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Effect of PDMS on the waterproofing performance and corrosion resistance of cement mortar
Applied Surface Science 507 (2020) 145016
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Full Length Article
Effect of PDMS on the waterproofing performance and corrosion resistance of cement mortar ⁎
T
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Fajun Wang , Sheng Lei, Junfei Ou , Wen Li School of Materials Engineering, Jiangsu University of Technology, Changzhou 213001, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cement Superhydrophobicity PDMS Corrosion resistance Durability Compressive strength
The currently used cement hydrophobic additives are either expensive or damageable during use, limiting their large-scale application on concrete. In this study, low-cost and commercially available polydimethylsiloxane (PDMS) is proposed as a cement admixture for the hydrophobic modification of cement mortar. The uniform distribution of PDMS in hardened cement mortar (HCM) attributes integral hydrophobicity to HCM. The PDMS modified HCM (M-HCM) surface exhibits superhydrophobicity, with a contact angle of 157.3° and a sliding angle of 8.7°. The superhydrophobic M-HCM is mechanically robust after being abraded for 110 m distance. In addition, the water absorption of the M-HCM reduced approximately 92.51%, and the corrosion resistance of the M-HCM obviously improved. The compressive strength and flexural strength of the M-HCM reduced by 30.9% and 18.1%, respectively, while the toughness of the M-HCM improved. Furthermore, the durability of the MHCM against weathering, sulfate attack, and leaching significantly improved in comparison with the U-HCM. This type of mechanically robust superhydrophobic M-HCM material can be used in the waterproof concrete and anti-corrosive concrete.
1. Introduction
spraying, brushing, impregnation, etc. For examples, Husni et al. prepared an ethanol dispersion containing fluoroalkyl silane modified rice husk ash particles. Concrete surface was first coated with a commercial adhesive, followed by spraying the dispersion on the surface of the adhesive. The as-prepared coating exhibited a contact angle (CA) for water of 152.3°. The water absorption decreased to 44%. Ismael et al. prepared a superhydrophobic coating on concrete surface using an aqueous emulsion containing siloxane oil, silica fume (or metakaolin), and PVA surfactant. The maximum CA obtained on concrete surface is 156°. Similar superhydrophobic coatings on the concrete surface such as carbon-based coating, polytetrafluoroethylene/polyether ether ketone (PTFE/PEEK) coating, siloxane oil modified nano silica coating, have also been reported. Silane and/or siloxane impregnation on concrete surface is another effective method to reduce the permeability of concrete. For examples, Xue et al. used silane (octyl triethoxysilane, liquid or aqueous emulsion) as an impregnating agent to modify the surface of concrete. The penetration depth of silane aqueous emulsion in concrete is 0.43 mm. The capillary water absorption of the modified cement mortar was only 5.4% of that of the unmodified cement mortar. In addition, the templating method was also used to prepare concrete with improved surface hydrophobicity [21,22]. However, the hydrophobic layer on concrete surface exhibited some problems, such as concrete cracking, coating peeling, coating aging, and weak mechanical
Portland cement is the most widely used building material in the world [1]. Because of its porous structure and hydrophilicity, water in the environment easily enters the pores of the concrete [2–4]. When water contains chloride ion, magnesium ion or sulfate ion, these ions will enter into concrete with water and cause erosion to concrete [2–4]. A large number of studies have shown that water penetration is the main cause of insufficient durability of concrete. Therefore, reducing water permeability can effectively prevent chloride ions, magnesium ions or sulfate ions from entering the concrete, thus improving the durability of concrete. Hydrophobic treatment is a common method to improve the waterproof and impermeability characteristics of concrete and has attracted extensive attention of researchers in recent years [5–10]. At present, there are two main methods for concrete hydrophobic treatment: (a) surface treatment method and (b) hydrophobic admixture method [1,2,5–10]. Surface treatment method is used to process the finished concrete surface, making the surface of concrete hydrophobic. Surface hydrophobic modification methods mainly include hydrophobic/super hydrophobic surface coating method [8–15], silane/siloxane impregnation method, [16–20] and template method [21,22]. The hydrophobic protective layer can be formed on the concrete surface by means of ⁎
Corresponding author. E-mail addresses: [email protected] (F. Wang), [email protected] (J. Ou).
https://doi.org/10.1016/j.apsusc.2019.145016 Received 11 May 2019; Received in revised form 1 December 2019; Accepted 7 December 2019 Available online 13 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 507 (2020) 145016
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robustness. The hydrophobic admixture method uses the hydrophobic modifier as an admixture for cement. The hydrophobic modifier is added during the mixing process of the fresh concrete [1,5–7,23]. It should be pointed out that silane and/or siloxane can be used as both surface hydrophobic modifier and integral modifier of concrete [1,5,6]. Silane is liquid at room temperature and can be used directly or processed into an environmentally friendly water-based emulsion [1,5]. In addition to silane, hydrophobic powders can also endow concrete with bulk hydrophobicity. For example, Li et al. produced superhydrophobic ground granulated blast furnace slag in a ball mill using stearic acid as a surface modifier. The capillary water absorption of concrete incorporated with superhydrophobic slag decreased by more than 90% [24]. Wong et al. used hydrophobic paper sludge ash (HPSA) as an admixture to modify concrete (replacing cement with 12% HPSA). The water absorption and water sorptivity of the modified concrete reduced by 84% and 86%, respectively [25]. The newly exposed surface of integral hydrophobic concrete is still hydrophobic when surface wear and/or cracking occur during service. Therefore, the long-term waterproofness of the integral hydrophobic concrete would be much higher than that of the surface hydrophobic concrete. However, these concrete integral hydrophobic additives are still insufficient in practical applications. The main shortcoming of silane is the high cost, limiting the extensive use of silane in concrete. Superhydrophobic powders typically float on the surface of the water and accumulate together. As a result, it is difficult to disperse them into cement mortar because of the existing of water, leading to uneven hydrophobicity of the concrete. In this study, polydimethylsiloxane (PDMS) was selected as a cement hydrophobic modifier to endow the concrete with integral hydrophobicity. Compared to small molecule silanes, PDMS has many advantages, such as cost effectiveness (its price is only 10% of octyl triethoxysilane), non-toxic, chemically inert, no volatile, and nonflammable characteristics. In addition, the appearance of concrete modified by PDMS would remain unchanged because of the transparency of PDMS. A large number of superhydrophobic coatings based on PDMS have been reported [26–28]. However, to the best of our knowledge, the use of PDMS as a concrete hydrophobic additive has not been reported so far. The main purpose of this study was to investigate the effect of PDMS on the processing performance of fresh cement mortar and the waterproof properties, corrosion resistance, as well as mechanical strength of hardened cement mortar.
further blended at a high speed for 3 min to ensure the homogeneous dispersion of PDMS in the cement mortar. The cement mortar was cast into molds of size 40 × 40 × 160 cm3 and 4.0 × 2 × 1.5 cm3. Finally, the molds were placed on a jolting table and compacted for 2 min. The samples were taken out from the molds after 24 h and placed in a curing chamber (temperature of 25 °C and relative humidity of 90%) for different periods (7, 14, and 28 days). The dosage of PDMS in the hardened cement mortar (HCM) is 0, 0.2%, 0.4, 0.6%, 0.8%, 1.0%, and 1.2% (by cement weight). The unmodified and PDMS modified HCM samples were denoted as U-HCM and M-HCM.
2. Materials and method
2.3.6. ATR-FTIR analysis The structural information of the surface organic components of the samples was analyzed using an attenuated total reflection Fourier transformed infrared spectrometer (ATR-FTIR, NICOLET 5700, USA).
2.3. Sample characterization 2.3.1. Standard consistency water consumption The standard consistency water consumptions of the cement pastes modified with different dosages of PDMS were measured using a Vicat apparatus according to the ISO9597-1989. 2.3.2. Fluidity The fluidity of the cement mortars modified with different dosages of PDMS was determined using a cement mortar fluidity tester (NLD-3) according to Chinese Standard JGJ 70–90. 2.3.3. Setting time The initial and final setting times of different contents of PDMS modified cement mortar was measured using a Vicat apparatus according to ASTM C191-08. 2.3.4. Surface wettability analysis After curing for 28 days, the mortar sample of size 4.0 × 2 × 1.5 cm3 was used for the measurement CA and sliding angle (SA) for water. The sample was polished with 280# sandpaper to remove impurities and construct a desired roughness on the surface of mortar. The CA and SA of different sample surfaces were measured using an optical contact angle tester (Krüss DSA 100, Germany). 2.3.5. SEM analysis The microstructures of the polished surface and fractured surface of mortar samples (size 4.0 × 2 × 1.5 cm3) cured for 28 days were observed by Field Emission Scanning Electron Microscopy (FESEM, FEI, QUANTA F250, USA).
