second, these passive protective materials are not intelligent enough to create an effective protection closed circuit with a monitoring-melting ice/snow-drainage-healing system. Therefore, they cannot cope with extreme cold weather conditions. In recent years, the utilization of superhydrophobic coatings in concrete has been substantially widened.24-27 According to the Wenzel’s model and the Young equation, superhydrophobic surfaces can achieve a very high contact angle (CA) by reducing the surface energy and by constructing rough surfaces.28-29 However, rough surfaces with low surface energy are highly vulnerable to stress and wear, thus making it difficult to preserve the stability of the hydrophobic layer for the entire lifespan of the building. Besides, it is very difficult to carry out the super-hydrophobicity of the building surface in an easy and cost-effective manner while preserving the anti-aging and environmentally friendly characteristics of the coating. Although immersion is an effective method, it is not suitable for existing constructions.

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The thermoelectricity, piezoresistive effect, high durability, and small-size performance of a number of materials including graphene, nano-silica and other small size effects have been widely studied.30-33 The utilization of coating schemes to prepare super-hydrophobic, heating, self-monitoring, and self-healing materials has also been extensively studied.34-35 However, conventional preparation schemes are extremely complex and lead to poor performance of materials in outdoor applications. Some of these schemes are rather expensive and limited to lab scale. For instance, in concrete engineering, materials with impregnated graphene (0.5–1 g/L) are unacceptable from the perspective of economic cost. Additionally, existing studies do not study these properties combined in single material while proposing a reasonable application for existing constructions. These passive protective materials still fail to cope with extreme cold weather and are not intelligent enough to create an effective protection closed circuit with a monitoring-melting ice/snow-drainage-healing system as well. In 2015, the domestic fly ash (FA) emissions reached 550 million tons in China, and this value kept increasing year after year. To promote the utilization of FA, The National Development and Reform Commission of the People's Republic of China revised the management measures for a comprehensive utilization of FA.36 In 2014, the utilization rate of FA reached 70.07% in China, and construction materials accounted for 81% of the overall FA production.37 19% of FA is catalogued as low grade and cannot be used because of the presence of irregular particles and the

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heterogeneous composition. To deal with the above issues, we herein developed a 5S multifunctional intelligent protective coating (5SC) with super-durable, super-hydrophobic, selfmonitoring, self-heating, and self-healing properties for existing construction applications. The 5SC consisted primarily of a layer of composite reduced Graphene Oxide coating (rGOC), super-durable/ superhydrophobic treated (SST) coating layers and two copper foil tapes as electrodes. Super-hydrophobic microspheres (SMs) with multi-scale roughness ranging from microns to nanometers were constructed by alkali-etched FA, silica fume (SF), and nano-silica (NS). The SST coating was fabricated via a room-temperature vulcanized (RTV) rubber as a carrier of the SMs. A stable superhydrophobic layer was deposited on the surface of the microspheres by using polymethylhydrosiloxane (PMHS) and fluoroalkylsilane (FAS) catalyzed by the alkali solution. The 5SC system is low cost and can be prepared from a wide range of sources all over the world. It was prepared by a one-pot method with H2 and ethanol-water steam as the only by-products, in line with the green chemistry approach. 5SC can withstand aging as conventional buildings while maintaining its superhydrophobic capacity. The amount of FAS and rGO used in the preparation of 5SC was 0.1% of that of the conventional internal blending method.23, 30 The coated substrates showed a very broad self-monitoring stress threshold (from ice and snow gravity and frost heave force) between 1 and 30000 N per unit area (0.01m3) with good robustness. Its self-

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healing time span was less than 1 min to knife cutting damage while the surface reached 60–80 ºC within 10 min under 50V DC. The operation time of 5SC to substrates varied from 30 min to several hours, making it suitable for large-scale protective treatments in existing building applications. 2. Experimental 2.1 Sample preparation 

Cement-based substrate The concrete matrix was prepared by using the mixture proportions illustrated in

Table S1. The cement paste samples were prepared with a water to cement ratio of 0.29. The fresh concrete was poured into molds of 100 × 100 × 100 cm and shaken for 60 s while the fresh cement paste was mold 2 × 2 × 2 cm peices. The molded samples were cured under a 98% relative humidity and 25°C. 28 days later, all the concrete samples were cut in half for the further tests. 

