Superhydrophobic Calcium Aluminate Cement with Super Mechanical

Article pubs.acs.org/IECR

Cite This: Ind. Eng. Chem. Res. 2019, 58, 10373−10382

Superhydrophobic Calcium Aluminate Cement with Super Mechanical Stability Fajun Wang,* Sheng Lei, Junfei Ou, Mingshan Xue,* Changquan Li, and Wen Li School of Materials Engineering, Jiangsu University of Technology, Changzhou 213001, PR China

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S Supporting Information *

ABSTRACT: In the present study, a simple and cost-effective method for the preparation of a superhydrophobic calcium aluminate cement (SCAC) with superstable mechanical stability is reported. Room-temperature vulcanized silicone rubber (RTV) is used as an internal hydrophobic additive in concrete. The as-prepared cement surface and all cross-sections exhibit superhydrophobicity. The mechanical stability of the superhydrophobic-cement sample was evaluated by multiple severe mechanical damages. The SCAC sample exhibits super mechanical stability against sandpaper abrasion under high pressure, long-time impinging with sand, repeated knife scratches, and electric-cutter cutting. Amazingly, the cement sample was ground into powder, which was also superhydrophobic. In addition, the SCAC sample shows improved corrosion resistance for rebar against Cl− in comparison with the superhydrophiliccement sample. Moreover, the application of superhydrophobic cement can be extended to oil−water separation using SCAC powder combined with an adhesive.

1. INTRODUCTION Cement-based composites are the most commonly used building materials and are widely used in public and individual architecture, road engineering, dams, bridges, and oil wells.1−3 Hardened cement stone is a complex composite material composed of various hydration products, aggregates, additives, and pores and is hydrophilic in nature.1,4−8 During the service life, these cement composites are inevitably affected by various kinds of water in the environment, such as the water in rivers, lakes, and oceans; rainwater; melting snow; and so on. Environmental water usually contains some dangerous ions corrosive to cement composites, such as Cl−, SO42−, and H+.2,9−11 For example, steel bars in reinforced concrete exposed to coastal marine environments and deicing salt are susceptible to the corrosion of chloride ions. Calcium hydroxide (CH) in Portland cement-concrete structures is susceptible to attack by sulfate, which in serious cases causes cracking.9 In winter, water absorbed in the pores of cement composites undergoes freeze−thaw cycles, which generates stress inside the cement and easily causes cracking of the cement structure.1,5 Therefore, the main damage of cementbased composites is basically related to water. If water adsorption can be prevented on the surface and voids of cement composites, the durability of cement-based composites will be greatly improved. Superhydrophobic materials have attracted extensive attention because of their wide application prospects in the fields of self-cleaning coatings, anticorrosion of metal materials, antiicing, drag reduction, oil−water separation reducing the adhesion of liquid food, and so forth.1,12−18 When a © 2019 American Chemical Society

superhydrophobic surface is in contact with water, most of the solid surface is separated from water by trapped air, which greatly reduces the adsorption of water on the solid surface. At present, artificial superhydrophobic surfaces are usually prepared by a coating method (i.e., spray-coating, dip-coating, etc.),15−21 chemical etching,22,23 freeze-drying,24 electrospinning,25 mechanical processing,26 a template method,27,28 or other such methods.29−31 Although superhydrophobic surfaces prepared by these methods have excellent water resistance, they cannot be used in practical applications because of some limitations, such as substrate dependency, the usage of expensive materials or equipment, safety and environmental issues, and so on. Particularly, most superhydrophobic surfaces are susceptible to mechanical contact, which has been identified as the main obstruction to the practical application of superhydrophobic surfaces.30,32 Therefore, it is urgent to find a solution to resolve the problem of mechanical damage of superhydrophobic materials. Recently, lots of mechanically stable superhydrophobic materials have been reported in the literature.32−35 However, they have limited potential in the field of cement-based composites. For example, Zhu et al. reported a mechanically durable superhydrophobic composite coating made from copper powder, silver nitrate, perfluorothiol, and ultrahighmolecular-weight polyethylene (UHMWP) as the raw Received: Revised: Accepted: Published: 10373

