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Portland cement based composites and superhydrophobic coatings
Construction and Building Materials 246 (2020) 118466
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Preparation and properties of foundry dust/Portland cement based composites and superhydrophobic coatings Fajun Wang a,b,⇑, Ting Xie b, Sheng Lei a,⇑, Junfei Ou b, Wen Li b, Mingshan Xue b, Daqi Huang c a
School of Materials Engineering, Jiangsu University of Technology, Changzhou 213001, PR China School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China c Yunxian Litong Renewable Resources Processing Co. LTD, Shiyan, 442599, PR China b
h i g h l i g h t s FD and PC based bulk composites and superhydrophobic coatings were prepared. The FD/PC composite with FD content of 10% have relatively high mechanical strength. The superhydrophobic coating was prepared using a simple brushing method. The superhydrophobic coatings exhibited mechanical robustness and easy repairability. The water absorption of the coating modified FD/PC composite is greatly reduced.
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Article history: Received 23 August 2019 Received in revised form 13 February 2020 Accepted 14 February 2020
Keywords: Foundry dust Portland cement Superhydrophobic coating Solid waste reuse Water absorption Mechanical robustness
a b s t r a c t In this study, Found dust (FD) and Portland cement (PC) based bulk composites and superhydrophobic coatings were prepared to utilize the solid waste FD and improve the water permeability. First, the compositions, structures, pore parameters, and mechanical properties of the FD/PC composites were tested and analyzed, indicating that the mechanical strength of the FD/PC composites decreased with increasing FD content. The flexural compressive strength and compressive strength of the FD/PC composites with an FD content of 10% reduced by only 8.5% and 9.8%, respectively, compared to that of the neat hardened cement paste (HCP). At greater than 10% FD content, the mechanical strength of the FD/PC composite material drastically reduced. Second, the FD and PC based superhydrophobic coatings were prepared using a simple brush-coating method by introducing polydimethylsiloxane as a hydrophobic admixture. The composite coating exhibits superhydrophobicity after stored at room temperature for 24 h. In addition, the mechanical robustness of the FD and PC composite based coating is excellent in the abrasion and peeling test and was further improved by double-layer coating method. Particularly, the superhydrophobic coating can be easily repaired after getting damaged. Furthermore, the water absorption of the superhydrophobic coating modified FD/PC composites reduced by more than 76% in comparison with that of the uncoated FD/PC composites. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction The large-scale use of natural resources for the development of human society has led to its gradual depletion together with increasing raw material cost [1]. In order to maintain the sustainable development of human society, new raw materials have to be developed to replace natural resources [2]. While achieving this goal, industrial production process also produces a large number of by-products, such as fly ash, slag, foundry waste sand, waste marble powder, and other solid waste [3-5]. When a large amount ⇑ Corresponding authors. E-mail addresses: [email protected] (F. Wang), [email protected] (S. Lei). https://doi.org/10.1016/j.conbuildmat.2020.118466 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
of solid waste is disposed in an outdoor storage yard or landfill, it will not only waste a lot of land resources, but also produce dust, heavy metal, toxic organic compounds, and other hazards in the environment [6]. Therefore, finding a safe and effective treatment method for the utilization/disposal of solid waste is urgently required. This study mainly focused on the utilization of solid waste. At present, the addition of solid waste to Portland cement to prepare cement-based composites (or concrete) is currently the most common method for recycling large amounts of solid waste [4,7-9]. There are many studies on the preparation of Portland cement concrete using solid waste [2-5]; however, they mainly focused on the effect of waste content on the microstructure, mechanical
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with FD/PC based superhydrophobic coatings to reuse FD are as follows: First, the consumption of FD increased (both bulk composite and surface coating contain FD); the mechanical strength of the FD/PC composite is high (due to the low content of FD in the composite); the water absorption of the FD/PC composite reduced sharply (due to superhydrophobic coating). The ‘‘waste based composite” + ‘‘waste based superhydrophobic coating” method proposed in this study might provide a new method for the recycling of various solid wastes in cement-based composites with low permeability.
strength, and permeability of waste concrete, but rarely addressed the durability of waste concrete. Increasing the durability of waste concrete can greatly improve the service life of concrete structures, reduce maintenance costs, and contribute to the rapid development of social economy [10]. The durability of concrete is closely related to its permeability. Water in the environment is easy to penetrate into the inner pores of concrete through the capillary pores in the concrete. Chloride ions in the water will corrode steel bars. Magnesium ions and sulfate ions in water will react with cement hydration products, cracking the concrete in severe cases [11]. Even pure water will cause freeze–thaw damage to concrete due to the volume expansion caused by icing in the winter [12]. Therefore, preventing water from penetrating and reducing water absorption is the most direct and effective method for improving the durability of concrete. The permeability of concrete has been increased by many methods including increasing the compactness of concrete, using water-repellent admixtures, surface isolation coatings, surface silane impregnation, and superhydrophobic coatings [13-18]; however, these methods have their own shortcomings. For example, the method of improving the compactness of concrete does not change the hydrophilic nature of concrete, and concrete still absorbs water. The use of integral hydrophobic additives reduces the concrete strength to a significant extent. The surface barrier coating method has poor weather resistance and is liable to failure due to chalking, peeling, and cracking. The surface silane impregnation method is too costly to be used on a large scale in the field of building materials. The main raw materials used in the preparation of superhydrophobic coatings include film-forming materials, hydrophobic agents, and fillers. The water contact angle (CA) on the superhydrophobic coating surface is greater than 150°, and the sliding angle (SA) is less than 10° [19-22]. Water is difficult to be adsorbed by the superhydrophobic coating. Therefore, the superhydrophobic coating method is the most effective method for reducing the permeability of concrete. At present, the main obstacles restricting the practical application of superhydrophobic coatings are high cost, low durability, and complicated manufacturing methods. Therefore, developing superhydrophobic coatings with low cost, easy preparation, and suitable durability for concrete surface is an important task. The foundry industry produces a huge amount of waste foundry sand, waste foundry slag, and foundry dust (FD) as the main waste products. Waste foundry sand and waste foundry slag as the concrete aggregate have been extensively investigated, but the application of FD in Portland cement concrete has been rarely investigated [2-5,23]. In this study, FD/PC composites and FD/PC composite based superhydrophobic coatings were prepared to utilize FD. First, FD/PC composites with different FD contents were prepared by partial FD instead of PC. Then, superhydrophobic coatings were prepared on the surface of the FD/PC composites by a simple brushing method using a mixture of FD, PC, PDMS, and water. The raw materials used included industrial waste FD, inexpensive PC, cost effective commercial product PDMS (5% of the price of n-octyltriethoxysilane), and water. The advantages of using FD/PC bulk composites combined
2. Materials and method 2.1. Materials Foundry dust (FD) was provided by Yunxian Litong Renewable Resources Processing Co. LTD (China). Ordinary Portland Cement (P.O 42.5) was provided by Huaxin Cement Co. LTD (China). Polydimethylsiloxane (PDMS, 1000 cp) was provided by Nanchang Yongqian Chemical Co. LTD (China). Methyl triethoxy silane (used as the curing agent) and dibutyltin dilaurate (used as the catalyst) were provided by Shanghai Macklin biochemical technology co., LTD, respectively. 2.2. Preparation of samples 2.2.1. Preparation of foundry dust/Portland cement composite Foundry dust/Portland cement (FD/PC) composites were prepared according to the formulations listed in Table 1. A desired amount of FD powders, PC powders, and tap water was mixed in a plastic beaker using a mechanical stirrer (JJ-5, China, according to Chinese Standard GB/T17671-1999) for 3 min. In brief, water, PC powders, and FD powders were successively added into an agitator kettle. The mixture was mixed for 1 min at a low speed (140 ± 5 rpm) and then for 2 min at a high speed (285 ± 10 rpm). The well mixed cement paste was cast in a plastic mould with the size of 40 mm 40 mm 160 mm, and then compacted using a vibration table. Subsequently, the sample was covered with a damp cotton cloth and placed in a closed plastic box to prevent evaporation of water. The prisms were demolded after 24 h, and then cured to different ages at 90% humidity and 25 °C,
Fig. 1. Mechanical abrasion test for the superhydrophobic surfaces.