2.1. Materials Portland cement (P·O 42.5) was provided by Anhui Conch Cement Co., Ltd. (China). Standard sand was purchased from Xiamen Aisiou standard sand co. LTD (China). Polypropylene fiber (PPF) with a length of 9 mm and a diameter of approximately 40 μm was provided by Jiangsu thermal insulation materials co. LTD (China). Hydroxyl terminated polydimethylsiloxane (PDMS, viscosity, 2000 cP) was purchased from Hubei Guishan new materials co. LTD. Tetraethoxysilane (TEOS) and dibutyl dilaurate (DBDL) were purchased from Shanghai Aladdin biochemical technology co., LTD and used as the curing agent and accelerator, respectively, to cure PDMS at room temperature The chemical structures of the raw materials and the cross linking reactions of PDMS are illustrated in Fig. S1 [see the Supporting Material (SM)].
2.3.7. Water absorption measurement Water absorptions of the U-HCM and M-HCM were measured according to the Chinese standard JC 474-2008 (water repellent admixture for mortar and concrete). 2.3.8. Pore structure measurement The pore structure of U-HCM and M-HCM was measured using a mercury intrusion porosimeter (POREMASTER 60, Quantachrome, USA). The hydration reaction was terminated by soaking the test sample in isopropanol after 28 days. 2.3.9. Electrochemical measurement The polarization curve of the samples was measured using an electrochemical workstation (CHI600E, Huachen, China). 45# carbon steel plate were embedded in cement mortars for the measurement of polarization curve. After 28 days of curing, the samples were immersed in 3.5 wt% NaCl aqueous solution for 1 day followed by naturally drying for 2 days. For the electrochemical measurement, a platinum electrode, a saturated calomel electrode (SCE), and a cement mortar
2.2. Preparation of samples 450 g cement, 1350 g standard sand, 2.25 g PPF, and 225 g tap water (the water cement ratio of 0.5) were blended in a cement mortar mixer (JJ-5, China) for 3 min. PDMS, TEOS, and DBDL with a weight ratio of 100:5:1 were mixed manually in a plastic cup. Afterwards, the liquid mixture of PDMS was poured into the mixer. The mixture was 2
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Fig. 1. (a) Effects of the PDMS contents on (a) the SCWC of cement paste, (b) the fluidity of cement mortar, (c) the initial and final setting times.
ammonium nitrate solution was used as the soaking solution to accelerate the dissolution of calcium. Deionized water was also used as the soaking solution to compare with the soaking result of the ammonium nitrate solution. The cubic samples (40 × 40 × 40 mm3) were used for leaching test. Prior to testing, the opposite sides of the cube were sealed with epoxy material to allow the leaching process to occur on the side of the sample. The samples were divided into four batches, including two batches of M-HCM samples and two batches of U-HCM samples, each containing 36 identical samples. The above two batches of M-HCM samples were soaked with ammonium nitrate solution (14 L) and deionized water (14 L). The two batches of UHCM samples were also soaked in the above solutions (in the other two tanks) for comparison. Three samples were taken from each tank for compressive strength test every 30 days, and the average value was taken as the test result.
coated with carbon steel plate were used as the counter electrode, reference electrode, and working electrode, respectively. The corrosion potential (Ecorr) and corrosion current density (Icorr) were obtained by the tafel curve fitting. 2.3.10. Mechanical strength measurement Compressive strength and flexural strength of the samples after different aging periods were measured according to the standard ISO 679:1989 (Test method for cement - determination of strength). 2.3.11. Durability test 2.3.11.1. Durability test of the superhydrophobcity against efflorescence. Five super hydrophobic samples (size 4.0 × 2 × 1.5 cm3, cured for 28 days before the test) were stored in the local natural environment (Changzhou, China) for one year. During the year, the samples were exposed to wind, sun, rain, dust, and snowfall. During the year, the samples were subjected to weathering under natural environmental conditions (wind, daylight, rain, dust, snow, and temperature changes). Samples were collected every 30 days, washed with tap water, and dried naturally for 2 h before surface wettability measurement. The surface CA and SA of five samples were tested, and the average value was taken as the test result.
3. Results and discussion 3.1. Variation in SCWC and fluidity The effect of the PDMS content on the SCWC of cement paste was investigated, and the results are shown in Fig. 1(a), indicating that the required water of the modified cement paste decreased in comparison with that of the un-modified cement paste. In addition, the higher the content of PDMS, the lower the required water of the PDMS modified cement paste. This phenomenon can be explained by the fact that liquid PDMS plays the role of lubricant in cement paste, reducing the SCWC of the cement paste. With increasing PDMS content in the cement paste, the water requirement of cement paste increases further, indicating that the higher the PDMS content, the stronger the lubrication effect. The effect of the PDMS content on the fluidity of cement mortar is shown in Fig. 1(b), indicating that the fluidity of the cement mortar increases after the modification of PDMS. The higher the content of PDMS, the better the fluidity of cement mortar. Fig. 1(c) shows that both the initial and final setting times of the PDMS modified cement paste increase with increasing PDMS content, indicating that the setting process of the cement paste delayed by the incorporation of PDMS. The effect of PDMS on the processability of cement paste is similar to that of silane [23]. The addition of PDMS can improve the fluidity and delay the setting process of the cement paste.
2.3.11.2. Durability test of mechanical properties against efflorescence. Twelve groups of prismatic samples (three samples in each group, 40 × 40 × 160 mm3 in size, cured for 28 days before test) were stored in the outdoor natural environment for one year. The flexural strength and compressive strength of the samples were tested after weathering under natural conditions for a certain period of time. The sampling test period was 30 days for a total of 10 cycles. The average of the three test results was taken as the final result. 2.3.11.3. Sulfate attack test. 14 L of sodium sulfate aqueous solutions with the mass percentage concentrations of 2%, 5%, and 10% were prepared as the corrosive sulfate solutions in three 15 L plastic containers (438 × 295 × 146 mm3). Three batches of M-HCM specimens (cubic sample, with a size of 40 × 40 × 40 mm3, cured for 28 days before the test) were placed in the above containers. Sodium sulfate solutions were renewed every ten days. The compressive strength of the samples was tested every 30 days, and the average value of the three samples was taken as the test value. The compressive strength of U-HCM samples after immersing in the sulfate solution was also tested for comparison according to the same method.
3.2. Surface wettability Fig. 2 shows the effect of the PDMS content on the surface wettabilities of different HCM samples. The surface of U-HCM is hydrophilic (CA = 0°). The CA of the M-HCM surface increases apparently with
2.3.11.4. Leaching test. M-HCM and M-HCM samples were subjected to leaching tests using the literature method [29,30]. A 6 mol/L 3
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chemical structures of U-HCM surface, M-HUM surface, and fractured surface of M-HUM were tested, and the results are depicted in Figs. 4 and 5, respectively. As shown in Fig. 4(a), the outside surface of U-HCM is relatively smooth at low magnification [200×, also see Fig. 3(a)]. As a comparison, both the outside surface and the fractured surface of MHCM are uneven at the same magnification [Fig. 4(d) and (g)]. At a medium magnification (1000×), the surface granules of the samples are similar in shape and size [Fig. 4(b) (e) and (h)]. In the fractured section of M-HCM, a PP fiber pulled out from M-HCM could be observed accidentally [Fig. 4(h)]. At high magnification [10,000, see Fig. 4(c), (f), and (i)], all of these surfaces exhibit micro/submicro multi-scale rough structures. Due to the hydrophilic nature of the hydration products of Portland cement, the rough surface of HCM shows superhydrophilicity [Fig. 3(a)]. The ATR-FTIR spectra of different HCM surfaces are shown in Fig. 5, indicating the presence of new absorption peaks on both the outside surface and the fractured surface. The peaks located at 1002.8 and 1260.1 cm−1 could be attributed to the Si–O and Si–CH3 groups derived from PDMS molecules [35]. The peak (2964.1 or 2957.2 cm−1) was attributed to the asymmetric stretch vibration of the CH3 group. The peaks at (2927.3 or 2913.1 cm−1) and (2857.6 or 2853.8 cm−1) were attributed to the asymmetric stretch vibration and symmetric stretch vibration of the CH2 group (derived from DBDL), respectively [36,37]. The ATR-FTIR results indicate that the PDMS molecules are chemically bonded to the surface of the cement hydration products (CHPs). The ATR-FTIR measurement and contact angle test results show that the PDMS molecules are evenly distributed in the whole cement mortar. The predicted chemical reactions between PDMS (including the curing agent) and CHP are illustrated in Fig. 6. First, the hydration products of Portland cement mainly consist of calcium silicate hydrate (C–S–H) and calcium hydroxide (CH) [35,38]. The curing agent TEOS was partially hydrolyzed to silanol under alkaline environment [Fig. 6(a)]. Second, the surface of the cement hydration product contains a large amount of hydroxyl groups. Both of the hydroxyl groups of PDMS [Fig. S1(a), the SM] and silanol [Fig. 6(a)] can react with the hydroxyl groups on the surface of CHP and bond on the surface of CHP by means of Si–O bond [Fig. 6(b)]. At the same time, the cross-linking reaction of PDMS occurs under the action of organotin catalyst (Sn-cat.) [Fig. 6(c) and Fig S1(b) of the SM]. Since the amount of PDMS is only 1.0% by cement weight, the cross-linked PDMS network structure mainly covered the surface of the CHP. As a result, a PDMS layer was chemically bonded to the surface of CHP [Fig. 6(c)]. Fig. 6(d) shows that PDMS molecules contain a large amount of methyl groups, which completely covers the hydroxyl groups on the surface of CHP, changing the CHP surface from hydrophilic to hydrophobic [Fig. 6(e)]. Since the PDMS is completely distributed throughout the cement mortar, the newly exposed surface of cement mortar is also hydrophobic when the outside surface is worn away [Fig. 6(f)].