SMs The NS dispersion liquid was prepared following a previous methodology of our

group.38 200 g FA, 200 g SF, and 300 mL water were mixed uniformly in a beaker. Then, 12 g NaOH were added to the turbid liquid while stirring (300 rpm) at room temperature for 5 h. 80 mL of the NS dispersion liquid, 4 g PMHS (hydrogen content 1.6 wt%) and 2 g FAS (1H,1H,1H,2H-perfluorodecyl trimethoxysilane) were subsequently added, and stirring was continued for 1 h, followed by sedimentation. The bottom precipitate was removed with a pipette, dried at 80 °C, ground and sieved

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(mesh aperture 0.6 mm) to obtain the SMs. 2.2 5S coating (5SC) 2.2.1 Reduced graphene oxide coating (rGOC) The rGO and dihydroxyl-terminated polydimethylsiloxane (PDMS) were mixed (15:100 in weight) uniformly by a roll mixer. The mixed paste rGO was applied to the surface of the substrate through an aluminum template, and the copper electrodes were connected to both ends of each rGOC. The thickness of the template was 0.5mm. 2.2.2 Super-durable and superhydrophobic treatment (SST) Firstly, the RTV rubber and the corresponding curing agent were uniformly mixed and uniformly mixed with 10% of the SMs to form a mixture. After the concrete substrate sample was subjected to the rGOC treatment, the above mixture was applied. Finally, a layer of SMs powder was sprayed before the surface was cured. 2.3 Superhydrophobic performance comparison To compare the hydrophobic properties of several existing mature techniques, cement substrates were treated with five different superhydrophobic treatment methods. The processing method is as follows: 

TEOS&PDMS The cement-based samples were dipped into a mixture of tetraethyl orthosilicate

(TEOS) and PDMS (3:1) at room temperature. Afterward, the samples were placed in

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20% wt. ammonia solution and reacted at 45 ºC for 1 h, and dried in an oven at 80 ºC. This method refers to the superhydrophobic scheme reported by Su et al. 39 

TEOS&PDMS&FAS We added 1 wt% FAS to the mixed solution of TEOS & PDMS to evaluate the

contribution of FAS to its hydrophobicity. 

Impregnation of FAS We refer to the ultra-durable and superhydrophobic concrete scheme reported by

Song et al.23, and incorporated 1 wt% FAS in a cement slurry with a water to cement ratio of 0.29. The samples were cured at 98% relative humidity and 25 ºC for 28 d. 

Common resin/rubber To compare the performance of the samples with SST, the same RTV rubber

without modification was applied to the surface of the substrate and subsequently tested. Meanwhile, epoxy resin (EP), polyurethane (PU), silicone, and acrylate homopolymer (AHP) were selected to compare the freeze-thaw resistance and aging resistance of the coating. 

Commercial superhydrophobic spray With the aim to compare with the mature superhydrophobic products on the

market, the Never Wet liquid-repelling treatment spray was used. 2.4 Characterization 

2D-3D Morphology Analysis The morphology and reactive products on the repair surfaces were observed and

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analyzed by scanning electron microscopy (SEM, FEI 3D), transmission electron microscopy (TEM, FEI acceleration voltage of 200 kV), and laser scanning confocal microscopy (LSCM, LSM700). Tapping-mode atomic-force microscopy (AFM) measurements were performed to evaluate the surface roughness and viscoelasticity of the surface. Both topological and phase images were recorded on a Dimension ICON model device. The data sampling rate was 10 MHz, whereas the scan size was 2500 μm2. 

Contact angle (CA) and roll off angle (RA) The static water CA and RA were measured by an OCA 15 plus contact angle

instrument (Germany, Dataphysics) using 5 μL water droplets. The data of samples were the average from 5 different locations. 

X-ray photoelectron spectroscopy (XPS) Chemical and elemental analyses of the polyester textile surfaces were obtained

by XPS on a Kratos Axis Ultra DLD (UK) device provided with an Al Kα (1486.6 eV) using amonochromatic X-ray source. 

Thermogravimetric analysis (TG) The thermal behavior of SMs was analyzed by TG with a Netzsch, TG209 F3

model. All the TG experiments were conducted as follows: 50 mg of sample, heating temperature 10 K/min, heating to 850 ºC, platinum crucible, and ambient 100 cm3 of N2. 

Infrared thermal imaging (ITI) and surface temperature measurements

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ITI measurements were carried out on FLIR T420 device with measurable temperature from - 20 to 650 ºC and fluctuations lower than 2 ºC. The surface temperature of the sample was measured by an omega four-channel data recording thermometer (HH309A), and each sample was tested simultaneously in four different zones. 