Table 1 Formulations of the FD/PC composites. Specimen code HCP0 HCP10 HCP20 HCP30
Mass (g) FD
PC
Water
Weight ratio FD/(PC + FD)
0 200 400 600
2000 1800 1600 1400
800 800 800 800
0/100 10/100 20/100 30/100
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mixture was added into the above cement paste mixture and mixed for 3 min to obtain a slurry. Coatings were prepared on various substrates using a brush-coating method [see the Supporting Material (SM), Movie S1]. 2.3. Characterization 2.3.1. Measurement of flexural strength and compressive strength Prism specimens of 40 mm 40 mm 160 mm size were made for testing the flexural strength. Cubic specimens of 40 mm 40 mm 40 mm size were made for testing the compressive strength. The compressive strength and flexural strength of the specimens were measured after the ages of 3, 7, 14, and 28 days. The average flexural strength of three prism samples and the average compressive strength of four cubic samples were used for strength analysis. The compressive strength and flexural strength were tested in accordance with the method specified in Chinese Standard GB/T 17671–1999. 2.3.2. Measurement of CA and SA Water CAs and SAs were measured using an optical contact Angle tester (Krüss, DSA100, Germany). 8 lL of deionized water was used as the test liquid. The SA of the sample surface was tested using a tilting table. The tilting table placed horizontally gradually inclined, and when the water droplet on the sample surface starts to roll, the angle at which the tilting table tilted is the SA. Both the CAs and SAs were tested at five different locations on the sample surface, and the average of each was used for CA and SA analyses. 2.3.3. Measurement of microstructures and surface topography The microstructures of the surfaces and cross-sections of the samples were observed by Field Emission Scanning Electron Microscopy (FESEM, QUANTA F250, FEI Company). The surface topography of the coating surface was observed using a surface profilometer (ContourGT-K, Bruker Company).
Fig. 2. Properties of FD powders. (a) XRD pattern; (b) FESEM image; (c) Granulometric distribution curve.
i.e. 3, 7, 14, and 30 days. The hardened cement pastes are abbreviated as HCPx, where represents the percentage of FD in the total mass of FD and PC (Table 1). 2.2.2. Preparation of foundry dust/Portland composite based superhydrophobic coatings 3 g of FD and 7 g of PC powder were mixed by hand in a mortar for 3 min, followed by the addition of 10 g of water, and mixing was continued for 3 min to obtain a cement paste mixture. At the same time, PDMS, curing agent, and catalyst were mixed evenly according to the mass ratio of 100:10:1. Then, 0.1 g PDMS
2.3.4. Measurement of phase structure and chemical composition The surface chemical elements of the samples were measured by X-ray photoelectron spectroscopy (XPS, Escalab250Xi). The phase structures of the powder and bulk samples were analyzed using an X-ray diffractometer (D8 Advance). The organic groups on the surface of the sample were analyzed by Fourier transform attenuated total reflection infrared spectroscopy (ATR-FTIR, VERTEX 70, Bruker Company). 2.3.5. Particle size distribution A laser particle size analyzer (Marlvern 3000) was used to analyze the particle size distribution of the powdres of FD. 2.3.6. Measurement of pore structure of hardened cement paste The pore structure of the hardened cement paste was analyzed by Mercury intrusion porosimetry (MIP, POREMASTER 60, Quantachrome). 2.3.7. Measurement of mechanical robustness The mechanical robustness of the superhydrophobic coating against abrasion was tested by the method shown in Fig. 1 [also see the SM, Movie S2]. Sandpaper (600#) was used for polishing
Table 2 XRF analysis of FD powder. Material
Na2O
MgO
Al2O3
SiO2
SO3
K2O
CaO
TiO2
Fe2O3
FD
3.51
10.33
21.74
46.60
0.90
2.36
7.85
1.02
5.69
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2.3.8. Measurement of water absorption The water absorption of FD/PC composites before and after the superhydrophobic coating modification was measured using the method described in international standard ISO 15148:2002(E). Three cubic specimens (100 100 100 mm3) were used for water absorption measurement. The lateral sides of each cubic specimen were sealed with epoxy. The specimens were dried at 105 °C for 24 h before the measurement. The bottom surface of the sample was immersed horizontally into the water at an immersing depth of 5 mm. The water absorbed by the specimen was determined by measuring the weight of the specimen after different soaking times. After that, the capillary water absorption of the specimen was determined by the following Equation [13]:
Fig. 3. XRD patterns for various FD/PC samples and FD powders.
the surface [16,24]. The test surface of the sample is in contact with the rough surface of the sandpaper. Samples were pressurized with weights (500 g and 100 g). The friction distance is approximately 15 cm for each cycle.
Dm A ¼ pffiffi t
ð1Þ
where A is the coefficient of water absorption, m is the amount of water absorbed by the specimen per unit of area (g/cm2), and t is the immersing time (hour).
Fig. 4. FESEM images of fracture surface of different FD/PC samples: (a), (a1), and (a2) HCP0; (b), (b1), and (b2) HCP10; (c), (c1), and (c2) HCP20; (d), (d1), and (d2) HCP30. Magnification: (a), (b), (c) and (d) 1,000; (a1), (b1), (c1) and (d1) 10,000; (a2), (b2), (c2) and (d2) 80,000 .
F. Wang et al. / Construction and Building Materials 246 (2020) 118466
3. Results and discussion
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500 mm is 39.06%. The content of particles less than 10 mm and larger than 500 mm were 12.21 and 1.45%, respectively.