Fig. 2. Effect of the PDMS content on the surface wettabilities of HCM.
increasing PDMS content. Moreover, the SA of the sample surface decreases rapidly with increasing PDMS content. The surface of the MHCM containing 1.0 wt% PDMS exhibits a CA of 157.3° and an SA of 8.7°, demonstrating its superhydrophobicity (see Movie S1, the SM). In addition, when the content of PDMS increases to 1.2 wt%, the CA and SA of the M-HCM surface did not change much. Therefore, the M-HCM sample modified with 1.0 wt% PDMS were used for further characterization. Fig. 3(a) shows a drop of water penetrating into the pores of the U-HCM surface, and the obtained CA value is 0°, indicating the hydrophilic nature of U-HCM. As a comparison, the water droplet showing spherical shape sits on the M-HCM surface, which is superhydrophobic [Fig. 3(b), water was dyed with methylene blue for good observation]. Movie S2 of the SM also shows that water droplets could roll away from the surface of M-HCM easily. Particularly, the newly exposed surface (fractured surface) of M-HCM is also superhydrophobic after being destroyed using a hammer [Fig. 3(c), and Movie S3, the SM], demonstrating the bulk hydrophobicity of the M-HCM. The M-HCM sample exhibits a mirror-like phenomenon after it was immersed into water [see Fig. S2 and Movie S4, SM] and the surface was completely dried after taken out of the water. The mirrorlike phenomenon is attributed to an air layer formed between the superhydrophobic M-HCM surface and water. Moreover, it was also observed that the PP fibers were embedded in the HCM matrix completely [Fig. 3(a)]. However, for the HCM modified by PDMS, a large number of PP fibers extended beyond the cement mortar [Fig. 3(b) and (c)]. The exposure of hydrophobic PP fibers on the surface increases the surface roughness of the M-HCM and is conducive to increasing the CA of the surface [Fig. 3(b)]. 3.3. Analysis of surface microstructure and surface chemistry It was generally accepted that both the surface microstructure and surface chemistry play an important role in determining the surface wettability of solid [31–34]. Therefore, the microstructures and surface
Fig. 3. Images of water droplets on different HCM surfaces. (a) U-HCM surface; (b) M-HCM surface; (c) fractured surface of M-HCM. 4
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Fig. 4. SEM images of different samples’ surfaces. (a), (b), and (c) surface of U-HCM at different magnifications; (d), (e), and (f) surface of M-HCM at different magnifications; (g), (h), and (i) fractured surface of M-HCM at different magnifications.
S6, SM]. Fig. 7(d) and (e) show that the surface CA of the sample remained above 150° and the SA remained below 10° during the 500 cycles of abrasion. Figs. S3(a), (d), and (g) show the surface microtopographies of M-HCM after 100, 200, and 500 abrasion cycles, respectively. Although the outermost layer of the sample is worn off after polishing, and the newly exposed surface is still very rough. The ARTFTIR spectra of the sample surface after 100, 200, and 500 abrasion cycles are shown in Fig. S4, exhibiting the presence of the absorption peaks corresponding to the asymmetric and symmetric stretch vibrations of methyl groups of the sample surface after 100 cycles of abrasion (surface a) at 2959.7 and 2882.9 cm−1, respectively. The absorption peaks located at 2927.4 and 2857.1 cm−1 could be attributed to the asymmetric and symmetric stretch vibrations of methylene groups, respectively. In addition, the absorption peaks located at 1258.1 and 1004.7 cm−1 could be attributed to the Si–CH3 and Si–O groups derived from the PDMS molecules, respectively. Similar absorption peaks were also observed on the surface of samples after being abraded for 200 and 500 cycles. The above ART-FTIR results show that the PDMS distributed throughout the M-HCM, attributing the M-HCM with bulk hydrophobicity. A piece of superhydrophobic M-HCM sample [Fig. S5(a), the SM] was cut in half, and the surface containing the slit remained superhydrophobic [Fig. S5(b), the SM]. In addition, the newly exposed surface after cutting is also superhydrophobic [Fig. S5(b), the SM].
Fig. 5. ATR-FTIR spectra of different sample surfaces.
3.4. Mechanical robustness The abrasion test was carried out by procedure developed in our laboratory. The sample was abraded back and forth on the surface of 280# sandpaper under an applied pressure of 5 kpa. The distance of each grinding cycle is about 22 cm, and the speed is approximately 22 cm/s [Fig. 7, and Movie S5 of the SM]. The surface of M-HCM exhibits a CA of 154.7° and an SA of 5.4° after 12 cycles of abrasion Movie
3.5. Water absorption analysis Fig. 8 shows the water absorption of the U-HCM and M-HCM as a 5
Applied Surface Science 507 (2020) 145016
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Fig. 6. Schematic diagram of the chemical reaction between PDMS and CHP.
following Washburn Equation [6]:
function of time, indicating that the water absorption rate is fast for UHCM in comparison with that of the M-HCM. In addition, the water absorption of the modified HCM is much lower than that of the unmodified one. The M-HCM absorbs only 7.49% of water absorbed by the U-HCM after the water absorption experiment. The water absorption of the U-HCM and M-HCM samples could be interpreted using the
P=
2γ cos θ rc
(1)
In Eq. (1), P is the capillary force; γ is the surface tension of water; θ is the contact angle for water and rc is the pore radius of capillary. P is
Fig. 7. (a)–(c) Abrasion test; (d) CA as a function of abrasion cycles; (e) SA as a function of abrasion cycles. The insets in (c) show the CA and SA measurement results. 6
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Fig. 8. Effect of test time on the water absorption of the U-HCM and M-HCM sample.
positive on the surface of hydrophilic concrete (CA < 90°), and water droplets can spontaneously infiltrate into the pore structures of the concrete. However, on the surface of hydrophobic concrete (CA > 90°), P is negative. Water droplets on the concrete surface are repulsed by the capillary pores and are difficult to enter the pore structure. Therefore, the hydrophobic modification of concrete is an effective method to reduce its water absorption. The pore structures of the U-HCM and M-HCM were characterized by MIP, and the results are depicted in Fig. 9. Fig. 9(a) shows that the total intruded volume of U-HCM and M-HCM were 0.1174 mL/g and 0.1081 mL/g, respectively, indicating that the pore volume of the HCM decreased by 7.9% after the PDMS modification. Fig. 9(b) shows that the threshold pore size of the M-HCM and the U-HCM samples are 388 and 424 nm, respectively. The threshold pore size of the M-HCM is significantly lower than that of the U-HCM, indicating that the PDMS modification reduces the connectivity of the capillary pores and makes the pore structure dense. The surface tension of water is 7.28 × 10−2 N/m at room temperature. The capillary force of the water droplet on the surface of M-HCM could be obtained from Eq. (1) and is about −0.3462 MPa. In other words, water can penetrate into the capillary pores of the M-HUM only when the water pressure > 0.3462 MPa was applied on the surface of M-HUM, which is equivalent to a pressure of 35.71 m of the water column. Therefore, we can predict that PDMS modified HCM has excellent waterproof performance when used in the range 0–35 m underwater.