March 4, 2019 May 22, 2019 May 23, 2019 June 7, 2019 DOI: 10.1021/acs.iecr.9b01188 Ind. Eng. Chem. Res. 2019, 58, 10373−10382

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Industrial & Engineering Chemistry Research materials.32 However, the preparation process is complicated and substrate-dependent, and an expensive and toxic fluorinecontaining reagent is used. Song et al. reported a super-robust superhydrophobic concrete that could be scratched by a knife and abraded by sandpaper. However, an expensive and dangerous fluorine reagent (i.e., fluoroalkylsilane, FAS) was also used.1 In addition, the wettability of the fractured surface (newly exposed surface after severe mechanical damage) of the concrete was not investigated. Generally speaking, spraying is the most promising method for preparing a superhydrophobic surface because of the advantages of simple equipment, convenient preparation, wide applicability of the substrates, and large-area fabrication.21 However, most superhydrophobic coatings have the disadvantage of being easily peeled off and cracking because the adhesion force between the coating and substrate is usually weak as a result of the inevitable use of lowsurface-energy materials.1,35 In addition, the reported mechanical stability of superhydrophobic materials was tested either under weak mechanical-damage conditions or by a single mechanical-damage method. Moreover, the mechanical stability of these superhydrophobic materials degrades easily once the surface encounters severe mechanical damage or multiple mechanical damages. Therefore, it is still a great challenge to develop superstable superhydrophobic materials that can withstand severe and multiple mechanical damages. Silane has been used as a hydrophobic modifier for cementitious composites for many years.1 On the one hand, silane can be added during the preparation of fresh concrete, as an integral waterproof material.1,36 On the other hand, silane can also be used as a modifying agent on the surface of concrete by impregnating. The concrete becomes hydrophobic after the modification of silane.37 As a result, the adsorption and permeability of aggressive aqueous solutions containing corrosive ions in hydrophobic concrete are remarkably reduced, and the durability of the concrete is improved. However, silane is too expensive to be widely used in concrete. In addition, small-molecule silanes are volatile and can cause environmental pollution during use. In the present work, a superhydrophobic calcium aluminate cement (SCAC) with super mechanical stability was prepared via a simple modification method using liquid silicone rubber as an internal additive. The raw materials used are all commercially available materials or reagents that are cheap and fluorine-free. Compared with Portland cement, aluminate cement has the advantages of a fast hardening speed, winter construction, and strong sulfate resistance.38−40 RTV is added during the mixing process. Its addition does not affect the mixing, molding, or hydration process of cement. No additional equipment and no post-treatment processes are required. Not only the surface but also the fractured surface of the as-prepared cement sample exhibits superhydrophobicity. The superhydrophobicity of the SCAC sample exhibits super mechanical stability against severe and multiple mechanical damages, including hammer beating, vigorously abrading with sandpaper, electric-cutter cutting, repeated knife scratching, and long-duration sand impinging. In addition, the SCAC sample showed excellent self-cleaning and anticorrosion properties. Particularly, the powder ground from bulk SCAC is also superhydrophobicity. Therefore, the SCAC powder can be adhered to different substrates (metal or a sponge) by using the method of “powder + adhesive”, thus expanding the applications of the superhydrophobic cement. The super mechanical stability of a superhydrophobic calcium aluminate

cement has not been reported to the best of the authors’ knowledge.