3.1. FD analysis 3.2. XRD analysis Fig. 2a shows the XRD pattern of the FD powders. As shown in the figure, the main phase of the FD powder is quartz and also contains a small amount of anorthite and illite. The chemical composition of the FD powders is summarized in Table 2 in the form of oxides. The content of SiO2, Al2O3, and MgO is 46.55%, 21.74%, and 10.33% respectively, and the total content is 78.62%. The total content of CaO and Fe2O3 is 13.54%, and the sum of other oxides is less than 10%. These oxides are chemically combined into structures of complex oxides such as illite and anorthite. Some toxic heavy metal elements such as lead, cadmium, and mercury were not detected by XRF, indicating that the content of these toxic metal elements in FD is very low, below the detection limit of XRF. The XRF result indicates that the FD powders are safe to be used as cement additive. The FESEM image shows that the FD powder is composed of irregular particles, and the particle sizes range from a few microns to a hundred microns (Fig. 2b). The particle content calculated from the particle size distribution curve between 10 and 100 mm is 47.28%, and that between 100 and
Ordinary Portland cement mainly comprises cement clinker and a small amount of gypsum retarder. The cement clinker is a mixture of tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetralcium ferroaluminate (C4AF). The hydration reaction of the cement powder and water produces hydrated calcium silicate gel, calcium hydroxide (portlandite), ettringite, and other hydration products [25,26]. The XRD pattern of pure HCP (Fig. 3) shows that the main hydration product, such as portlandite and ettringite were detected by XRD. The calcite phase in the XRD spectrum originates from the carbonation process of the hardened cement paste. In addition, the hydration of Portland cement is a long-term process. During the one-year age, the hydration reaction of C3S is only about 80%, while the hydration process of C2S is slower. Therefore, there are also phases such as C3S, C2S, and C4AF, which are not fully hydrated in the HCP of 28 days old. The XRD spectra of FD/PC composites are very similar to that of pure HCP. No diffraction peaks disappeared or new ones appeared,
Fig. 5. Surfaces FESEM images of different FD/PC samples: (a) HCP0; (b) HCP10; (c) HCP10; (d) HCP30. Magnification: (a), (b), (c) and (d) 1,000; (a1), (b1), (c1) and (d1) 10,000; (a2), (b2), (c2) and (d2) 80,000 .
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indicating that FD only acted as filler in HCP and there was no chemical reaction between them. In addition, with increasing content of FD in the FD/PC composite, the diffraction peak intensity of the portlandite phase significantly reduced, because of decreasing cement content in the FD/PC composites. 3.3. FESEM images analysis of the FD/PC compoistes The FESEM images of the fracture surfaces of different FD/PC samples are shown in Fig. 4. The FEM morphologies of the cross section of various FD/PC composites were similar at 1000x magnification (Fig. 4a–c). At high FD content, the irregular FD particles were observed in the SEM images (Fig. 4c and d). At high magnification (10,000 and 80,000 times), the micron-scale pores in the FD/ PC composites are clearly visible (the highlight section in Fig. 4a1, b1, c1, and d1). In addition, irregular cement gel (Fig. 4a2 and c2) and hydrated product (crystal) with regular shape (Fig. 4b2 and d2) were also observed in high magnification SEM images. The FESEM images of the as-prepared surfaces for different FD/ PC samples are shown in Fig. 5. Compared to the fracture surfaces, the surfaces of the FD/PC composites are smoother. The surface of the FD/PC composite with 30% FD content shows a large number of voids, whereas the surfaces of other FD/PC composites are relatively compact. However, in the high magnification SEM image, micron-sized pores were observed on the surface of all the FD/PC composites.
with the FD content of 10%, 20%, and 30% decreased by 8.5%, 33.8%, and 53.1% at 28 days, while the corresponding flexural strength decreased by 9.8%, 26.5%, and 53.0%, respectively. The FD/PC composite with an FD content of 10% maintained the mechanical strength of more than 90% of the HCP, with a 28-day compressive strength larger than 40 MPa and a flexural strength larger than 10 MPa. Therefore, the FD/PC composites with an FD content of 10% can realize the recycling of FD in the cementbased composites and decrease the production cost of the cement products. The FD/PC composites with high FD contents (i.e., 20% and 30%) have higher reuse efficiency for FD; however their mechanical strengths are poor. They are recommended to be used in non-load-bearing building materials such as rendering mortars or cement based coatings to reduce costs.
3.4. Pore structure analysis The pore parameters of the FD/PC composites tested by MIP are shown in Fig. 6. The pore volume of the FD/PC composites with the FD content of 0%, 10%, 20%, and 30% were 0.1225, 0.1041, 0.1451, and 0.1648 mL/g, respectively (Fig. 6a). Compared to pure HCP, adding a small amount of FD (10%) is conducive to the densification of the composite structure, while adding more FD (20% and 30%) increases the pore volume of the composite. The pore distribution diagram of the composites shows that the pores in the FD/PO composites mainly consist of nanopores and submicron pores, with a total content of more than 85%. Therefore, these two types of pores will have an important impact on the properties of the FD/PC composites such as permeability, water absorption, and mechanical strength. The composite of HCP10 has the highest nanopore content and the lowest submicron pore content. In addition, the content of nanopores in the composite increases with increasing FD content (from 10% to 30%), while the content of submicron pores decreases with increasing FD content. Another important pore parameter is the threshold pore size, which is closely related to the connectivity of pores in HCP and the permeability of cementbased materials. All the FD/PC composites exhibit double peaks in the differential curves (Fig. 6c). The pore corresponding to the peak in the curve is the threshold pore of the FD/PC composite. The threshold pores of the four composites were 134.7, 26.3, 133.2, and 114.8 nm, respectively (Fig. 6c). Therefore, the threshold pores of HCP10 composites are the lowest in size, whereas that of other composites is close [13,27]. The results show that the pores in the HCP10 composite have lower connectivity and denser pore structure. 3.5. Compressive strength and flexural strength analysis The compressive strength and flexural strength of the FD/PC composites with different FD contents at the ages of 3, 7, 14, and 28 days are shown in Fig. 7a and b, respectively. The compressive strength and flexural strength of all the FD/PC composites increase with increasing age, but decrease with increasing FD content. Compared to HCP, the compressive strength of the FD/PC composites
Fig. 6. Pore diameter distributions of various FD/PC composites obtained from MIP measurement. (a) cumulative pore volume; (b) pore diameter distribution; (c) differential intrusion volume.
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Fig. 7. Compressive strength and flexural strength of FD/PC composites at different ages.