M-HCM hereafter) are depicted in Fig. 10(a). The Ecorr and Icorr of the CS/U-HCM are −231.0 mV and 6.78 × 10−7 A·cm−2, respectively, while the Ecorr and Icorr of the CS/M-HCM are −72.04 mV and 4.10 × 10−8 A·cm−2, respectively. The results show that the CS/MHCM has a higher Ecorr and lower Icorr than that of the CS/U-HCM. It was generally accepted that a lower Icorr corresponds to a higher corrosion protection capability and a lower corrosion rate [39]. Based on these measurements, the corrosion resistance of the M-HCM obviously improved than that of the U-HCM. The effect of the test cycles on the corrosion potential of the CS/U-HCM and the CS/M-HCM was also measured. After each test cycle, the sample was immersed in 3.5 wt% NaCl aqueous solution for 1 day followed by naturally drying for 2 days. It was observed that the Ecorr of CS/M-HCM maintained a high value at each test cycle, demonstrating its long-term anti-corrosion performance of the CS/M-HCM.
3.6. Electrochemical measurements
3.7. Mechanical properties
The polarization curves of carbon steel plate embedded in U-HCM (abbreviated as CS/U-HCM hereafter) and M-HCM (abbreviated as CS/
The development of mechanical strength of the U-HCM and M-HCM are summarized in Fig. 11, indicating that the compressive strength of
Fig. 10. (a) Polarization curves of the steel plate embedded in U-HCM and MHCM respectively; (b). Effect of the test cycles on the corrosion potential of the steel plate embedded in U-HCM and M-HCM.
Fig. 9. Effect of PDMS modification on (a) the cumulative intrusion volume distribution and (b) pore size distribution. 7
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Fig. 11. Effect of age on the mechanical strength of the U-HCM and M-HCM: (a) compressive strength; (b) flexural strength; (b) F/C ratio.
damage to the M-HCM samples, and the increase in the mechanical strength of samples benefited from the fact that hydration degree became more perfect with increasing age, while the effect of short-term weathering on U-HCM samples was less. In contrast, sulfate corrosion is one of the main reasons for the lack of durability of Portland cement concrete. Sulfate reacts with the hydrated products of Portland cement such as calcium hydroxide (CH) and hydrated calcium silicate (C–S–H) to form expensive component ettringite and gypsum [Reactions (2) and (3)]:[40]
U-HCM increases rapidly within 28 days and reaches 42.82 MPa at the age of 28 days [Fig. 11(a)]. However, the compressive strength development of the U-HCM becomes very slow after 28 days and achieves only 43.26 MPa at the age of 90 days. For the M-HCM, the compressive strength shows an apparent increase throughout the aging period (3–90 days). The compressive strength of the M-HCM is 22.43 MPa and 29.85 MPa at the aging periods of 28 and 90 days, respectively. Therefore, the modification of PDMS will reduce the compressive strength of the HCM. The compressive strength at 90 days of M-HCM decreased by 30.9% compared to that of U-HCM. Similarly, the M-HCM has a lower flexural strength at each age than the U-HCM [Fig. 11(b)]. The 90-day flexural strength of M-HCM is 8.32 MPa, which is 18.1% lower than that of the U-HCM (10.16 MPa). The ratio of the flexural strength to the compressive strength (F/C ratio) can be used to evaluate the toughness of cement matrix composites and the corresponding results are depicted in Fig. 11(c). Clearly, the F/C ratio of both the U-HCM and M-HCM decreases with increasing age. However, the F/C ratio of the M-HCM at each aging period is always larger than that of the UHCM. Hence, the result indicates that the modification of PDMS will increase the toughness of the cement-based composites.
Ca (OH )2 + Na2 SO4 + 2H2 O → CaSO4 ·2H2 O + 2NaOH
(2)
3(CaSO4 ·2H2 O) + 4CaO·Al2 O3·12H2 O + 14H2 O → 3CaO ·Al2 O3·3CaSO4 ·31H2 O + Ca (OH )2
(3)
In serious cases, the concrete structure will swell, crack and/or peel, and the mechanical properties of concrete will deteriorate. In this study, the water absorption of the modified HCM is much lower than that of the unmodified one (Fig. 8). Therefore, the resistance of the MHCM against sulfate attack is better than that of the U-HCM as shown in Fig. 12(c)], indicating that the compressive strength of the U-HCM decreases significantly with increasing soaking time. In addition, the higher the sulfate concentration is, the more the compressive strength decreases. In contrast, the compressive strength of the M-HCM does not change significantly with increasing immersion time, indicating that the M-HCM has excellent resistance to sulfate attack. In the process of long-term contact with water, the hydration product of concrete, calcium hydroxide, can slowly dissolve and filter out with the flow of water, decreasing the alkalinity and increasing the porosity of the concrete. Once the calcium leach accumulates to a certain extent, it will cause damage to the concrete structure. In the accelerated leaching test adopted in this study, the compressive strength of the M-HCM remained basically unchanged after being immersed in ammonium nitrate solution for a long time (300 days). In contrast, the compressive strength of the H-HCM decreased by 79.8% after only six soaking cycles (180 days in total). In other words, the superhydrophobic M-HCM prepared in this study has good durabilities, which can prolong the service life of the concrete structures in various erosion environments, thereby reducing the maintenance cost and is conducive to the sustainable development.
3.8. Durability The durability of the cement concrete is a decisive factor that determines the service life of concrete structure. In practical application, it is necessary for concrete material to have good resistance to weathering, sulfate erosion, and calcium leaching during long-term service in the outdoor environment. Therefore, the durability of pdms-modified superhydrophobic concrete was systematically tested, and the results are listed in Fig. 12. Therefore, the durability of the PDMS modified superhydrophobic M-HCM was systematically tested, and the results are shown in Fig. 12. Fig. 12(a) shows that the superhydrophobic M-HUM can maintain its superhydrophobicity after weathering in the outdoor environment for one year, and its surface CAs are always > 150°, while the SAs are < 10°. The excellent durability of the superhydrophobic properties of the samples can be attributed to the stability of PDMS. PDMS is a type of polymer material with excellent comprehensive properties. It has high chemical stability, very low water absorption, and can withstand repeated high- and low-temperature changes and long-term ultraviolet radiation. In the electric power sector, it is widely used for making silicone rubber insulators to improve the anti-pollution flashover ability of insulators. Fig. 12(b) shows the change in the mechanical strength of M-HCM and U-HCM samples after one-year weathering. Both of the compressive strength and flexural strength of M-HCM samples showed a slight upward trend during the test period, while the strength of U-HCM samples showed a general downward trend. The above results showed that efflorescence had almost no
4. Conclusions In summary, an integral superhydrophobic HCM was developed using PDMS as the cement waterproof admixture. The preparation process of M-HCM is exactly the same as that of the common cementbased composites. All the materials used are inexpensive and abundant commercial raw materials. The fluidity of the cement mortar increased, the water demand reduced, and the setting time prolonged after adding 8
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Fig. 12. Durability measurements for different samples. (a) Durability test of the superhydrophobcity of the M-HCM samples against efflorescence. Durability tests of the compressive strength and flexural strength for the M-HCM and H-HCM samples against (b) efflorescence, (c) sulfate attack, (d) calcium leaching by the homemade accelerated calcium leaching test.
References
PDMS. The as-prepared M-HCM surface possessed superhydrophobicity (CA = 157.3° and SA = 8.7°) after being simply abraded by sandpaper. The superhydrophobic M-HCM surface was mechanically robust against repeated abrading attributed to the integral hydrophobicity of the MHCM. The waterproof property and corrosion resistance of the M-HCM improved effectively attributed to the superhydrophobicity of the MHCM, which can effectively prevent the water and corrosive aqueous solution from entering the M-HCM. In addition, both the compressive strength and flexural strength of the HCM modified with PDMS decreased. However, the toughness of the HCM improved. Moreover, the M-HCM also possesses excellent durabilities against efflorescence, sulfate attack, and calcium leaching. Hence, the PDMS modified cement mortar can be used in concrete materials with waterproof and/or anticorrosion requirements, such as bathroom floor and concrete pavement that require the use of deicing salts and coastal concrete projects.