2. EXPERIMENTAL SECTION 2.1. Materials. Room-temperature vulcanized silicone rubber (RTV, viscosity of 10 000 cP) was provided by Shanghai Nanzheng Silicon Material Company, Ltd. Tetraethoxysilane (TEOS, curing agent) and dibutyltin dilaurate (DD, catalyst) were purchased from Shanghai Zhenggong Silicon Material Company, Ltd. Calcium aluminate cement (CAC) was purchased from Zhengzhou Jiainite Aluminate Salt Company, Ltd. Standard sand (ISO0679) was purchased from Xiamen Aisiao Standard Sand Company, Ltd. Standard sand consists of coarse sand, medium sand, and fine sand (as per ISO0679). Quartz sand (80−120 mesh) was purchased from East China Yuanyang Quartz Sand Plant. Polypropylene fiber (PPF) with a length of 9 mm and diameter of about 40 μm was kindly provided by Langfang Dekai Insulation Material Company, Ltd. Graphite nanoplatelet carbon nanotube composite aqueous slurry (GNCNS, industrial grade, content: 5 wt %, 1:1 weight ratio) was purchased from Shanghai Aladdin Biochemical Technology Company, Ltd. A polyurethane sponge (PUS) and silicon carbide sandpaper (Eagle brand) of different grades were purchased from a local market. 2.2. Preparation. CAC powder (1000 g), water (300 g, water to cement ratio of 0.3), and the desired amount of filler were premixed in a beaker using a mechanical stirrer for 3 min. Subsequently, 10 g of RTV, 0.5 g of TEOS, and 0.05 g of DD were added to the beaker successively under constant stirring. The mixing process continued for a further 10 min to obtain an uniform mixture of cement. Then, the mixture was poured into rectangular molds (40 mm long, 30 mm wide, and 15 mm thick) made of silicone rubber and compacted using a joint table. The samples were removed from the molds after 24 h and cured at a temperature of 20 °C and a humidity of 90% for 28 days. Finally, each surface of the sample was abraded using a sandpaper before characterization. 2.3. Characterization. Contact angles (CAs) and rolling angles (SAs) were measured using an optical-contact-angle tester (DSA100, KRUSS). The surface microstructures of various samples were observed by a field-emission scanning electron microscope (FE-SEM, FEI-QUANTA F250). The surface chemical compositions of the samples were analyzed using a Thermo ESCALAB 250XI X-ray photoelectron spectrometer and a NICOLET 5700 Fourier-transforminfrared-spectroscopy (FT-IR) instrument (Nicolet). The crystal structures of the samples were measured with a D8 ADVANCE X-ray diffractometer (Bruker). 2.4. Mechanical Stability. The mechanical stability of the superhydrophobic samples was evaluated comprehensively according to the methods reported in the literature,33−35 including a sandpaper-abrasion test, a knife-scratch test, a sandimpacting test, and an electric-cutter test. The sandpaperabrasion test and sand-impacting test are illustrated in Figure S1 (Supporting Information). 2.5. Corrosion Resistance. The polarization curve was measured in 3.5 wt % NaCl aqueous solution using an electrochemical workstation (GAMRY, Reference 3000). A platinum plate and a saturated calomel electrode were used as contour and reference electrodes, respectively. A 45# steel sheet embedded in the cement sample was used as the working electrode. 10374

DOI: 10.1021/acs.iecr.9b01188 Ind. Eng. Chem. Res. 2019, 58, 10373−10382

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Industrial & Engineering Chemistry Research

Figure 1. Photos of water droplets on the surfaces of different samples: (a) unmodified CAC, (b−e) modified CAC surfaces, (f) SCAC sample before hammer beating, (g) SCAC sample after hammer beating, and (h) fractured surface of the SCAC sample after hammer beating.

Figure 2. SEM images of various samples at different magnifications. (a1,a2) Surface of CAC sample abraded with 600# sandpaper. (b1,b2) Surface of SCAC sample abraded with 800# sandpaper. (c1,c2) Surface of SCAC sample abraded with 600# sandpaper. (d1,d2) Surface of SCAC sample abraded with 320# sandpaper. (e1,e2) Surface of SCAC sample abraded with 220# sandpaper. (f1,f2) Fractured surface of SCAC sample.