3.6. Surface wettability analysis In order to reduce the water absorptions of the FD/PC composites, the surface of FC/PC composites was superhydrophobically coated. In addition to being used to prepare bulk cement-based composites, FD can also be used to prepare cement-based coatings, thereby expanding the field of FD reuse. In this study, the main raw materials used in the preparation of superhydrophobic coatings are still PC and FD, as well as small amounts of water repellent such as PDMS. The surface wettabilities of various FD/PC composites before and after coating treatment were tested, and the results are shown in Fig. 8, indicating that all of the as-prepared FD/PC composites show superhydrophilicities and their surfaces’ CAs are 0°. After superhydrophobic coating treatment, water droplets could not spread on the surface of FD/PC composites. The water CA and SA of the coating are 154.1° and 6.1°, respectively (taking HCP-C surface as an example), indicating that the coating surface is superhydrophobic. When 20 mL of water was poured onto the surface of the superhydrophobic coating, the beaded water flow away rapidly on the surface of the coating (see Movie S3, in the SM). In practical applications, the water contacted by superhydrophobic surfaces is usually aqueous solutions or dispersions containing various impurities such as acid rain, brine, milk, and fruit juice. Therefore, superhydrophobic surfaces are required to have superlyophobic properties [28-30]. Therefore, the hydrophobicity of the coating was measured using a laboratory solution of saline water (5% sodium chloride
Fig. 8. Surface CAs and SAs measurement for different FD/PC composites before and after coating treatment.
solution), acidic water (dilute hydrochloric acid with a pH of 5), and alkaline water (sodium hydroxide solution with a pH of 9) to simulate the water in the natural environment. Fig. 9a shows the photograph of the liquid droplets on the surface of the coating, indicating that all of the above liquids exhibit CAs greater than 150° on the surface of the coating. In addition, liquid foods commonly used in daily life, such as Coca Cola, fruit juice, honey, soy sauce and tea, also have liquid CAs above 150° on the coating surface (Fig. 9b). The superhydrophobic surface has a self-cleaning function, and a large amount of dust accumulated on the surface is completely removed from the surface of the coating by the impacted water droplets (see Movie S4, the SM). Artificial slurry is prepared using diatomite as contamination. When the slurry is dumped on the coating, the coating cannot be polluted by the slurry as shown in the Movie S5 of the SM.
3.7. Surface rough structure and chemical component analysis The surface micro-structures of the FD/PC composite based superhydrophobic coating were observed by FESEM, and the images are shown in Fig. 10a and Fig. 10b, exhibiting very rough
Fig. 9. Surface lyophobicity measurement for the FD/PC composite based superhydrophobic coating.
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surface. Irregular FD particles and a large number of micron scale holes were observed on the coating surface. The 2D and 3D profile images of the coating surface show that the surface is rugged, just like the mountains and valleys on Earth. The average roughness (Ra) of the coating surface was approximately 4977 nm, belonging to the micron-scale roughness. Superhydrophobic surface usually contains groups such as ACH3, ACF3, and -Si-CH3 to ensure low surface energy of the coating [31-33]. Therefore, the surface chemical composition of the FC/PC composite based superhydrophobic coating was measured. The XPS survey spectrum indicates that the presence of oxygen, carbon, calcium, and silicon elements on the surface of the superhydrophobic coating. Elemental oxygen and silicon can be attributed to the cement hydration products, FD and PDMS, all of which contain elemental oxygen and silicon. Elemental calcium can be attributed to cement hydration products and FD. Three distinct peaks located at the binding energies of 284.8, 286.7, and 289.9 eV were observed in the C region, indicating three types of carbon sources on the coating surface, namely, carbonate (CaCO3) produced during the carbonation process of
HCP in air, carbon adsorption on HCP surface in air (often containing polar and non-polar carbon), and carbon introduced from PDMS (-Si-CH3). Obviously, the XPS peak at 289.9 eV was attributed to calcium carbonate. Obviously, the XPS peak at 286.7 eV is attributed to the polar carbon adsorbed on the HCP surface (C@ @O and/or COR) [31]. The XPS peak at 284.8 eV is mainly attributed to the silicon methyl group in PDMS, and a small part of it might come from the adsorption of non-polar carbon in the air (C–C). The ATP-FTIR spectrum of the superhydrophobic coating showed the presence of carbonate, methyl, and Si–O bonds on the surface of the coating, and this result is consistent with the XPS test results Fig. 11.
3.8. Surface mechanical robustness analysis The mechanical robustness of superhydrophobic coatings is one of the key factors to determine their applicability in practical engineering fields [16,24]. At present, there is no standard
Fig. 10. (a) and (b) FESEM images of the FD/PC composite based superhydrophobic coating; (c) 2-D surface profile measurement of the coating; (d) 3-D surface profile measurement of the coating.
Fig. 11. (a) XPS survey spectrum and (b) high resolution C 1 s spectrum of the FD/PC composite based superhydrophobic coating; (c) ATR-FTIR curve of the coating.
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equipment or method for measuring the mechanical robustness of superhydrophobic materials. The stability of the superhydrophobic materials is evaluated by measuring the changes in the surface CA and SA after various mechanical damage tests by artificial methods such as sandpaper abrading, double-sided adhesive adhesion and peeling, gravel impact, and knife cutting. Among these methods, sandpaper sanding is the most commonly used method for characterizing the mechanical robustness of superhydrophobic surfaces in the literature, because some important experimental parameters such as pressure, roughness of sandpaper, and abrasion distance (or cycles) of sanding are easily quantified. In the present
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work, the robustness of the superhydrophobic coating against mechanical damage was evaluated by sandpaper sanding and adhesion-peeling method, and the results are summarized in Fig. 12. Fig. 12(a) shows that the superhydrophobic coating of the single layer (brushing only once) still has superhydrophobicity after six cycles of polishing under a pressure of 3.1 kpa with a surface CA of 153.2° and SA of 6.9°. With the prolongation of polishing cycles, the surface CA of the coating decreases rapidly, while the SA cannot be measured (droplets can not roll). In other words, the surface of the coating loses its superhydrophobicity. When the superhydrophobic coatings are polished under low pressure
Fig. 12. Effect of abrading and peeling cycles on the surface wettability of the FD/PC composite based superhydrophobic coatings. (a) one layer coating, abrading test under a pressure of 3.1 KPa; (b) one layer coating, peeling test under a pressure of 2 MPa; (a1) one layer coating, abrading test under a pressure of 0.62 KPa; (b1) one layer coating, peeling test under a pressure of 31 KPa; (a2) two-layer coating, abrading test under a pressure of 3.1 KPa; (b2) two-layer coating, peeling test under a pressure of 2 MPa.