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CRediT authorship contribution statement Fajun Wang: Conceptualization, Methodology. Sheng Lei: Data curation, Writing - original draft. Junfei Ou: Writing - review & editing. Wen Li: Project administration. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.145016. 9
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Full Length Article
Effect of PDMS on the waterproofing performance and corrosion resistance of cement mortar ⁎
T
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Fajun Wang , Sheng Lei, Junfei Ou , Wen Li School of Materials Engineering, Jiangsu University of Technology, Changzhou 213001, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cement Superhydrophobicity PDMS Corrosion resistance Durability Compressive strength
The currently used cement hydrophobic additives are either expensive or damageable during use, limiting their large-scale application on concrete. In this study, low-cost and commercially available polydimethylsiloxane (PDMS) is proposed as a cement admixture for the hydrophobic modification of cement mortar. The uniform distribution of PDMS in hardened cement mortar (HCM) attributes integral hydrophobicity to HCM. The PDMS modified HCM (M-HCM) surface exhibits superhydrophobicity, with a contact angle of 157.3° and a sliding angle of 8.7°. The superhydrophobic M-HCM is mechanically robust after being abraded for 110 m distance. In addition, the water absorption of the M-HCM reduced approximately 92.51%, and the corrosion resistance of the M-HCM obviously improved. The compressive strength and flexural strength of the M-HCM reduced by 30.9% and 18.1%, respectively, while the toughness of the M-HCM improved. Furthermore, the durability of the MHCM against weathering, sulfate attack, and leaching significantly improved in comparison with the U-HCM. This type of mechanically robust superhydrophobic M-HCM material can be used in the waterproof concrete and anti-corrosive concrete.
1. Introduction
spraying, brushing, impregnation, etc. For examples, Husni et al. prepared an ethanol dispersion containing fluoroalkyl silane modified rice husk ash particles. Concrete surface was first coated with a commercial adhesive, followed by spraying the dispersion on the surface of the adhesive. The as-prepared coating exhibited a contact angle (CA) for water of 152.3°. The water absorption decreased to 44%. Ismael et al. prepared a superhydrophobic coating on concrete surface using an aqueous emulsion containing siloxane oil, silica fume (or metakaolin), and PVA surfactant. The maximum CA obtained on concrete surface is 156°. Similar superhydrophobic coatings on the concrete surface such as carbon-based coating, polytetrafluoroethylene/polyether ether ketone (PTFE/PEEK) coating, siloxane oil modified nano silica coating, have also been reported. Silane and/or siloxane impregnation on concrete surface is another effective method to reduce the permeability of concrete. For examples, Xue et al. used silane (octyl triethoxysilane, liquid or aqueous emulsion) as an impregnating agent to modify the surface of concrete. The penetration depth of silane aqueous emulsion in concrete is 0.43 mm. The capillary water absorption of the modified cement mortar was only 5.4% of that of the unmodified cement mortar. In addition, the templating method was also used to prepare concrete with improved surface hydrophobicity [21,22]. However, the hydrophobic layer on concrete surface exhibited some problems, such as concrete cracking, coating peeling, coating aging, and weak mechanical
Portland cement is the most widely used building material in the world [1]. Because of its porous structure and hydrophilicity, water in the environment easily enters the pores of the concrete [2–4]. When water contains chloride ion, magnesium ion or sulfate ion, these ions will enter into concrete with water and cause erosion to concrete [2–4]. A large number of studies have shown that water penetration is the main cause of insufficient durability of concrete. Therefore, reducing water permeability can effectively prevent chloride ions, magnesium ions or sulfate ions from entering the concrete, thus improving the durability of concrete. Hydrophobic treatment is a common method to improve the waterproof and impermeability characteristics of concrete and has attracted extensive attention of researchers in recent years [5–10]. At present, there are two main methods for concrete hydrophobic treatment: (a) surface treatment method and (b) hydrophobic admixture method [1,2,5–10]. Surface treatment method is used to process the finished concrete surface, making the surface of concrete hydrophobic. Surface hydrophobic modification methods mainly include hydrophobic/super hydrophobic surface coating method [8–15], silane/siloxane impregnation method, [16–20] and template method [21,22]. The hydrophobic protective layer can be formed on the concrete surface by means of ⁎
Corresponding author. E-mail addresses: [email protected] (F. Wang), [email protected] (J. Ou).
https://doi.org/10.1016/j.apsusc.2019.145016 Received 11 May 2019; Received in revised form 1 December 2019; Accepted 7 December 2019 Available online 13 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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robustness. The hydrophobic admixture method uses the hydrophobic modifier as an admixture for cement. The hydrophobic modifier is added during the mixing process of the fresh concrete [1,5–7,23]. It should be pointed out that silane and/or siloxane can be used as both surface hydrophobic modifier and integral modifier of concrete [1,5,6]. Silane is liquid at room temperature and can be used directly or processed into an environmentally friendly water-based emulsion [1,5]. In addition to silane, hydrophobic powders can also endow concrete with bulk hydrophobicity. For example, Li et al. produced superhydrophobic ground granulated blast furnace slag in a ball mill using stearic acid as a surface modifier. The capillary water absorption of concrete incorporated with superhydrophobic slag decreased by more than 90% [24]. Wong et al. used hydrophobic paper sludge ash (HPSA) as an admixture to modify concrete (replacing cement with 12% HPSA). The water absorption and water sorptivity of the modified concrete reduced by 84% and 86%, respectively [25]. The newly exposed surface of integral hydrophobic concrete is still hydrophobic when surface wear and/or cracking occur during service. Therefore, the long-term waterproofness of the integral hydrophobic concrete would be much higher than that of the surface hydrophobic concrete. However, these concrete integral hydrophobic additives are still insufficient in practical applications. The main shortcoming of silane is the high cost, limiting the extensive use of silane in concrete. Superhydrophobic powders typically float on the surface of the water and accumulate together. As a result, it is difficult to disperse them into cement mortar because of the existing of water, leading to uneven hydrophobicity of the concrete. In this study, polydimethylsiloxane (PDMS) was selected as a cement hydrophobic modifier to endow the concrete with integral hydrophobicity. Compared to small molecule silanes, PDMS has many advantages, such as cost effectiveness (its price is only 10% of octyl triethoxysilane), non-toxic, chemically inert, no volatile, and nonflammable characteristics. In addition, the appearance of concrete modified by PDMS would remain unchanged because of the transparency of PDMS. A large number of superhydrophobic coatings based on PDMS have been reported [26–28]. However, to the best of our knowledge, the use of PDMS as a concrete hydrophobic additive has not been reported so far. The main purpose of this study was to investigate the effect of PDMS on the processing performance of fresh cement mortar and the waterproof properties, corrosion resistance, as well as mechanical strength of hardened cement mortar.
further blended at a high speed for 3 min to ensure the homogeneous dispersion of PDMS in the cement mortar. The cement mortar was cast into molds of size 40 × 40 × 160 cm3 and 4.0 × 2 × 1.5 cm3. Finally, the molds were placed on a jolting table and compacted for 2 min. The samples were taken out from the molds after 24 h and placed in a curing chamber (temperature of 25 °C and relative humidity of 90%) for different periods (7, 14, and 28 days). The dosage of PDMS in the hardened cement mortar (HCM) is 0, 0.2%, 0.4, 0.6%, 0.8%, 1.0%, and 1.2% (by cement weight). The unmodified and PDMS modified HCM samples were denoted as U-HCM and M-HCM.
2. Materials and method
2.3.6. ATR-FTIR analysis The structural information of the surface organic components of the samples was analyzed using an attenuated total reflection Fourier transformed infrared spectrometer (ATR-FTIR, NICOLET 5700, USA).
2.3. Sample characterization 2.3.1. Standard consistency water consumption The standard consistency water consumptions of the cement pastes modified with different dosages of PDMS were measured using a Vicat apparatus according to the ISO9597-1989. 2.3.2. Fluidity The fluidity of the cement mortars modified with different dosages of PDMS was determined using a cement mortar fluidity tester (NLD-3) according to Chinese Standard JGJ 70–90. 2.3.3. Setting time The initial and final setting times of different contents of PDMS modified cement mortar was measured using a Vicat apparatus according to ASTM C191-08. 2.3.4. Surface wettability analysis After curing for 28 days, the mortar sample of size 4.0 × 2 × 1.5 cm3 was used for the measurement CA and sliding angle (SA) for water. The sample was polished with 280# sandpaper to remove impurities and construct a desired roughness on the surface of mortar. The CA and SA of different sample surfaces were measured using an optical contact angle tester (Krüss DSA 100, Germany). 2.3.5. SEM analysis The microstructures of the polished surface and fractured surface of mortar samples (size 4.0 × 2 × 1.5 cm3) cured for 28 days were observed by Field Emission Scanning Electron Microscopy (FESEM, FEI, QUANTA F250, USA).