3D scale analysis The 3D scale of the SST coating was analyzed by 3D X-ray microscopy (Zeiss

Xradia 510 Versa with X-ray source: 30–160 kV transmission closed-tube X-ray source; detector: scintillator + optical lens + CCD, number of pixels: 2048×2048) with a minimum voxel size of 70 nm. 3.Results 3.1. Morphology, Chemical Composition, and Formation Mechanism of the SMs FA is the main solid waste released from coal-fired power plants at high temperature, while SF is comprised of spherical particulate dust produced by the industrial electric furnace during the high-temperature melting of industrial silicon and ferrosilicon.40-41 FA , SF and NS are mostly composed of SiO2 or a partial solid solution containing Al. Thus, the surface of the particles mostly contain -Si-O-Si- and Si-OH bonds.42-43 However, since these products are subjected to a high temperature processing, the amount of these silanol species able to chemically bond to molecules decreases dramatically. The alkali etching process was, therefore, used to increase the number of these reaction sites for functional molecules as well as the surface

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roughness of the microspheres. After the alkali etching treatment, both the specific surface area and the number of silicon hydroxyl groups exposed on the microspheres increased.38, 43 Therefore, the amount of functional molecules immobilized by FA and SF drastically increased to 6.5 and 29.2 wt.%, respectively. The super-hydrophobic properties of SMs were enhanced by the presence of modified molecules and the alkali etching treatment. As shown in Figures 1 a and b, alkali etching was highly effective in promoting surface roughness for both FA and SF (control experiments are included in the Supporting Information). The petal morphology of the etched FA surface was likely explained by the different distribution of internal Si and Al elements and the orientation reaction with NaOH. As shown in Figure 1c, SF microspheres were wrapped by abundant NS particles added at the end of the reaction. As shown in Figure 1d, a considerable number of microspheres with a diameter lower than 100 nm were attached to the surface of the dried FA, and the equivalent NS particles were also attached to the microspheres. The thermogravimetry (TG) results (Figure 2a) indicated that only 3–5 wt.% of functional molecules (PMHS and FAS) can be immobilized on SF and FA without any treatment. These SMs of different sizes (from dozens of μm to several hundred nm as shown in Figure S-1) showed enough roughness after the hydrophobic treatment to maintain the superhydrophobicity over the inside and outside surfaces.44 As shown in Figure 2b, the Na content increased with the duration of the etching treatment up to 60 min and decreased thereafter. The F content increased with the reaction time. Thus, once the

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microspheres were etched, a certain amount of Na+ was attached to the surface by ionic bond. During the rubbing process, SMs showed super-hydrophobic properties due to their different scales from nanometer to micrometer, Thus, SMs can be continuously exposed to the surface to provide superhydrophobic properties. The XPS high-resolution spectrum shown in Figure 2c also revealed that the amount of -CF2and -CF3 functional groups increased as the reaction progressed. XPS results revealed that the surface of the SMs was mainly composed of Na, O, F, C, and Si (Figure S-3b). The NaOH adsorbed by the active hydroxyl groups or a silica gel on the surface (Figures 3 a1, and a2 and R1) accounted for most of the Na detected. Na+ was removed after the super-hydrophobic treatment by PMHS and FAS, while the content in inarched FAS continuously increased after this treatment (Figure 3a3 and R2). Therefore, NaOH seems to play a major role i.e., catalyst in the reaction. We employed molecular dynamics to simulate the interaction of the functionalized molecules PMHS-OH and FAS-OH after hydrolysis on the active microspheres (Figures 3, b1–b4). These studies revealed that the polar hydroxyl groups of PMHS and FAS can interact with the surface -OH groups generated by NaOH on the microspheres via hydrogen bonds. As the reaction progresses, the SMs gradually changed from a highly polar state to a rough, composite microsphere with ultra-low surface energy. This chain action promoted the formation of more hydrogen bonds and was accelerated under an alkaline environment. Therefore, the super-hydrophobic SMs (Figures 3a4, R3, and R4) were self-assembled with a film of low surface energy

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groups such as -CH3 and -CF3. The preparation and working process of the 5SC are shown in Figures 4a and b. 3.2. Super-hydrophobicity and Durability of the SST Coating in Various Scales To further test the wear resistance of the SST coating, the surface layer was abraded with an 800# sandpaper at a pressure of 800 Pa while monitoring CA and RA along the cumulative abrasion distance from 0 to 20 m (Figure 5a). For an initial friction distance of 2 m, the CA of the SST sample increased up to 153.2° and levelled off above 150° for an abrasion distance of 20 m. The RA increased continuously above 10° when the friction distance was 16 m. This result was explained by the peeling off of the coatings. However, all SST samples showed excellent hydrophobicity. Thus, the frictional surface abraded for 20 m remained super-hydrophobic with an area retention of 99%. When the SST surface was intact, a certain amount of rough SMs (Figure 5b1) was attached to the coating. Interestingly, the frictional surface morphology of the samples (Figures 5b2) clearly indicated that the interface was a rough surface composed of particles of tens of nanometers in diameter when the SST was subjected to different frictional damages. This phenomenon proves that, as long as the SST coating remained attached, the multiscale SMs on the surface will construct sufficient roughness for its superhydrophobicity. The coating of these particles contained -CF3/-CF2- or -CH3 from FAS and PMHS and a minor amounts of RTV rubber with small surface energy. Freeze-thaw damage and acid solution erosion, the most serious material degradation