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(i.e., 0.62 MPa), the coatings remain superhydrophobic even after 50 polishing cycles (Fig. 12a1). The superhydrophobic coating loses its superhydrophobicity after six cycles of grinding under a pressure of 3.1 KPa, which can be attributed to the wear of the coating. The greater the pressure, the longer the distance, the more severe the wear of the superhydrophobic surface. In order to enhance the robustness of the superhydrophobic coating, a double layer coating was prepared. The double-layer coating remained superhydrophobicity after 10 cycles of abrasions under an abrasion pressure of 3.1 KPa (Fig. 12a2). The 3 M double-sided adhesive was adhered on the coating under a pressure of 2 MPa [10,16]. After the pressure was kept for 2 min, the double-sided adhesive was peeled off, and the CA and SA of the coating were measured (Fig. 13). The robustness of the coating against peeling is summarized in Fig. 12b, b1, and b2, indicating that the peeling stability of single layer superhydrophobic coating is only for five cycles under 2 MPa. However, the coating has a peeling stability larger than 100 cycles under the pressure of 31 KPa. In addition, the double-layer superhydrophobic coating has a peeling stability of more than 10 cycles under the adhesion pressure of 2 MPa, which is significantly higher than that of the single-layer superhydrophobic coating Therefore, mechanical robustness can be effectively improved by increasing the number of layers of superhydrophobic coating. Another important property for superhydrophobic surfaces is the repairability [34]. In nature, the lotus leaf maintains a stable superhydrophobic surface by secreting a wax layer of low surface energy. However, for artificially prepared superhydrophobic surfaces, simulating the function of lotus leaf to secrete low surface energy substances automatically is difficult. An alternative approach is to repair the degraded superhydrophobic surface in a simple manner. Our superhydrophobic coatings were prepared by a very simple brushing method and cured at room temperature. Therefore, the damaged coating can be easily repaired by repeating the preparation process. In order to demonstrate the repair process
Fig. 14. Repair test for the FD/PC composite based superhydrophobic coating. (a) picture of water droplets on the damaged coating surface; (b) picture of water droplets on the repaired coating surface.
of the superhydrophobic coating, the surface of the coating was destroyed using a blade (Movie S6 in the SM,). The area immediately loses its superhydrophobicity after the coating is damaged (Fig. 14a). However, after a simple brushing process and curing at room temperature for 24 h, the damaged coating area restored its superhydrophobicity (Fig. 14b and the SM, Movie S7). 3.9. Water absorption analysis The water absorption measurement results of various FD/PC composites before and after superhydrophobic coating modification are listed in Fig. 15 [15,35]. Fig. 15a shows that the water absorption of FD/PC composites without superhydrophobic coating is closely related to the pore volume of the composites (also see Fig. 5a). The larger the total pore volume, the higher the water absorption rate and the faster the water absorption rate of the composites. The 96-hour water absorption of the FD/PC composite with an FD content of 10% was 3358 g/m2, which was the lowest in the composites. After the superhydrophobic coating treatment, the water absorption of the FD/PC composites decreased significantly, and their water absorption curves almost coincided [Fig. 15b]. The 96-hour water absorption of the FD/PC composites modified with superhydrophobic coatings fluctuated between 710 and 815 g/ m2, with a maximum value of 814 g/m2. When a water droplet was placed on the surface of the superhydrophobic coating (also see Fig. 8), the surface wettability could be described by the following Equation: [36]
coshw ¼ f 1 coshr f 2
Fig. 13. Mechanical robustness against peeling of the superhydrophobic coating.
ð2Þ
where hw is the apparent CA on a rough surface of a solid and hw = 155.3° (rough surface of PDMS), hr is the CA on a smooth surface of the same solid and hr = 110° (smooth surface of PDMS), and f1 and f2 are the area fractions of the water droplet in contact with solid and air of the superhydrophobic coating, respectively (f1 + f2 = 1). Both f1 and f2 can be determined by Equation (2) and were found as 0.1291 and 0.8609, respectively. The result indicates that 12.91% of the surface area of the superhydrophobic coating contacts with water. During the soaking period, water can gradually penetrate into the pores from the contacting areas with
F. Wang et al. / Construction and Building Materials 246 (2020) 118466
11
Fig. 15. Water absorption curves for different FD/PC composites before and after superhydrophobic coating modification.
increasing soaking time. As a result, the FD/PC composites with superhydrophobic coating absorb water, and their water absorption reduced by more than 76% after the superhydrophobic coating treatment. Therefore, superhydrophobic coating treatment is an effective method to reduce the water absorption of the cementbased composites.
Acknowledgements The authors acknowledge the financial support by the Science and Technology Support Program (Social Development) of Changzhou City (Grant No. CE20195038). Appendix A. Supplementary data
4. Conclusions In this study, FD/PC based composites and superhydrophobic coatings were prepared by partially replacing Portland cement with FD. The mechanical strength of the FD/PC composite decreased with increasing FD content. However, the FD/PC composite still exhibited a relatively high mechanical strength (the compressive strength is 31.70 kPa and the flexural strength is 8.32 kPa), which could ensure the application of FD/PC composite in the field of some construction materials. Superhydrophobic coatings based on the FD/PC composites were prepared by simply brushing the admixture of FD, PC, PDMS, and water on substrates. The coating exhibits superhydrophobicity after curing at room temperature for 24 h. The superhydrophobic coating possesses excellent mechanical robustness against abrasion and peeling tests. The mechanical robustness of the superhydrophobic coatings was further improved by double-layer coating method. In addition, the superhydrophobic coating can be easily repaired after getting damaged. Moreover, the water absorption of the FD/PC composites reduced by more than 76% after the superhydrophobic coating treatment. The FD/PC composite modified by the FD/PC composite based superhydrophobic coating exhibited high mechanical strength, low water absorption, and high utilization rate of FD and thus is expected to be widely used in the field of building materials. CRediT authorship contribution statement Fajun Wang: Conceptualization. Ting Xie: Data curation, Writing - original draft. Sheng Lei: Data curation, Writing - original draft. Junfei Ou: Writing - review & editing. Wen Li: Project administration. Mingshan Xue: Methodology. Daqi Huang: Methodology. 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.
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Preparation and properties of foundry dust/Portland cement based composites and superhydrophobic coatings Fajun Wang a,b,⇑, Ting Xie b, Sheng Lei a,⇑, Junfei Ou b, Wen Li b, Mingshan Xue b, Daqi Huang c a
School of Materials Engineering, Jiangsu University of Technology, Changzhou 213001, PR China School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China c Yunxian Litong Renewable Resources Processing Co. LTD, Shiyan, 442599, PR China b
h i g h l i g h t s FD and PC based bulk composites and superhydrophobic coatings were prepared. The FD/PC composite with FD content of 10% have relatively high mechanical strength. The superhydrophobic coating was prepared using a simple brushing method. The superhydrophobic coatings exhibited mechanical robustness and easy repairability. The water absorption of the coating modified FD/PC composite is greatly reduced.