2.1. Materials Portland cement (P·O 42.5) was provided by Anhui Conch Cement Co., Ltd. (China). Standard sand was purchased from Xiamen Aisiou standard sand co. LTD (China). Polypropylene fiber (PPF) with a length of 9 mm and a diameter of approximately 40 μm was provided by Jiangsu thermal insulation materials co. LTD (China). Hydroxyl terminated polydimethylsiloxane (PDMS, viscosity, 2000 cP) was purchased from Hubei Guishan new materials co. LTD. Tetraethoxysilane (TEOS) and dibutyl dilaurate (DBDL) were purchased from Shanghai Aladdin biochemical technology co., LTD and used as the curing agent and accelerator, respectively, to cure PDMS at room temperature The chemical structures of the raw materials and the cross linking reactions of PDMS are illustrated in Fig. S1 [see the Supporting Material (SM)].
2.3.7. Water absorption measurement Water absorptions of the U-HCM and M-HCM were measured according to the Chinese standard JC 474-2008 (water repellent admixture for mortar and concrete). 2.3.8. Pore structure measurement The pore structure of U-HCM and M-HCM was measured using a mercury intrusion porosimeter (POREMASTER 60, Quantachrome, USA). The hydration reaction was terminated by soaking the test sample in isopropanol after 28 days. 2.3.9. Electrochemical measurement The polarization curve of the samples was measured using an electrochemical workstation (CHI600E, Huachen, China). 45# carbon steel plate were embedded in cement mortars for the measurement of polarization curve. After 28 days of curing, the samples were immersed in 3.5 wt% NaCl aqueous solution for 1 day followed by naturally drying for 2 days. For the electrochemical measurement, a platinum electrode, a saturated calomel electrode (SCE), and a cement mortar
2.2. Preparation of samples 450 g cement, 1350 g standard sand, 2.25 g PPF, and 225 g tap water (the water cement ratio of 0.5) were blended in a cement mortar mixer (JJ-5, China) for 3 min. PDMS, TEOS, and DBDL with a weight ratio of 100:5:1 were mixed manually in a plastic cup. Afterwards, the liquid mixture of PDMS was poured into the mixer. The mixture was 2
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Fig. 1. (a) Effects of the PDMS contents on (a) the SCWC of cement paste, (b) the fluidity of cement mortar, (c) the initial and final setting times.
ammonium nitrate solution was used as the soaking solution to accelerate the dissolution of calcium. Deionized water was also used as the soaking solution to compare with the soaking result of the ammonium nitrate solution. The cubic samples (40 × 40 × 40 mm3) were used for leaching test. Prior to testing, the opposite sides of the cube were sealed with epoxy material to allow the leaching process to occur on the side of the sample. The samples were divided into four batches, including two batches of M-HCM samples and two batches of U-HCM samples, each containing 36 identical samples. The above two batches of M-HCM samples were soaked with ammonium nitrate solution (14 L) and deionized water (14 L). The two batches of UHCM samples were also soaked in the above solutions (in the other two tanks) for comparison. Three samples were taken from each tank for compressive strength test every 30 days, and the average value was taken as the test result.
coated with carbon steel plate were used as the counter electrode, reference electrode, and working electrode, respectively. The corrosion potential (Ecorr) and corrosion current density (Icorr) were obtained by the tafel curve fitting. 2.3.10. Mechanical strength measurement Compressive strength and flexural strength of the samples after different aging periods were measured according to the standard ISO 679:1989 (Test method for cement - determination of strength). 2.3.11. Durability test 2.3.11.1. Durability test of the superhydrophobcity against efflorescence. Five super hydrophobic samples (size 4.0 × 2 × 1.5 cm3, cured for 28 days before the test) were stored in the local natural environment (Changzhou, China) for one year. During the year, the samples were exposed to wind, sun, rain, dust, and snowfall. During the year, the samples were subjected to weathering under natural environmental conditions (wind, daylight, rain, dust, snow, and temperature changes). Samples were collected every 30 days, washed with tap water, and dried naturally for 2 h before surface wettability measurement. The surface CA and SA of five samples were tested, and the average value was taken as the test result.
3. Results and discussion 3.1. Variation in SCWC and fluidity The effect of the PDMS content on the SCWC of cement paste was investigated, and the results are shown in Fig. 1(a), indicating that the required water of the modified cement paste decreased in comparison with that of the un-modified cement paste. In addition, the higher the content of PDMS, the lower the required water of the PDMS modified cement paste. This phenomenon can be explained by the fact that liquid PDMS plays the role of lubricant in cement paste, reducing the SCWC of the cement paste. With increasing PDMS content in the cement paste, the water requirement of cement paste increases further, indicating that the higher the PDMS content, the stronger the lubrication effect. The effect of the PDMS content on the fluidity of cement mortar is shown in Fig. 1(b), indicating that the fluidity of the cement mortar increases after the modification of PDMS. The higher the content of PDMS, the better the fluidity of cement mortar. Fig. 1(c) shows that both the initial and final setting times of the PDMS modified cement paste increase with increasing PDMS content, indicating that the setting process of the cement paste delayed by the incorporation of PDMS. The effect of PDMS on the processability of cement paste is similar to that of silane [23]. The addition of PDMS can improve the fluidity and delay the setting process of the cement paste.
2.3.11.2. Durability test of mechanical properties against efflorescence. Twelve groups of prismatic samples (three samples in each group, 40 × 40 × 160 mm3 in size, cured for 28 days before test) were stored in the outdoor natural environment for one year. The flexural strength and compressive strength of the samples were tested after weathering under natural conditions for a certain period of time. The sampling test period was 30 days for a total of 10 cycles. The average of the three test results was taken as the final result. 2.3.11.3. Sulfate attack test. 14 L of sodium sulfate aqueous solutions with the mass percentage concentrations of 2%, 5%, and 10% were prepared as the corrosive sulfate solutions in three 15 L plastic containers (438 × 295 × 146 mm3). Three batches of M-HCM specimens (cubic sample, with a size of 40 × 40 × 40 mm3, cured for 28 days before the test) were placed in the above containers. Sodium sulfate solutions were renewed every ten days. The compressive strength of the samples was tested every 30 days, and the average value of the three samples was taken as the test value. The compressive strength of U-HCM samples after immersing in the sulfate solution was also tested for comparison according to the same method.
3.2. Surface wettability Fig. 2 shows the effect of the PDMS content on the surface wettabilities of different HCM samples. The surface of U-HCM is hydrophilic (CA = 0°). The CA of the M-HCM surface increases apparently with
2.3.11.4. Leaching test. M-HCM and M-HCM samples were subjected to leaching tests using the literature method [29,30]. A 6 mol/L 3
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chemical structures of U-HCM surface, M-HUM surface, and fractured surface of M-HUM were tested, and the results are depicted in Figs. 4 and 5, respectively. As shown in Fig. 4(a), the outside surface of U-HCM is relatively smooth at low magnification [200×, also see Fig. 3(a)]. As a comparison, both the outside surface and the fractured surface of MHCM are uneven at the same magnification [Fig. 4(d) and (g)]. At a medium magnification (1000×), the surface granules of the samples are similar in shape and size [Fig. 4(b) (e) and (h)]. In the fractured section of M-HCM, a PP fiber pulled out from M-HCM could be observed accidentally [Fig. 4(h)]. At high magnification [10,000, see Fig. 4(c), (f), and (i)], all of these surfaces exhibit micro/submicro multi-scale rough structures. Due to the hydrophilic nature of the hydration products of Portland cement, the rough surface of HCM shows superhydrophilicity [Fig. 3(a)]. The ATR-FTIR spectra of different HCM surfaces are shown in Fig. 5, indicating the presence of new absorption peaks on both the outside surface and the fractured surface. The peaks located at 1002.8 and 1260.1 cm−1 could be attributed to the Si–O and Si–CH3 groups derived from PDMS molecules [35]. The peak (2964.1 or 2957.2 cm−1) was attributed to the asymmetric stretch vibration of the CH3 group. The peaks at (2927.3 or 2913.1 cm−1) and (2857.6 or 2853.8 cm−1) were attributed to the asymmetric stretch vibration and symmetric stretch vibration of the CH2 group (derived from DBDL), respectively [36,37]. The ATR-FTIR results indicate that the PDMS molecules are chemically bonded to the surface of the cement hydration products (CHPs). The ATR-FTIR measurement and contact angle test results show that the PDMS molecules are evenly distributed in the whole cement mortar. The predicted chemical reactions between PDMS (including the curing agent) and CHP are illustrated in Fig. 6. First, the hydration products of Portland cement mainly consist of calcium silicate hydrate (C–S–H) and calcium hydroxide (CH) [35,38]. The curing agent TEOS was partially hydrolyzed to silanol under alkaline environment [Fig. 6(a)]. Second, the surface of the cement hydration product contains a large amount of hydroxyl groups. Both of the hydroxyl groups of PDMS [Fig. S1(a), the SM] and silanol [Fig. 6(a)] can react with the hydroxyl groups on the surface of CHP and bond on the surface of CHP by means of Si–O bond [Fig. 6(b)]. At the same time, the cross-linking reaction of PDMS occurs under the action of organotin catalyst (Sn-cat.) [Fig. 6(c) and Fig S1(b) of the SM]. Since the amount of PDMS is only 1.0% by cement weight, the cross-linked PDMS network structure mainly covered the surface of the CHP. As a result, a PDMS layer was chemically bonded to the surface of CHP [Fig. 6(c)]. Fig. 6(d) shows that PDMS molecules contain a large amount of methyl groups, which completely covers the hydroxyl groups on the surface of CHP, changing the CHP surface from hydrophilic to hydrophobic [Fig. 6(e)]. Since the PDMS is completely distributed throughout the cement mortar, the newly exposed surface of cement mortar is also hydrophobic when the outside surface is worn away [Fig. 6(f)].