2.6. Oil−Water Separation. The superhydrophobic CAC bulk samples were ground into fine powders (passed through a 400 mesh stainless-steel sieve). RTV (5 g), TEOS (0.25 g), and DD (0.03 g) were dissolved in 100 mL of hexane at room temperature to form a transparent silicone rubber solution. Then, 5 g of superhydrophobic CAC powder was added to the solution, which was ball-milled for 10 min to form a uniform dispersion. The dispersion was sprayed on a piece of PUS three times. Finally, a superhydrophobic and superoleophilic PUS sample was obtained after being cured at ambient conditions overnight.

products of cement. However, the sample surface exhibits superhydrophobicity after the addition of only 1 wt % RTV (see Figure 1b−d and Movie S1). The CA is 154.2°, and the SA is 7.8°. In addition, not only the outside surface but also the newly exposed surface is superhydrophobic (see Figure 1f−h and Movie S1). In other words, the modified CAC is a body superhydrophobic material. The water droplet bounces on both the surface and the fractured section (see Movie S2). An 8 μL water droplet suspended on the tip of a needle cannot be adsorbed by the surface of the superhydrophobic cement easily. A bulk SCAC sample was ground into powder using a planetary ball mill. To our surprise, the powders also exhibit excellent hydrophobicity (see Figure S2a,c). This a very important performance in practical applications, because each part of the cement is superhydrophobic and can resist nearly any mechanical damage.

3. RESULTS AND DISCUSSION 3.1. Surface-Wettability Analysis. Figure 1a shows the surface wettability of the CAC sample (see also Movie S1). One can see that the CAC sample without modification is hydrophilic because of the hydrophilic nature of hydration 10375


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Figure 3. XPS spectra of unmodified CAC surface, modified CAC surface, and fractured surface of modified CAC. (a) Survey spectrum, (b) C 1s spectrum, (c) O 1s spectrum, and (d) Si 2p spectrum.

Figure 4. (a) FT-IR spectra of modified and unmodified CAC samples. (b) XRD analysis of modified and unmodified CAC samples.

3.2. Surface-Microstructure Analysis. The surface microstructures of various samples were measured and are depicted in Figure 2. A lot of scratches (see Figure 2a1) and protrusions (see Figure 2a2) can be observed on the abraded surface of the CAC sample (abraded with 600# sandpaper). The unmodified CAC-sample surface exhibits a water CA of 0°. As a comparison, the surface of the modified CAC sample shows very similar microstructures (abraded with 600# sandpaper, see Figure 2c1,c2). It exhibits a surface CA of 155.3° and an SA of 7.5° for water (see insets in Figure 2c1,c2, respectively), which demonstrates its superhydrophobicity. In addition, the finer the sandpaper, the smaller the surface microprotrusions of the polished sample (see Figure 2b1−e2). Moreover, the surface superhydrophobicities of the SCAS samples are nearly independent of sandpaper grade. All of the surfaces have CAs larger than 150° and SAs lower than 10° when the grade of sandpaper decreases from 800# to 220#,

which indicates that the superhydrophobic CAC material is easy to fabricate. 3.3. Chemical-Component Analysis. The XPS survey spectra of different sample surfaces were depicted in Figure 3a. It reveals that oxygen, carbon, calcium, silicon, and aluminum are the main elements on all of the three samples’ surfaces. The high-resolution C 1s spectra of all the above samples exhibit a main peak centered at 284.8 eV. The C 1s peak of the CACsample surface can be attributed to carbon adsorption. In addition, the carbonation of CAC cement can also occur according the following reaction:41,42 CaO·Al 2O3 ·10H 2O + CO2 → CaCO3 + 2Al(OH)3 + 7H 2O

(1)

3CaO·Al 2O3 ·6H 2O + 3CO2 → 3CaCO3 + 2Al(OH)3 + 3H 2O 10376

(2)