a r t i c l e
i n f o
Article history: Received 23 August 2019 Received in revised form 13 February 2020 Accepted 14 February 2020
Keywords: Foundry dust Portland cement Superhydrophobic coating Solid waste reuse Water absorption Mechanical robustness
a b s t r a c t In this study, Found dust (FD) and Portland cement (PC) based bulk composites and superhydrophobic coatings were prepared to utilize the solid waste FD and improve the water permeability. First, the compositions, structures, pore parameters, and mechanical properties of the FD/PC composites were tested and analyzed, indicating that the mechanical strength of the FD/PC composites decreased with increasing FD content. The flexural compressive strength and compressive strength of the FD/PC composites with an FD content of 10% reduced by only 8.5% and 9.8%, respectively, compared to that of the neat hardened cement paste (HCP). At greater than 10% FD content, the mechanical strength of the FD/PC composite material drastically reduced. Second, the FD and PC based superhydrophobic coatings were prepared using a simple brush-coating method by introducing polydimethylsiloxane as a hydrophobic admixture. The composite coating exhibits superhydrophobicity after stored at room temperature for 24 h. In addition, the mechanical robustness of the FD and PC composite based coating is excellent in the abrasion and peeling test and was further improved by double-layer coating method. Particularly, the superhydrophobic coating can be easily repaired after getting damaged. Furthermore, the water absorption of the superhydrophobic coating modified FD/PC composites reduced by more than 76% in comparison with that of the uncoated FD/PC composites. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction The large-scale use of natural resources for the development of human society has led to its gradual depletion together with increasing raw material cost [1]. In order to maintain the sustainable development of human society, new raw materials have to be developed to replace natural resources [2]. While achieving this goal, industrial production process also produces a large number of by-products, such as fly ash, slag, foundry waste sand, waste marble powder, and other solid waste [3-5]. When a large amount ⇑ Corresponding authors. E-mail addresses: [email protected] (F. Wang), [email protected] (S. Lei). https://doi.org/10.1016/j.conbuildmat.2020.118466 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
of solid waste is disposed in an outdoor storage yard or landfill, it will not only waste a lot of land resources, but also produce dust, heavy metal, toxic organic compounds, and other hazards in the environment [6]. Therefore, finding a safe and effective treatment method for the utilization/disposal of solid waste is urgently required. This study mainly focused on the utilization of solid waste. At present, the addition of solid waste to Portland cement to prepare cement-based composites (or concrete) is currently the most common method for recycling large amounts of solid waste [4,7-9]. There are many studies on the preparation of Portland cement concrete using solid waste [2-5]; however, they mainly focused on the effect of waste content on the microstructure, mechanical
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with FD/PC based superhydrophobic coatings to reuse FD are as follows: First, the consumption of FD increased (both bulk composite and surface coating contain FD); the mechanical strength of the FD/PC composite is high (due to the low content of FD in the composite); the water absorption of the FD/PC composite reduced sharply (due to superhydrophobic coating). The ‘‘waste based composite” + ‘‘waste based superhydrophobic coating” method proposed in this study might provide a new method for the recycling of various solid wastes in cement-based composites with low permeability.
strength, and permeability of waste concrete, but rarely addressed the durability of waste concrete. Increasing the durability of waste concrete can greatly improve the service life of concrete structures, reduce maintenance costs, and contribute to the rapid development of social economy [10]. The durability of concrete is closely related to its permeability. Water in the environment is easy to penetrate into the inner pores of concrete through the capillary pores in the concrete. Chloride ions in the water will corrode steel bars. Magnesium ions and sulfate ions in water will react with cement hydration products, cracking the concrete in severe cases [11]. Even pure water will cause freeze–thaw damage to concrete due to the volume expansion caused by icing in the winter [12]. Therefore, preventing water from penetrating and reducing water absorption is the most direct and effective method for improving the durability of concrete. The permeability of concrete has been increased by many methods including increasing the compactness of concrete, using water-repellent admixtures, surface isolation coatings, surface silane impregnation, and superhydrophobic coatings [13-18]; however, these methods have their own shortcomings. For example, the method of improving the compactness of concrete does not change the hydrophilic nature of concrete, and concrete still absorbs water. The use of integral hydrophobic additives reduces the concrete strength to a significant extent. The surface barrier coating method has poor weather resistance and is liable to failure due to chalking, peeling, and cracking. The surface silane impregnation method is too costly to be used on a large scale in the field of building materials. The main raw materials used in the preparation of superhydrophobic coatings include film-forming materials, hydrophobic agents, and fillers. The water contact angle (CA) on the superhydrophobic coating surface is greater than 150°, and the sliding angle (SA) is less than 10° [19-22]. Water is difficult to be adsorbed by the superhydrophobic coating. Therefore, the superhydrophobic coating method is the most effective method for reducing the permeability of concrete. At present, the main obstacles restricting the practical application of superhydrophobic coatings are high cost, low durability, and complicated manufacturing methods. Therefore, developing superhydrophobic coatings with low cost, easy preparation, and suitable durability for concrete surface is an important task. The foundry industry produces a huge amount of waste foundry sand, waste foundry slag, and foundry dust (FD) as the main waste products. Waste foundry sand and waste foundry slag as the concrete aggregate have been extensively investigated, but the application of FD in Portland cement concrete has been rarely investigated [2-5,23]. In this study, FD/PC composites and FD/PC composite based superhydrophobic coatings were prepared to utilize FD. First, FD/PC composites with different FD contents were prepared by partial FD instead of PC. Then, superhydrophobic coatings were prepared on the surface of the FD/PC composites by a simple brushing method using a mixture of FD, PC, PDMS, and water. The raw materials used included industrial waste FD, inexpensive PC, cost effective commercial product PDMS (5% of the price of n-octyltriethoxysilane), and water. The advantages of using FD/PC bulk composites combined
2. Materials and method 2.1. Materials Foundry dust (FD) was provided by Yunxian Litong Renewable Resources Processing Co. LTD (China). Ordinary Portland Cement (P.O 42.5) was provided by Huaxin Cement Co. LTD (China). Polydimethylsiloxane (PDMS, 1000 cp) was provided by Nanchang Yongqian Chemical Co. LTD (China). Methyl triethoxy silane (used as the curing agent) and dibutyltin dilaurate (used as the catalyst) were provided by Shanghai Macklin biochemical technology co., LTD, respectively. 2.2. Preparation of samples 2.2.1. Preparation of foundry dust/Portland cement composite Foundry dust/Portland cement (FD/PC) composites were prepared according to the formulations listed in Table 1. A desired amount of FD powders, PC powders, and tap water was mixed in a plastic beaker using a mechanical stirrer (JJ-5, China, according to Chinese Standard GB/T17671-1999) for 3 min. In brief, water, PC powders, and FD powders were successively added into an agitator kettle. The mixture was mixed for 1 min at a low speed (140 ± 5 rpm) and then for 2 min at a high speed (285 ± 10 rpm). The well mixed cement paste was cast in a plastic mould with the size of 40 mm 40 mm 160 mm, and then compacted using a vibration table. Subsequently, the sample was covered with a damp cotton cloth and placed in a closed plastic box to prevent evaporation of water. The prisms were demolded after 24 h, and then cured to different ages at 90% humidity and 25 °C,
Fig. 1. Mechanical abrasion test for the superhydrophobic surfaces.