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mechanisms in construction, were studied to evaluate the coating protection performance of our materials. We compared the mass losses of cement samples with different protection methods after being immersed in HCl (38 wt%) and H2SO4 (70 wt%) solutions at 50 °C for varying times. The impregnation of FAS did not affected significantly the long-term erosion resistance in an acidic solution since the alkaline nature of concrete did not change (Figure 6a). Samples with other super-hydrophobic surface treatments completely lost their protection (Figure S–8). The mass loss of the SST samples after one month of immersion erosion was as low as 3–5%. Although RTV rubber has the same chemical structure of the SST coating matrix, the sample underwent serious damage due to its poor hydrophobic properties and low gas permeability. The EP resins and silicon were limited by their small strain/stress thresholds and did not cope the frost heave stress. The SST coating showed robust and excellent resistance to freeze-thaw corrosion and maintained its super-hydrophobic properties (mass loss below 3% after 80 freeze-thaw cycles in salt solution, Figures 6b and S–9). As shown in Figure 6c, both samples exhibited excellent superhydrophobicity before the friction tests. After the friction tests, knife scraping and hammering, the hydrophobicity of the commercial super-hydrophobic spray treatment was partially lost. Once the entire surface was abraded with a sandpaper, the surface almost completely lost its hydrophobicity. In contrast, the SST-treated surface performed well in a simple friction robustness test. More than 99% of the teased area preserved a CA of 150° and a roll angle lower than 10°. The specific robustness

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testing process can be found in the Supporting Information (videos S1 and S2). To correlate the surface physical properties of SST with its superhydrophobicity, additional roughness parameters were measured and correlated with the CA results. The traditional surface topography is generally described by roughness parameters such as Ra (Sa), Rp (Sp), and Rv (Sv), although these parameters alone do not reflect specific physical properties of the surface such as the peaks and valleys of the surface topography.45-46 The Abbott–Firestone curve can be used to describe surface granules, peaks, and valleys in detail by plotting the cumulative distribution of profile heights as a function of the amount (%) of material.47-48 The areal material ratio function of the tested surface (i.e., roughness bearing ratio), defined as the cumulative probability density function of the surface profile's height - ordinates z(x, y), can be calculated by integrating the profile trace.49 The relationship of unit space volume ∆V, tested area A, and profile height z are described in Formula S–1. Therefore, the additional height parameters can be calculated by using Formula S–1 to S–8. Where Sp is the maximum peak height, Sv is the maximum valley depth, St is the maximum height of the profile, Sa is the arithmetical mean deviation of the assessed profile, Sq is the root-mean-square deviation of the surface, and Ssk and Sku are the skewness and the kurtosis of peaks, respectively.50 The Abbott–Firestone curves of the sample (Figure 7a) became highly stable with the roughness bearing ratio decreasing by ca. 40%. After repeated testing, the CA of the SST decreased significantly after 2–4 friction cycles. The three-

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dimensional (3D) analysis (Figures 7b1 and b2) of the roughness phase revealed that the apparent morphology of the sample filled with SMs shifted to a relatively flat structure. The specific roughness parameters described in Formula S2–S8 are shown in Table S–3. One single friction can greatly damage the convex structure proved by the decrease of Sp, Sa, and Sq to a stable value after the first cycle.51 However, the CA did not decrease significantly thereafter, such that the super-hydrophobicity of the SST surface affected the Ssk value to a larger extent. The Ssk value (Formula S–7) represents the ratio of the average cubic value of the ordinate value of the height of each point on the measured surface to the cube of the Sq value of the reference plane. This parameter provides an indication of the asymmetry of the surface height distribution histogram and can be the representatives of peaks or valleys. As shown in Figure S–7, Ssk was higher than 0 revealing the presence of visible spikes of SMs bestrewed on the SST surface. Since most of the solid materials were below the reference plane, Ssk could be used to differentiate surfaces with similar Sa values. Ssk lower than 0 revealed a curved surface supported by a hole, and most of the solid materials were distributed over the reference surface. These results indicated that most of the microspheres were worn and destroyed, and the hydrophobicity was mainly produced by residual microspheres and the hydrophobic structure of the worn edges. Sku provides an indication of the steepness of the surface topography. The lower Sku, the wider the surface height distribution curve is.52 The SST coating showed very high super-hydrophobicity values for Ssk values higher than 0. The 3D