Fig. 2. Effect of the PDMS content on the surface wettabilities of HCM.
increasing PDMS content. Moreover, the SA of the sample surface decreases rapidly with increasing PDMS content. The surface of the MHCM containing 1.0 wt% PDMS exhibits a CA of 157.3° and an SA of 8.7°, demonstrating its superhydrophobicity (see Movie S1, the SM). In addition, when the content of PDMS increases to 1.2 wt%, the CA and SA of the M-HCM surface did not change much. Therefore, the M-HCM sample modified with 1.0 wt% PDMS were used for further characterization. Fig. 3(a) shows a drop of water penetrating into the pores of the U-HCM surface, and the obtained CA value is 0°, indicating the hydrophilic nature of U-HCM. As a comparison, the water droplet showing spherical shape sits on the M-HCM surface, which is superhydrophobic [Fig. 3(b), water was dyed with methylene blue for good observation]. Movie S2 of the SM also shows that water droplets could roll away from the surface of M-HCM easily. Particularly, the newly exposed surface (fractured surface) of M-HCM is also superhydrophobic after being destroyed using a hammer [Fig. 3(c), and Movie S3, the SM], demonstrating the bulk hydrophobicity of the M-HCM. The M-HCM sample exhibits a mirror-like phenomenon after it was immersed into water [see Fig. S2 and Movie S4, SM] and the surface was completely dried after taken out of the water. The mirrorlike phenomenon is attributed to an air layer formed between the superhydrophobic M-HCM surface and water. Moreover, it was also observed that the PP fibers were embedded in the HCM matrix completely [Fig. 3(a)]. However, for the HCM modified by PDMS, a large number of PP fibers extended beyond the cement mortar [Fig. 3(b) and (c)]. The exposure of hydrophobic PP fibers on the surface increases the surface roughness of the M-HCM and is conducive to increasing the CA of the surface [Fig. 3(b)]. 3.3. Analysis of surface microstructure and surface chemistry It was generally accepted that both the surface microstructure and surface chemistry play an important role in determining the surface wettability of solid [31–34]. Therefore, the microstructures and surface
Fig. 3. Images of water droplets on different HCM surfaces. (a) U-HCM surface; (b) M-HCM surface; (c) fractured surface of M-HCM. 4
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Fig. 4. SEM images of different samples’ surfaces. (a), (b), and (c) surface of U-HCM at different magnifications; (d), (e), and (f) surface of M-HCM at different magnifications; (g), (h), and (i) fractured surface of M-HCM at different magnifications.
S6, SM]. Fig. 7(d) and (e) show that the surface CA of the sample remained above 150° and the SA remained below 10° during the 500 cycles of abrasion. Figs. S3(a), (d), and (g) show the surface microtopographies of M-HCM after 100, 200, and 500 abrasion cycles, respectively. Although the outermost layer of the sample is worn off after polishing, and the newly exposed surface is still very rough. The ARTFTIR spectra of the sample surface after 100, 200, and 500 abrasion cycles are shown in Fig. S4, exhibiting the presence of the absorption peaks corresponding to the asymmetric and symmetric stretch vibrations of methyl groups of the sample surface after 100 cycles of abrasion (surface a) at 2959.7 and 2882.9 cm−1, respectively. The absorption peaks located at 2927.4 and 2857.1 cm−1 could be attributed to the asymmetric and symmetric stretch vibrations of methylene groups, respectively. In addition, the absorption peaks located at 1258.1 and 1004.7 cm−1 could be attributed to the Si–CH3 and Si–O groups derived from the PDMS molecules, respectively. Similar absorption peaks were also observed on the surface of samples after being abraded for 200 and 500 cycles. The above ART-FTIR results show that the PDMS distributed throughout the M-HCM, attributing the M-HCM with bulk hydrophobicity. A piece of superhydrophobic M-HCM sample [Fig. S5(a), the SM] was cut in half, and the surface containing the slit remained superhydrophobic [Fig. S5(b), the SM]. In addition, the newly exposed surface after cutting is also superhydrophobic [Fig. S5(b), the SM].
Fig. 5. ATR-FTIR spectra of different sample surfaces.
3.4. Mechanical robustness The abrasion test was carried out by procedure developed in our laboratory. The sample was abraded back and forth on the surface of 280# sandpaper under an applied pressure of 5 kpa. The distance of each grinding cycle is about 22 cm, and the speed is approximately 22 cm/s [Fig. 7, and Movie S5 of the SM]. The surface of M-HCM exhibits a CA of 154.7° and an SA of 5.4° after 12 cycles of abrasion Movie
3.5. Water absorption analysis Fig. 8 shows the water absorption of the U-HCM and M-HCM as a 5
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Fig. 6. Schematic diagram of the chemical reaction between PDMS and CHP.
following Washburn Equation [6]:
function of time, indicating that the water absorption rate is fast for UHCM in comparison with that of the M-HCM. In addition, the water absorption of the modified HCM is much lower than that of the unmodified one. The M-HCM absorbs only 7.49% of water absorbed by the U-HCM after the water absorption experiment. The water absorption of the U-HCM and M-HCM samples could be interpreted using the
P=
2γ cos θ rc
(1)
In Eq. (1), P is the capillary force; γ is the surface tension of water; θ is the contact angle for water and rc is the pore radius of capillary. P is
Fig. 7. (a)–(c) Abrasion test; (d) CA as a function of abrasion cycles; (e) SA as a function of abrasion cycles. The insets in (c) show the CA and SA measurement results. 6
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Fig. 8. Effect of test time on the water absorption of the U-HCM and M-HCM sample.
positive on the surface of hydrophilic concrete (CA < 90°), and water droplets can spontaneously infiltrate into the pore structures of the concrete. However, on the surface of hydrophobic concrete (CA > 90°), P is negative. Water droplets on the concrete surface are repulsed by the capillary pores and are difficult to enter the pore structure. Therefore, the hydrophobic modification of concrete is an effective method to reduce its water absorption. The pore structures of the U-HCM and M-HCM were characterized by MIP, and the results are depicted in Fig. 9. Fig. 9(a) shows that the total intruded volume of U-HCM and M-HCM were 0.1174 mL/g and 0.1081 mL/g, respectively, indicating that the pore volume of the HCM decreased by 7.9% after the PDMS modification. Fig. 9(b) shows that the threshold pore size of the M-HCM and the U-HCM samples are 388 and 424 nm, respectively. The threshold pore size of the M-HCM is significantly lower than that of the U-HCM, indicating that the PDMS modification reduces the connectivity of the capillary pores and makes the pore structure dense. The surface tension of water is 7.28 × 10−2 N/m at room temperature. The capillary force of the water droplet on the surface of M-HCM could be obtained from Eq. (1) and is about −0.3462 MPa. In other words, water can penetrate into the capillary pores of the M-HUM only when the water pressure > 0.3462 MPa was applied on the surface of M-HUM, which is equivalent to a pressure of 35.71 m of the water column. Therefore, we can predict that PDMS modified HCM has excellent waterproof performance when used in the range 0–35 m underwater.