Table 1 Formulations of the FD/PC composites. Specimen code HCP0 HCP10 HCP20 HCP30
Mass (g) FD
PC
Water
Weight ratio FD/(PC + FD)
0 200 400 600
2000 1800 1600 1400
800 800 800 800
0/100 10/100 20/100 30/100
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mixture was added into the above cement paste mixture and mixed for 3 min to obtain a slurry. Coatings were prepared on various substrates using a brush-coating method [see the Supporting Material (SM), Movie S1]. 2.3. Characterization 2.3.1. Measurement of flexural strength and compressive strength Prism specimens of 40 mm 40 mm 160 mm size were made for testing the flexural strength. Cubic specimens of 40 mm 40 mm 40 mm size were made for testing the compressive strength. The compressive strength and flexural strength of the specimens were measured after the ages of 3, 7, 14, and 28 days. The average flexural strength of three prism samples and the average compressive strength of four cubic samples were used for strength analysis. The compressive strength and flexural strength were tested in accordance with the method specified in Chinese Standard GB/T 17671–1999. 2.3.2. Measurement of CA and SA Water CAs and SAs were measured using an optical contact Angle tester (Krüss, DSA100, Germany). 8 lL of deionized water was used as the test liquid. The SA of the sample surface was tested using a tilting table. The tilting table placed horizontally gradually inclined, and when the water droplet on the sample surface starts to roll, the angle at which the tilting table tilted is the SA. Both the CAs and SAs were tested at five different locations on the sample surface, and the average of each was used for CA and SA analyses. 2.3.3. Measurement of microstructures and surface topography The microstructures of the surfaces and cross-sections of the samples were observed by Field Emission Scanning Electron Microscopy (FESEM, QUANTA F250, FEI Company). The surface topography of the coating surface was observed using a surface profilometer (ContourGT-K, Bruker Company).
Fig. 2. Properties of FD powders. (a) XRD pattern; (b) FESEM image; (c) Granulometric distribution curve.
i.e. 3, 7, 14, and 30 days. The hardened cement pastes are abbreviated as HCPx, where represents the percentage of FD in the total mass of FD and PC (Table 1). 2.2.2. Preparation of foundry dust/Portland composite based superhydrophobic coatings 3 g of FD and 7 g of PC powder were mixed by hand in a mortar for 3 min, followed by the addition of 10 g of water, and mixing was continued for 3 min to obtain a cement paste mixture. At the same time, PDMS, curing agent, and catalyst were mixed evenly according to the mass ratio of 100:10:1. Then, 0.1 g PDMS
2.3.4. Measurement of phase structure and chemical composition The surface chemical elements of the samples were measured by X-ray photoelectron spectroscopy (XPS, Escalab250Xi). The phase structures of the powder and bulk samples were analyzed using an X-ray diffractometer (D8 Advance). The organic groups on the surface of the sample were analyzed by Fourier transform attenuated total reflection infrared spectroscopy (ATR-FTIR, VERTEX 70, Bruker Company). 2.3.5. Particle size distribution A laser particle size analyzer (Marlvern 3000) was used to analyze the particle size distribution of the powdres of FD. 2.3.6. Measurement of pore structure of hardened cement paste The pore structure of the hardened cement paste was analyzed by Mercury intrusion porosimetry (MIP, POREMASTER 60, Quantachrome). 2.3.7. Measurement of mechanical robustness The mechanical robustness of the superhydrophobic coating against abrasion was tested by the method shown in Fig. 1 [also see the SM, Movie S2]. Sandpaper (600#) was used for polishing
Table 2 XRF analysis of FD powder. Material
Na2O
MgO
Al2O3
SiO2
SO3
K2O
CaO
TiO2
Fe2O3
FD
3.51
10.33
21.74
46.60
0.90
2.36
7.85
1.02
5.69
4
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2.3.8. Measurement of water absorption The water absorption of FD/PC composites before and after the superhydrophobic coating modification was measured using the method described in international standard ISO 15148:2002(E). Three cubic specimens (100 100 100 mm3) were used for water absorption measurement. The lateral sides of each cubic specimen were sealed with epoxy. The specimens were dried at 105 °C for 24 h before the measurement. The bottom surface of the sample was immersed horizontally into the water at an immersing depth of 5 mm. The water absorbed by the specimen was determined by measuring the weight of the specimen after different soaking times. After that, the capillary water absorption of the specimen was determined by the following Equation [13]:
Fig. 3. XRD patterns for various FD/PC samples and FD powders.
the surface [16,24]. The test surface of the sample is in contact with the rough surface of the sandpaper. Samples were pressurized with weights (500 g and 100 g). The friction distance is approximately 15 cm for each cycle.
Dm A ¼ pffiffi t
ð1Þ
where A is the coefficient of water absorption, m is the amount of water absorbed by the specimen per unit of area (g/cm2), and t is the immersing time (hour).
Fig. 4. FESEM images of fracture surface of different FD/PC samples: (a), (a1), and (a2) HCP0; (b), (b1), and (b2) HCP10; (c), (c1), and (c2) HCP20; (d), (d1), and (d2) HCP30. Magnification: (a), (b), (c) and (d) 1,000; (a1), (b1), (c1) and (d1) 10,000; (a2), (b2), (c2) and (d2) 80,000 .
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3. Results and discussion
5
500 mm is 39.06%. The content of particles less than 10 mm and larger than 500 mm were 12.21 and 1.45%, respectively.
3.1. FD analysis 3.2. XRD analysis Fig. 2a shows the XRD pattern of the FD powders. As shown in the figure, the main phase of the FD powder is quartz and also contains a small amount of anorthite and illite. The chemical composition of the FD powders is summarized in Table 2 in the form of oxides. The content of SiO2, Al2O3, and MgO is 46.55%, 21.74%, and 10.33% respectively, and the total content is 78.62%. The total content of CaO and Fe2O3 is 13.54%, and the sum of other oxides is less than 10%. These oxides are chemically combined into structures of complex oxides such as illite and anorthite. Some toxic heavy metal elements such as lead, cadmium, and mercury were not detected by XRF, indicating that the content of these toxic metal elements in FD is very low, below the detection limit of XRF. The XRF result indicates that the FD powders are safe to be used as cement additive. The FESEM image shows that the FD powder is composed of irregular particles, and the particle sizes range from a few microns to a hundred microns (Fig. 2b). The particle content calculated from the particle size distribution curve between 10 and 100 mm is 47.28%, and that between 100 and
Ordinary Portland cement mainly comprises cement clinker and a small amount of gypsum retarder. The cement clinker is a mixture of tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetralcium ferroaluminate (C4AF). The hydration reaction of the cement powder and water produces hydrated calcium silicate gel, calcium hydroxide (portlandite), ettringite, and other hydration products [25,26]. The XRD pattern of pure HCP (Fig. 3) shows that the main hydration product, such as portlandite and ettringite were detected by XRD. The calcite phase in the XRD spectrum originates from the carbonation process of the hardened cement paste. In addition, the hydration of Portland cement is a long-term process. During the one-year age, the hydration reaction of C3S is only about 80%, while the hydration process of C2S is slower. Therefore, there are also phases such as C3S, C2S, and C4AF, which are not fully hydrated in the HCP of 28 days old. The XRD spectra of FD/PC composites are very similar to that of pure HCP. No diffraction peaks disappeared or new ones appeared,
Fig. 5. Surfaces FESEM images of different FD/PC samples: (a) HCP0; (b) HCP10; (c) HCP10; (d) HCP30. Magnification: (a), (b), (c) and (d) 1,000; (a1), (b1), (c1) and (d1) 10,000; (a2), (b2), (c2) and (d2) 80,000 .