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Industrial & Engineering Chemistry Research Both the outside surface and the fractured surface of the SCAC sample exhibit stronger C 1s peaks in comparison with the unmodified one. The CAC sample is composed of multiple hydration products, such as CAH10 (CaO·Al2O3·10H2O), C2AH8 (2CaO·Al2O3·8H2O), AH (Al(OH)3), and a small amount of silicate.43,44 The surface is rough and contains lots of hydroxy groups, which leads to the superhydrophilicity of the sample (see Figure 1a and Movie S1). The contents of both Si and C on the SCAC-sample surface increase, apparently because of the modification of RTV. The outer layer of the sample was polished with sandpaper, which resulted in the loss of the RTV-modified layer on the surface. As a result, the amount of silane bound on the outer surface is less than that on the fractured surface. Moreover, the oxygen content of the CAC surface is significantly higher than that of either the outer surface or the fractured surface of the SCAC sample, as depicted in Figure 3c. This can be attributed to the fact that the oxygen content of RTV is less than that of cement, so the oxygen content on the sample surface decreases after modification. The unmodified CAC surface exhibits a distinct peak situated at 289.4 eV, which is assigned to the Ca−CO3 group (Figure 3b). The peak intensity corresponding to the Ca−CO3 group decreases apparently after silane modification, which indicates that the interaction between silane and cement and the carbonation of cement are competitive processes (also see Figure S2a,b). The FT-IR spectra of modified and unmodified CAC-sample surfaces were measured using the attenuated-total-reflectance method (see Figure 4a). The two samples, showing similar absorption spectra, indicate these samples have the same hydration products. In other words, the incorporation of RTV has little effect on hydration of cement. The broad peak located at 3469.6 cm−1 and a shoulder peak situated at 3530.1 cm−1 are assigned to O−H groups of various hydration products of cement.45 The peaks observed at 2919.1 and 2851.2 cm−1 are assigned to the stretching vibration of C−H in RTV.24 The adsorption band centered at 1481.3 cm−1 is assigned to the characteristic absorption peak of CaCO3.46 The new peak located at 1101.2 cm−1 is attributed to the Si−O vibration of RTV chemically bound on the surface of CAC.47,48 The crystal structures of CAC and SCAC samples were analyzed using the XRD technique, and the corresponding patterns are depicted in Figure 4b. In the XRD patterns, the compound CA (CaO·Al2O3, PDF 23-1036) and CA2 (CaO· 2Al2O3, PDF 76-0706) were identified.39,40 In addition, one can see that the two samples show nearly the same XRD patterns, indicating that the addition of silane has little effect on the hydration products of calcium aluminate cement. The chemical reactions between the cement-hydration product and RTV are illustrated in Figure S3. 3.4. Mechanical Stability. The samples were subjected to various severe mechanical damage, and then the CAs of the damaged sample surfaces were measured to evaluate the mechanical stability of the superhydrophobic samples. Sandpaper abrading is the most commonly used method to detect the mechanical stabilities of superhydrophobic materials. Low applied pressures or weights (10 kPa, 600 Pa, and 1100 Pa; 53 g), short abrasion distances or few abrasion cycles (3, 6, and 2 m), and fixed sandpaper grades (1500 mesh, 1500# and 360#, and 800 mesh) were used in these methods.1,25,30,32−35,49,51 In the present case, the applied pressure was as high as 44.5 kPa (using 5 kg weight), and the abrasion distance was as long as 50 m (corresponding to 50 cycles of abrasion, see Figures 5a

Figure 5. Mechanical-stability tests. (a1−a3) Sandpaper-abrasion test. (b1−b2) Electric-cutter test. (c1−c7) Sand-impact test. (d1−d3) Knife-scratching test.