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spatial structure of SST (Figures 7c and e and Figure S6) revealed SMs particles with relatively high specific surface area filled both inside and outside the RTV substrate. Also, a hollow and a porous structure formed by SMs aggregates (Figures 7d and e) was observed, and this structure accounted for the inner-outer superhydrophobic properties of the SMs which supported results regarding hydrophobicity and durability.53-54 As shown in the particle count distribution of the statistical data (Figure 7 f), the median diameter of the particles was centered at ca. 1200 and 2000 nm, respectively. These multi-scale particles with low surface energy showed sufficient roughness to provide superhydrophobic properties at any scale and interface of 5SC. 3.3. Electrical, Thermal and Healing Properties of 5SC Easy use of materials and methods is critical for the suitability of this system in large-scale engineering applications. Coatings have been widely studied because they are the simplest assemblies used for the protection of existing constructions. In order to further simplify the application of the 5S system, we followed three steps. First, we prepared low-cost conductive line-copper conductive tapes (electrodes) easily attached to the surface. Secondly, the material was coated with a conductive coating (rGO coating). It can also be applied to any surface, which increases its range of applications, other than planes and facades. Finally, the surface coverage of the SST coating was finished. Rigid protective coatings mostly fail because of their very limited capability of

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ultimate strain. Therefore, super-hydrophobic films hardly prevent harmful media from entering the interior after surface cracking of substrate. Therefore, we studied and quantified the super-hydrophobic properties of SST coatings under various strains. As shown in Figures 8a and c, the CA and RA of the SST film were monitored separately while varying the tensile strain from 0 to 100%. RA increased as the RTV rubber area exposed to the surface increased and levelled off at 10° for an 80% tensile strain. The CA remained above 150° for a strain of 100%. In addition, we left a 5 mm notch at the edge of the SST film. While stretching the film to the twice of the original length, the length of the notch did not change significantly, and the SST coating therefore revealed a good crack resistance. The surface viscoelasticity tests carried out by the mechanical mode of atomic force microscopy (Figure 8 b) on hardened cement paste (CP), the SST coating, and the RTV coating revealed that the SST coating provided more stiffness than the RTV rubber, while the deformation was more stable under the action of micro stress. Compared to CP, the SST coating clearly exhibited higher dispersive force and good resilience to fracture energy. Thus, the SST coating can prevent cracking during the fracture of the cement matrix and prolong the protection of the substrate. The good resilience of the SST layer provided 5SC material with piezoresistive and self-healing propertie. By applying pressures of 0–3000 N to the 5SC treated concrete substrates (Figures 9a, b, and Figure S-11), the 5SC feedbacked the pressure-positive resistance signal with an excellent sensitivity and robustness. Since the internal rGOC could itself form a current path for the

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continuous phase, the cross-sectional area of the path decreased upon pressure, resulting in higher resistance. Once the pressure was relieved, the internal pressure generated by the SST forced the rGOC back to its original shape. As the pressure increased from 0 to 300 N, the stress and the resistance signal became non-linear, and this was related to the decreasing cross-sectional area of the conductive layer of graphene and the increasing contact probability between the adjacent graphene sheets.55 By correlating the recorded resistance change signal (Figure 9c and d) with the pressure with a model, two regular processes of variation can be obtained as follows:

{

―0.51114 ― 0.07245𝑥 ― 3.93 × 10 ―4𝑥2 + 7.1576 × 10 ―7𝑥3 𝑥 ≤ 1000 𝑦 = ―53402.2 + 53.34987𝑥 𝑥 > 1000(Formula 1)

where y is the predicted stress (N) and x is the absolute value (Ω) of the measured resistance change. The seepage zone ranged ca. from 20 to 300 N. Both regression models showed good correlation and can provide reference for 5SC self-monitoring. During the electrical test, the 5SC material was very sensitive to stress with a detectable threshold ranging from 1 to 30000 N. Even after the concrete matrix was cracked, the surface of the 5SC material did not change significantly and maintained a good robustness (see Supplemental Information). The self-healing ability has become one of the most important research topics in the field of corrosion protection in recent years. This ability is beneficial to prolong the service life of the coating, showing a great economic value and development potential. To evaluate this property, we cut rGOC in different areas using a knife to