M-HCM hereafter) are depicted in Fig. 10(a). The Ecorr and Icorr of the CS/U-HCM are −231.0 mV and 6.78 × 10−7 A·cm−2, respectively, while the Ecorr and Icorr of the CS/M-HCM are −72.04 mV and 4.10 × 10−8 A·cm−2, respectively. The results show that the CS/MHCM has a higher Ecorr and lower Icorr than that of the CS/U-HCM. It was generally accepted that a lower Icorr corresponds to a higher corrosion protection capability and a lower corrosion rate [39]. Based on these measurements, the corrosion resistance of the M-HCM obviously improved than that of the U-HCM. The effect of the test cycles on the corrosion potential of the CS/U-HCM and the CS/M-HCM was also measured. After each test cycle, the sample was immersed in 3.5 wt% NaCl aqueous solution for 1 day followed by naturally drying for 2 days. It was observed that the Ecorr of CS/M-HCM maintained a high value at each test cycle, demonstrating its long-term anti-corrosion performance of the CS/M-HCM.
3.6. Electrochemical measurements
3.7. Mechanical properties
The polarization curves of carbon steel plate embedded in U-HCM (abbreviated as CS/U-HCM hereafter) and M-HCM (abbreviated as CS/
The development of mechanical strength of the U-HCM and M-HCM are summarized in Fig. 11, indicating that the compressive strength of
Fig. 10. (a) Polarization curves of the steel plate embedded in U-HCM and MHCM respectively; (b). Effect of the test cycles on the corrosion potential of the steel plate embedded in U-HCM and M-HCM.
Fig. 9. Effect of PDMS modification on (a) the cumulative intrusion volume distribution and (b) pore size distribution. 7
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Fig. 11. Effect of age on the mechanical strength of the U-HCM and M-HCM: (a) compressive strength; (b) flexural strength; (b) F/C ratio.
damage to the M-HCM samples, and the increase in the mechanical strength of samples benefited from the fact that hydration degree became more perfect with increasing age, while the effect of short-term weathering on U-HCM samples was less. In contrast, sulfate corrosion is one of the main reasons for the lack of durability of Portland cement concrete. Sulfate reacts with the hydrated products of Portland cement such as calcium hydroxide (CH) and hydrated calcium silicate (C–S–H) to form expensive component ettringite and gypsum [Reactions (2) and (3)]:[40]
U-HCM increases rapidly within 28 days and reaches 42.82 MPa at the age of 28 days [Fig. 11(a)]. However, the compressive strength development of the U-HCM becomes very slow after 28 days and achieves only 43.26 MPa at the age of 90 days. For the M-HCM, the compressive strength shows an apparent increase throughout the aging period (3–90 days). The compressive strength of the M-HCM is 22.43 MPa and 29.85 MPa at the aging periods of 28 and 90 days, respectively. Therefore, the modification of PDMS will reduce the compressive strength of the HCM. The compressive strength at 90 days of M-HCM decreased by 30.9% compared to that of U-HCM. Similarly, the M-HCM has a lower flexural strength at each age than the U-HCM [Fig. 11(b)]. The 90-day flexural strength of M-HCM is 8.32 MPa, which is 18.1% lower than that of the U-HCM (10.16 MPa). The ratio of the flexural strength to the compressive strength (F/C ratio) can be used to evaluate the toughness of cement matrix composites and the corresponding results are depicted in Fig. 11(c). Clearly, the F/C ratio of both the U-HCM and M-HCM decreases with increasing age. However, the F/C ratio of the M-HCM at each aging period is always larger than that of the UHCM. Hence, the result indicates that the modification of PDMS will increase the toughness of the cement-based composites.
Ca (OH )2 + Na2 SO4 + 2H2 O → CaSO4 ·2H2 O + 2NaOH
(2)
3(CaSO4 ·2H2 O) + 4CaO·Al2 O3·12H2 O + 14H2 O → 3CaO ·Al2 O3·3CaSO4 ·31H2 O + Ca (OH )2
(3)
In serious cases, the concrete structure will swell, crack and/or peel, and the mechanical properties of concrete will deteriorate. In this study, the water absorption of the modified HCM is much lower than that of the unmodified one (Fig. 8). Therefore, the resistance of the MHCM against sulfate attack is better than that of the U-HCM as shown in Fig. 12(c)], indicating that the compressive strength of the U-HCM decreases significantly with increasing soaking time. In addition, the higher the sulfate concentration is, the more the compressive strength decreases. In contrast, the compressive strength of the M-HCM does not change significantly with increasing immersion time, indicating that the M-HCM has excellent resistance to sulfate attack. In the process of long-term contact with water, the hydration product of concrete, calcium hydroxide, can slowly dissolve and filter out with the flow of water, decreasing the alkalinity and increasing the porosity of the concrete. Once the calcium leach accumulates to a certain extent, it will cause damage to the concrete structure. In the accelerated leaching test adopted in this study, the compressive strength of the M-HCM remained basically unchanged after being immersed in ammonium nitrate solution for a long time (300 days). In contrast, the compressive strength of the H-HCM decreased by 79.8% after only six soaking cycles (180 days in total). In other words, the superhydrophobic M-HCM prepared in this study has good durabilities, which can prolong the service life of the concrete structures in various erosion environments, thereby reducing the maintenance cost and is conducive to the sustainable development.
3.8. Durability The durability of the cement concrete is a decisive factor that determines the service life of concrete structure. In practical application, it is necessary for concrete material to have good resistance to weathering, sulfate erosion, and calcium leaching during long-term service in the outdoor environment. Therefore, the durability of pdms-modified superhydrophobic concrete was systematically tested, and the results are listed in Fig. 12. Therefore, the durability of the PDMS modified superhydrophobic M-HCM was systematically tested, and the results are shown in Fig. 12. Fig. 12(a) shows that the superhydrophobic M-HUM can maintain its superhydrophobicity after weathering in the outdoor environment for one year, and its surface CAs are always > 150°, while the SAs are < 10°. The excellent durability of the superhydrophobic properties of the samples can be attributed to the stability of PDMS. PDMS is a type of polymer material with excellent comprehensive properties. It has high chemical stability, very low water absorption, and can withstand repeated high- and low-temperature changes and long-term ultraviolet radiation. In the electric power sector, it is widely used for making silicone rubber insulators to improve the anti-pollution flashover ability of insulators. Fig. 12(b) shows the change in the mechanical strength of M-HCM and U-HCM samples after one-year weathering. Both of the compressive strength and flexural strength of M-HCM samples showed a slight upward trend during the test period, while the strength of U-HCM samples showed a general downward trend. The above results showed that efflorescence had almost no
4. Conclusions In summary, an integral superhydrophobic HCM was developed using PDMS as the cement waterproof admixture. The preparation process of M-HCM is exactly the same as that of the common cementbased composites. All the materials used are inexpensive and abundant commercial raw materials. The fluidity of the cement mortar increased, the water demand reduced, and the setting time prolonged after adding 8
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Fig. 12. Durability measurements for different samples. (a) Durability test of the superhydrophobcity of the M-HCM samples against efflorescence. Durability tests of the compressive strength and flexural strength for the M-HCM and H-HCM samples against (b) efflorescence, (c) sulfate attack, (d) calcium leaching by the homemade accelerated calcium leaching test.
References
PDMS. The as-prepared M-HCM surface possessed superhydrophobicity (CA = 157.3° and SA = 8.7°) after being simply abraded by sandpaper. The superhydrophobic M-HCM surface was mechanically robust against repeated abrading attributed to the integral hydrophobicity of the MHCM. The waterproof property and corrosion resistance of the M-HCM improved effectively attributed to the superhydrophobicity of the MHCM, which can effectively prevent the water and corrosive aqueous solution from entering the M-HCM. In addition, both the compressive strength and flexural strength of the HCM modified with PDMS decreased. However, the toughness of the HCM improved. Moreover, the M-HCM also possesses excellent durabilities against efflorescence, sulfate attack, and calcium leaching. Hence, the PDMS modified cement mortar can be used in concrete materials with waterproof and/or anticorrosion requirements, such as bathroom floor and concrete pavement that require the use of deicing salts and coastal concrete projects.
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CRediT authorship contribution statement Fajun Wang: Conceptualization, Methodology. Sheng Lei: Data curation, Writing - original draft. Junfei Ou: Writing - review & editing. Wen Li: Project administration. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.145016. 9
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