and 6a). In addition, sandpaper grade varied from 220# to 800# (Figure 6a). Electric-cutting machines (ECMs, Figure 5b1) can cause serious mechanical damage to many materials, such as wood, stone, metal, ceramic, and so on. Figure 5b2 shows an SCAC sample that was cut by the ECM with a weight loss of 14.8%. However, the freshly exposed surface was still superhydrophobic. A quantitative study shows that the SCAC sample maintains its superhydrophobicity even when half of its weight is cut by the ECM (Figure 6a), demonstrating the super mechanical stability of its superhydrophobicity. Sand impinging is another commonly used method for checking the mechanical stabilities of superhydrophobic surfaces. Li et al. reported a spray-coated superhydrophobic coating that could retain its superhydrophobicity after 1 h of sand impinging from a height of 30 cm.50 Xu et al. prepared a transparent and superhydrophobic silica coating. The coating exhibited a CA larger than 150° after being impacted by sand from a height of 20 cm for 50 s.34 In our sand-impinging test, 500 g of standard sand (containing coarse sand, medium sand, and fine sand) flowed to the tilted sample surface (see Figure 5c1 and Movie S4) from a height of 100 cm. It should be noted that one impinging cycle corresponds to an impinging time of about 12 s. After 10 cycles of impinging (corresponding to about 120 s), the superhydrophobicity of the sample surface changes little (see Figures 5c1−c7 and 6c). Finally, the surface of the superhydrophobic sample was repeatedly scratched along the longitudinal and transverse directions with an art knife (see Figure 5d1−d3 and Movie S5). To our surprise, although the surface of the sample was seriously damaged by the knife, and significant scratches and even powder appeared, the surface was still superhydrophobic. Quantitative testing indicated that the sample surface could withstand more than 100 cycles of 10377


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Figure 6. CA as a function of (a) sandpaper grade and abrasion cycle, (b) weight loss caused by electric cutting, (c) impinging cycle, and (d) scratching cycle.

Figure 7. (a) Polarization curves of different samples. (b) Chemical stability of SCAC samples against various corrosive aqueous solutions. (c) Corrosion model of an SCAC sample in aqueous solution containing chloride ions.

3.5. Corrosion Resistance. Reinforced concrete is susceptible to corrosion in corrosive environments. For example, chloride ions in seawater can penetrate the pores in cement concrete, reaching the surface of the steel and corroding the steel. Therefore, improving the corrosion resistance of reinforced concrete to chloride ions has important practical value. The potentiodynamic-polarization curves of bare steel, the steel−CAC sample, and the steel−SCAC sample are shown in Figure 7a (also see Movie S6). It can be clearly observed that the polarization curve of bare steel is located at the upper left of the graph, whereas the curve of the steel− SCAC sample moved to the lower right of the graph, which indicates that the steel−SCAC sample possessed the lowest corrosion-current density (Icorr) and the highest corrosion potential (Ecorr) among the three samples. It can be clearly seen from the graph that the polarization curve of the blank stainless-steel sample is located at the upper left of the graph, whereas the polarization curve of the stainless steel−superhydrophobic-cement sample is located at the lower right of the graph. The values of Icorr and Ecorr obtained from the fitting results of the polarization cures are summarized in Table 1. The corrosion potential of the stainless steel−superhydrophobic-cement sample is the highest (Ecorr= −0.48 V), and its

knife scratching without losing its superhydrophobicity (Figure 6d). To the best of our knowledge, materials that can withstand the multiple severe mechanical damages described above and still maintain their superhydrophobicity have not been reported in the literature. The SCAC sample possesses peculiar mechanical stability, which can be attributed to the following two factors. First, the hydrated cement stone is composed of a large amount of hydration products (crystals), gel (amorphous), unhydrated cement powder, and voids, and its surface is a nonsmooth surface with microscopic roughness.39,40 The hardened cement paste is composed of a large number of hydration products interlaced with each other. When the cement stone is destroyed by external forces, such as breakage or wear, its newly exposed surface also tends to form microscopic roughness (see the SEM images of a fractured surface in Figure 2f1,f2). Second, liquid RTV is added in the mixing process of cement and water. Hence, there are four kinds of chemical reactions in the process of cement mixing and hydration. That is, the hydration of cement, the hydrolysis of TEOS, the combination of RTV with cement-hydration products, and the condensation between RTV and curing agent.6,14 RTV binds to the surfaces of cement-hydration products and makes them hydrophobic. The newly exposed surface of cement due to mechanical damage contains both microscopic roughness and low-surface-energy material of RTV, so it is easy to have superhydrophobicity. Grinding the superhydrophobic sample (SCAC sample) into powder is the most extreme mechanical damage to the sample, but the obtained powder also has superhydrophobicity (see Figure S1a,c).