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evaluate the healing on resistance value of 5SC material (see Figure S12). The speculative self-healing mechanism diagram is schematically described in Figure 10a. The resistance changes were continuously monitored and compared them with those of the rGOC without the SST layer and the conductive tape. In comparison, 5SC did not show this transition, and the resistance decreased more than 90% within 2 min and levelled off to 110% of the original one over time (Figure 10b). Before the conductive layer was cut, the three samples showed very stable electrical signals. After the damage, the resistance of rGOC without SST and the conductive tape suddenly increased to tens of thousands of ohms with a great dispersion over time (Figure 10c and d). The above results illustrated that SST was required for the 5SC material to preserve self-healing abilities. When the incision is generated, the pressure brought by the surface layer forces the rGO around the incision to fuse with each other, forming a continuous phase and completing the healing process eventually. However, no spillage of rGO from the surface of the SST coating occurred during testing, and this was ascribed to the viscoelastic properties exhibited by RTV (see Figure S12). The Infrared thermal imaging (ITI) results (Figure 10e) also revealed that the scratches did not affect significantly the heating power and robustness of the coating. As a result of the high local resistance, the scratched areas showed higher heating powers, which contributed to the softening and self-healing of rGOC. Two-dimensional electron fluids has been recently reported to provide graphene with healing effects via the quantum effect, allowing electrons to move rapidly in all directions at low

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temperatures.56 A slow increase in temperature causes the electrons to disperse, and the two electrons rebound from each other in a completely different way.57-58 The excellent conductivity of graphene provided rGOC with sensitive piezoresistive properties, while the more abundant functional groups on the rGO surface provided higher contact resistance and stable heating performance to this material. Melting and drainage of ice or snow layers is the most effective way to verify self-heating and superhydrophobic characteristics of the 5SC system. With this aim, we used four-point sampling to test the temperature change rates of the coated surface in various cases. The samples were first cooled at -20 ºC for 5 h and then subjected to a DC 50V. A noticeable temperature rise was observed on the 5SC surface (40 ºC) within 10 min, and this temperature increased to 60–80 ºC within 60 min (Figure 11a). After 1000 aging cycles, the material maintained excellent robustness with a heat efficiency loss of ca. 5% (Figure 11b). At temperatures below -20 ºC, a low voltage was sufficient for the 5SC to melt ice and snow and protect the substrate (Figure 11c). Figure 11d summarizes the melting ice and drainage process of 5SC: When the coating resistance signal exceeded the set threshold of the programmable logic controller (PLC), the self-heating was triggered. The surface rapidly risen above the freezing point with heating and melting of the ice cubes and the as-formed water droplets rolled off the super-hydrophobic surface. Finally, the whole circuit was automatically disconnected and entered the self-monitoring state. The entire melting process lasted only 20 min. The specific self-heating effects can be viewed in the

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Supporting Information (videos S3 and S4). Therefore, the 5SC can effectively withstand the freezing damage to the structure, and the early response to snow and ice in the roof, bridge deck, and other areas etc. can effectively prevent injuries and economic losses caused by the falling of a heavy layer. After an artificial light aging test with a total irradiation time of more than 3000 h, both the intact and abraded surfaces of the 5SC preserved their super-hydrophobicity without any aging sign (e.g., yellowing, bubbling, or embrittlement, Figure S10). According to the National Meteorological Information Center, based on an average annual radiation of 4000 MJ/m2 in China59, the SST coating was robust enough to maintain superhydrophobic properties outdoor for at least 1 year. 4. Conclusions Herein, we propose a multi-functional coating with piezoresistive, self-healing, and super-hydrophobic properties. This material represents an intelligent construction of protective closed-circuit systems for melting ice/snow-drain through an external PLC and solar power. Through qualitative and quantitative analyses and simulation studies, we revealed the mechanism of action of SMs and several important parameters of 5SC. Conventional cement-based materials are hardened through a liquid phase with massive Ca2+ participating in the formation of a C–S–H gel, which results in its essentially hydrophilic nature. Ca2+ serve as important ion bridge for the C–S–H gel skeleton and appears in the form of hydrophilic minerals such as Ca (OH)2, CaCO3, and