Table 1. Corrosion-Current Densities (Icorr) and Corrosion Potentials (Ecorr) of Different Samples

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sample

Icorr (A·cm−2)

Ecorr (V)

steel steel−CAC steel−SCAC

2.65 × 10−5 2.32 × 10−5 3.10 × 10−7

−0.97 −0.56 −0.48


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Figure 8. FEM images of different SCAC samples at different magnifications. (a,b) CAC−SS, (c,d) CAC−QS, (e,f) CAC−GC, and (g,h) CAC− PPF. The insets show the CA- and SA-measurement results of various sample surfaces.

Figure 9. Self-cleaning behavior of the SCAC surface.

corrosion-current density is the lowest (Icorr = 3.10 × 10−7A· cm−2). Generally speaking, the lower the corrosion-current density is, the lower the corrosion rate of the corresponding material.22,23 Therefore, the corrosion resistance of the steel− SCAC sample is significantly improved in comparison with that of the steel−CAC sample. The inset in Figure 7a shows that an SCAC sample immersed in water (3.5 wt % NaCl aqueous solution) exhibits a mirrorlike surface because lots of air bubbles encompass the surface of the SCAC sample. Figure 7b shows the relationship between the surface CA and SA of the SCAC sample, and the pH value of the corrosive aqueous solution. The SCAC-sample surface exhibits superhydrophobicity for corrosive aqueous solutions covering a wide range of pH values (from 1 to 14), which demonstrates its wide environment adaptability. One can see that spherical solution droplets of acid (pH = 1.0, HCl), salt (3.5 wt % NaCl, pH = 7.0), and alkali (pH = 14.0, NaOH) sit on the surface of SCAC sample (see the inset in Figure 7b). The improved corrosion resistance of the SCAC sample against Cl− can be interpreted using the corrosion model depicted in Figure 7c according to the equation:52−54

surface is separated by air. Therefore, corrosive chloride ions in water can only erode superhydrophobic CAC in limited areas, which greatly improves the corrosion resistance of superhydrophobic CAC compared with that of hydrophilic CAC (see Figure 7c). In fact, the RTV-modified CAC sample is very stable to various organic solvents, such as acetone, chloroform, and DMF. After soaking for 1 h in the above solvents and drying naturally, the surfaces of the RTV-modified CAC samples still exhibited superhydrophobicity. 3.6. Expanding Applications. In practical application, cement usually needs various additives, such as sand of various particle sizes, fly ash, organic fibers (stainless-steel fiber, PP fiber, or PVA fiber), water reducers, and so on. Additives can improve the fluidity of cement mortar, reduce the cost of cement products, and improve the performance of cement concrete. In this work, four typical cement additives were used: standard sand (1000 g), quartz sand (1000 g), graphite nanoplatelet carbon nanotube composite aqueous slurry (0.5 g of solid content), and PP fiber (5 g). Cement composites containing different additives were also modified with RTV in the same way during mixing. Figure 8a,b shows the surface microstructures of the modified cement composite containing SS. One can see that the surface contains lots of microsized scratches, protrusions, pores, and fissures. Similar microtopographies can be also observed on the surfaces of the other cement composites (see Figure 8c−h). In addition, all cement-composite surfaces are superhydrophobic, with high CAs (>150°) and low SAs (10°). This result indicates that the RTV-modification method in this work is universal, endowing superhydrophobicity to the surfaces of the cement composites (including newly exposed surfaces) without affecting the use of other cement additives. The surface of the SCAC sample has self-cleaning properties. As shown in Figure 9, a few drops of water on the surface

cos θr = f1 cos θ − f2

where θr and θ are the apparent CA of a rough surface and the intrinsic CA of a smooth surface, respectively; f1 and f 2 represent the area fractions of solid surface and air in contact with air, respectively, and f1 + f 2 = 1. In the present case, θ is about 98° (using the value of CA for a smooth glass surface modified by MTES and OTES, weight ratio of 1), and θr is about 154° (see Figure 7b). The values of f1 and f 2 were calculated to be about 0.1176 and 0.8824, respectively. When the superhydrophobic CAC is immersed in water, only 11.76% of the surface contacts the water, whereas 88.24% of the 10379