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ettringite.8,

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Once cement is modified by adding hydrophobic modifiers, the

complexation of a large amount of active hydroxyl groups with Ca2+ was delayed, hindering hydration of the cement, and causing delayed coagulation and performance degradation.61 As one of the superhydrophobic properties, creating more low surface energy areas did not contribute to the densification of the cement stone. As a result, the internal doping and impregnation methods cannot be used for preparing cement-based materials. Besides, the fresh section of the substrate always exposes polar groups, resulting in a decrease or complete loss of the hydrophobic properties. In the alkali etching system of SST, NaOH reached the liquid phase from the surface of the microspheres while hydrophobically modifying by PMHS and FAS to complete the catalytic process. Thus, only a minor fraction of Na+ remained on the surface of the SMs. Moreover, Na was not the main constituent of the gel skeleton. Regarding the preparation method, we designed an environmentally friendly synthesis of SMs through the utilization of source-rich industrial waste. The microspheres and FAS & PMHS are self-assembled into a core-shell structure in which polar molecules/structures are encapsulated by non-polar molecules/groups. The key challenge of the 5S system is the robustness, super-hydrophobic, and selfheating properties of the entire coating. These two properties can be maintained under various harsh environments to meet the anti-freezing, snow melting, and drainage characteristics of existing buildings. The SST coating showed highly stable superhydrophobic properties under the main failure modes of building materials

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including physical friction, stress-strain, photoaging, acid corrosion, freeze-thaw aging. This material provided stable protection for concrete substrates by utilization of a multiscale microsphere structure. The SST coating prepared herein can be used on any building surface. The coating dry time can be also adjusted (from 15 min to several hours). Therefore, the entire 5S system can be used in a wide range of applications, and its versatility and operability are also important novel points in addition to its various excellent functions. Since the SST coating divides the rGOC into smaller regions, its excellent viscoelasticity and crack resistance protects the rGOC while allowing sensitive changes and rapid recovery of its resistance values. With regards to the thermal and electrical properties of the material, 5SC showed excellent durability and robustness under extreme conditions with good monitoring sensitivity and broad monitoring thresholds from 1 to 30000 N (0.01 m3) of frost heave or ice weight. PDMS brings rGOC good chemical inertness, flame retardancy, and fluidity within -50–200 ºC, which directly results in possibility of self-healing properties. 5SC does not show a significant resistance transition after cutting damage, and 90% of the original resistance recovers within 2 min. At 50 V DC, the heating rate of 5SC was steadily maintained at 6 ºC /min for the first 10 min. The seepage zone ranged approximately from 20 to 300 N. The unique electrical conductivity of rGO provide rGO coatings with electrical

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conductivity and piezoresistive properties in the plastic coating stage. However, these properties can only be achieved at a concentration of 10 wt% of rGO. When larger areas of rGO coating are used, the interface between the substrate and the surface SST layer were detached easily. To prevent this issue, a strip coating tape solution was used. This solution provided the 5S system with higher pressure sensitivity and self-healing properties. Thus, the bonding area of the SST coating on the substrate increases substantially while providing a certain pressure to the rGO conductive layer to force its internal interconnection. Composite carbon nanotubes could be used in the future to reduce the amount of rGO in the 5S system and to improve various properties of these materials.

AUTHOR INFORMATION Corresponding Author *Yunsheng Zhang. E-mail: [email protected] **Yantao Jia. E-mail: [email protected]

NOTE The authors declare no conflict of interest.

SUPPORTING INFORMATION Table S-1 The rGO performance parameters. Table S-2 Mixture proportions of the concrete matrix. Table S-3 Surface roughness parameters of SST coating after various friction

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cycle. Figure S-1 Particle size distribution and morphology of nano-silica (a1 and a2), silica fume and fly ash. Figure S-2 Morphology of FA and SF before alkali etching and morphology of rGO. Figure S-3 The morphology of the SST superhydrophobic system composed of FA, SF and nano-silica after drying, wherein the alkali etching time is different. XPS spectra of alkali etched FA& SF& nano-silica after superhydrophobic treatment for various time. Figure S-4 Friction testing of different superhydrophobic treatment solutions. Contact angle and roll angle results before and after sample wear. SEM images of cross section and friction surface of SST coating. Figure S-5 AFM corresponding 2D image, 3D surface structure, and section roughness profile of the pristine surface of RTV and SST coating. Figure S-6 Three-dimensional reconstruction of the SST coating structure and distribution of different components. Figure S-7 Sketches depicting surfaces with negative and positive skew. The roughness trace is on the left, and the bearing area curve (Abbott-Firestone curve) is on the right. Figure S-8 Images of cement samples by immersion erosion treated with TEOS & PDMS, TEOS & PDMS & FAS, impregnation of FAS, coated EP, RTV rubber,

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SST coating and commercial superhydrophobic spray at 50 °C, using HCl (38%wt.) and H2SO4 (70%wt.), photograph of the protection of HCl (38%wt.) by SST coating of cement samples. Figure S-9 Concrete samples before and after the freeze-thaw cycle using RTV coating, SST coating, silicone coating, and high-strength epoxy coating. Figure S-10 Concrete samples before and after artificial photoaging experiments using various coating. Figure S-11 Resistance value signal versus forward compression (0 to 30000N). Figure S-12 Images of 5SC treated concrete sample before and after knife cutting. Images of 5SC treated concrete sample after the 30,000 N pressure test. Surface image of 5SC without SMs.