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Figure 10. (a1,a2) Oil and water droplets on the surface and side face of a pristine PU sponge. Hexadecane was used as an example of oil and dyed red. A droplet of a 3.5 wt % NaCl aqueous solution was dyed green for clear observation. The oil CA is 0°, and the water CA is 110.6°. (a3) Oil and water droplets on the surfaces of an SCAC-particle-modified PU sponge (SPUS). The oil CA is 0°, and the water CA is 161.8°. (b1−b6) Process of oil−water separation using the SPUS. (c1) Absorption capacity of the SPUS. (c2) Separation efficiency of the SPUS.

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remove the dust attached to the surface in a few seconds, keeping the surface clean. The superhydrophobicity of cement can be extended to the surfaces of other materials by using powder and adhesives, thus expanding the application field of superhydrophobic cement. Before use, the SCAC sample was ground into powder (see Figure S1a,c). A superhydrophobic surface can be fabricated on a flat substrate quickly by using a double-sided adhesive (see Figure S1b,d and Movie S7). For complex substrates, we can use liquid adhesives, such as RTV. Figure 10a1−a3 shows that PUS exhibits superoleophilicity and superhydrophobicity after modification with SCAC particles (using a solution of RTV as the adhesive). The obtained SPUS can separate oil from the water surface with high absorption capacity (>23 g/g), high separation efficiency (>90%), good reusability (20 cycles), and wide suitability (for different types of oils).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01188. Schematic diagram of sandpaper-abrasion test and sandimpacting test, photos of water droplets on the surfaces of SCAC powders and superhydrophobic coating, and chemical reactions between cement-hydration product and RTV (PDF) Surface wettability of samples with water (AVI) Water droplet bouncing on the superhydrophobic surface of SCAC (AVI) Nonstick surface (AVI) Sand-impact test(AVI) Knife-scratching test (AVI) Mirrorlike surface formed when the SCAC sample was immersed in water (AVI) Water droplets rolling off the superhydrophobic surface fabricated using double-sided adhesive and SCAC powder (AVI) Bouncing behavior of a droplet on the surface of SCAC after the abrasion test (AVI)

4. CONCLUSIONS In summary, superhydrophobic calcium aluminate cement (SCAC) was prepared according to the traditional cementproduct-preparation process without any postmodifications. RTV was added to the mixtures of cement and water before molding. The surface, any fractured surface, and even the powder of SCAC are superhydrophobic, with water-contact angles larger than 150° and rolling angles lower than 10°. The SCAC retained its superhydrophobicity after severe and multiple mechanical damages, such as sandpaper abrasion (44.5 kPa, 100 cm, and 10 cycles), sand impacting (500g, height of 100 cm, and 10 cycles), electric cutting (repeated), and knife scratches (>100 cycles). Furthermore, the SCAC showed improved corrosion resistance for rebar compared with that of the unmodified CAC. Additionally, the superhydrophobic SCAC powder can be adhered to the surfaces of various substrates using the method of “powder + adhesives”, thus expanding the application field of SCAC. Therefore, the superhydrophobic calcium aluminate cement has potential applications in building waterproof materials, building coastal cement-concrete structures, and other fields.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F.W.). *E-mail: [email protected] (M.X.). ORCID

Fajun Wang: 0000-0002-9437-8148 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge with pleasure the financial support of this work by the Natural Science Foundation of China (Grant Nos. 51662032 and 11864024). 10380


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Superhydrophobic Calcium Aluminate Cement with Super Mechanical

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