ACKNOWLEDGMENTs The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Nos. 51678143, 51808189, and 51878153), the 973 National Basic Research Program of China (No. 2015CB655102) and the China Scholarship Council. The authors would also like to thank Xiaoyun Song from the Qingdao Institute of Marine Geology (China Geological Survey) for assistance with the experiments, and Haihua Yu, Yunfei Xia, and Xiuhang Xu for providing experimental equipment and solutions.

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Figure 1 Morphology of FA (a) and SF (b) after alkali etching for 5 h; morphology of SF wrapped by NS after 5 h of alkali etching (not dried) (c); morphology of the SST super-hydrophobic system composed of FA, SF, and NS after drying (d).

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Figure 2 FAS and PMHS contents in FA and SF before and after the alkali treatment by TG analyze (a); XPS spectra of alkali etched FA& SF& NS after a superhydrophobic treatment of different durations at amplification region of Na1s and F1s (b); high-resolution XPS spectra of C1s of alkali etched FA& SF& NS after a superhydrophobic treatment for 5, 10, and 60 min (c)

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Figure 3 Synthesis mechanism and molecular dynamics model of SST- SMs (a1–a4 and b1–b4) and chemical reaction equations (R1–R4).

Figure 4 A brief workflow (a) and structure (b) of the 5SC system, where B1 is an external power source (for powering the 5SC) containing a solar charging device, B2 is a monitoring battery, and S1 and S2 are automatic switches.

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Figure 5 CA and RA of the SST sample after rubbing test at different distances using 800# sandpaper and 800 Pa pressure (a). Morphology of SST super-hydrophobic layer surface (without friction) (b1) and after fiction test (800m sandpaper, 800 Pa) for 2m (b2). 42


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Figure 6 Mass loss results of cement samples by immersion erosion treated with TEOS & PDMS, TEOS & PDMS & FAS, impregnation of FAS, coated EP, RTV rubber, SST coating, and commercial superhydrophobic spray at 50 °C, using HCl (38 wt.%) and H2SO4 (70 wt.%) (a). Mass loss results of the concrete samples after the freeze-thaw cycle in the artificially prepared seawater solution, coated with EP, RTV rubber, silicone rubber, SST coating, and PU(b). Before the friction test, the two samples performed good superhydrophobic properties; then used sandpaper (800#) abrasion, knife scratch, and hammer beat in different areas (dotted line area), hydrophobicity tested again; finally, abrase the whole surface (dotted line area) with sandpaper and then tested the hydrophobic properties (c).

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Figure 7 Roughness bearing ratio (Abbott–Firestone curves) function and material volume per unit area relevant to profile height (a). 3D perspective and water CA of SST coating surface after friction test for 1 (b1) and 4 (b2) cycles; 3D reconstructed SST coating forward section (c) (green: surface particles; red: internal particles; blue: RTV matrix) and particle edge vector enlargement (d) and 3d structure enlargement (e); particle size distribution of SMs (f).

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Figure 8 Super-hydrophobic properties (a) and images (c) of the SST layer in its state and in its stretched states. Surface viscoelasticity of cement matrix, SST coating, and RTV coating (b).

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Figure 9 Correspondence between the resistance signal and forward pressure in the range of 0–10 N (a), and 2.5–20 N (b); signal data (c) and linear regression models (d) of resistance value signals versus the forward pressure.

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Figure 10 Speculative self-healing mechanism diagram of the 5SC system (a); resistance signal versus time response to cutting failure of 5SC (b), rGOC without SST (c) and conductive tape (d), the red curve is the smoothed data; ITI changes of 5SC after cutting damage (e).

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Figure 11 5SC heating efficiency (4 points sampling) tested (DC, 50V, test ambient temperature 13 ºC, indoor) after 1 cycle (a) and 1000 cycles (b); the 5SC heating efficiency (4 points sampling) tested in coolant of -22 ºC (c) (DC, 50V); the process (d) of melting and draining ice cubes of unpowered (left) and energized (DC, 50V) 5SC concrete.

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TOC

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5S Multifunctional Intelligent Coating with ... - ACS Publications

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