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indicating that FD only acted as filler in HCP and there was no chemical reaction between them. In addition, with increasing content of FD in the FD/PC composite, the diffraction peak intensity of the portlandite phase significantly reduced, because of decreasing cement content in the FD/PC composites. 3.3. FESEM images analysis of the FD/PC compoistes The FESEM images of the fracture surfaces of different FD/PC samples are shown in Fig. 4. The FEM morphologies of the cross section of various FD/PC composites were similar at 1000x magnification (Fig. 4a–c). At high FD content, the irregular FD particles were observed in the SEM images (Fig. 4c and d). At high magnification (10,000 and 80,000 times), the micron-scale pores in the FD/ PC composites are clearly visible (the highlight section in Fig. 4a1, b1, c1, and d1). In addition, irregular cement gel (Fig. 4a2 and c2) and hydrated product (crystal) with regular shape (Fig. 4b2 and d2) were also observed in high magnification SEM images. The FESEM images of the as-prepared surfaces for different FD/ PC samples are shown in Fig. 5. Compared to the fracture surfaces, the surfaces of the FD/PC composites are smoother. The surface of the FD/PC composite with 30% FD content shows a large number of voids, whereas the surfaces of other FD/PC composites are relatively compact. However, in the high magnification SEM image, micron-sized pores were observed on the surface of all the FD/PC composites.
with the FD content of 10%, 20%, and 30% decreased by 8.5%, 33.8%, and 53.1% at 28 days, while the corresponding flexural strength decreased by 9.8%, 26.5%, and 53.0%, respectively. The FD/PC composite with an FD content of 10% maintained the mechanical strength of more than 90% of the HCP, with a 28-day compressive strength larger than 40 MPa and a flexural strength larger than 10 MPa. Therefore, the FD/PC composites with an FD content of 10% can realize the recycling of FD in the cementbased composites and decrease the production cost of the cement products. The FD/PC composites with high FD contents (i.e., 20% and 30%) have higher reuse efficiency for FD; however their mechanical strengths are poor. They are recommended to be used in non-load-bearing building materials such as rendering mortars or cement based coatings to reduce costs.
3.4. Pore structure analysis The pore parameters of the FD/PC composites tested by MIP are shown in Fig. 6. The pore volume of the FD/PC composites with the FD content of 0%, 10%, 20%, and 30% were 0.1225, 0.1041, 0.1451, and 0.1648 mL/g, respectively (Fig. 6a). Compared to pure HCP, adding a small amount of FD (10%) is conducive to the densification of the composite structure, while adding more FD (20% and 30%) increases the pore volume of the composite. The pore distribution diagram of the composites shows that the pores in the FD/PO composites mainly consist of nanopores and submicron pores, with a total content of more than 85%. Therefore, these two types of pores will have an important impact on the properties of the FD/PC composites such as permeability, water absorption, and mechanical strength. The composite of HCP10 has the highest nanopore content and the lowest submicron pore content. In addition, the content of nanopores in the composite increases with increasing FD content (from 10% to 30%), while the content of submicron pores decreases with increasing FD content. Another important pore parameter is the threshold pore size, which is closely related to the connectivity of pores in HCP and the permeability of cementbased materials. All the FD/PC composites exhibit double peaks in the differential curves (Fig. 6c). The pore corresponding to the peak in the curve is the threshold pore of the FD/PC composite. The threshold pores of the four composites were 134.7, 26.3, 133.2, and 114.8 nm, respectively (Fig. 6c). Therefore, the threshold pores of HCP10 composites are the lowest in size, whereas that of other composites is close [13,27]. The results show that the pores in the HCP10 composite have lower connectivity and denser pore structure. 3.5. Compressive strength and flexural strength analysis The compressive strength and flexural strength of the FD/PC composites with different FD contents at the ages of 3, 7, 14, and 28 days are shown in Fig. 7a and b, respectively. The compressive strength and flexural strength of all the FD/PC composites increase with increasing age, but decrease with increasing FD content. Compared to HCP, the compressive strength of the FD/PC composites
Fig. 6. Pore diameter distributions of various FD/PC composites obtained from MIP measurement. (a) cumulative pore volume; (b) pore diameter distribution; (c) differential intrusion volume.
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Fig. 7. Compressive strength and flexural strength of FD/PC composites at different ages.
3.6. Surface wettability analysis In order to reduce the water absorptions of the FD/PC composites, the surface of FC/PC composites was superhydrophobically coated. In addition to being used to prepare bulk cement-based composites, FD can also be used to prepare cement-based coatings, thereby expanding the field of FD reuse. In this study, the main raw materials used in the preparation of superhydrophobic coatings are still PC and FD, as well as small amounts of water repellent such as PDMS. The surface wettabilities of various FD/PC composites before and after coating treatment were tested, and the results are shown in Fig. 8, indicating that all of the as-prepared FD/PC composites show superhydrophilicities and their surfaces’ CAs are 0°. After superhydrophobic coating treatment, water droplets could not spread on the surface of FD/PC composites. The water CA and SA of the coating are 154.1° and 6.1°, respectively (taking HCP-C surface as an example), indicating that the coating surface is superhydrophobic. When 20 mL of water was poured onto the surface of the superhydrophobic coating, the beaded water flow away rapidly on the surface of the coating (see Movie S3, in the SM). In practical applications, the water contacted by superhydrophobic surfaces is usually aqueous solutions or dispersions containing various impurities such as acid rain, brine, milk, and fruit juice. Therefore, superhydrophobic surfaces are required to have superlyophobic properties [28-30]. Therefore, the hydrophobicity of the coating was measured using a laboratory solution of saline water (5% sodium chloride
Fig. 8. Surface CAs and SAs measurement for different FD/PC composites before and after coating treatment.
solution), acidic water (dilute hydrochloric acid with a pH of 5), and alkaline water (sodium hydroxide solution with a pH of 9) to simulate the water in the natural environment. Fig. 9a shows the photograph of the liquid droplets on the surface of the coating, indicating that all of the above liquids exhibit CAs greater than 150° on the surface of the coating. In addition, liquid foods commonly used in daily life, such as Coca Cola, fruit juice, honey, soy sauce and tea, also have liquid CAs above 150° on the coating surface (Fig. 9b). The superhydrophobic surface has a self-cleaning function, and a large amount of dust accumulated on the surface is completely removed from the surface of the coating by the impacted water droplets (see Movie S4, the SM). Artificial slurry is prepared using diatomite as contamination. When the slurry is dumped on the coating, the coating cannot be polluted by the slurry as shown in the Movie S5 of the SM.
3.7. Surface rough structure and chemical component analysis The surface micro-structures of the FD/PC composite based superhydrophobic coating were observed by FESEM, and the images are shown in Fig. 10a and Fig. 10b, exhibiting very rough
Fig. 9. Surface lyophobicity measurement for the FD/PC composite based superhydrophobic coating.
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surface. Irregular FD particles and a large number of micron scale holes were observed on the coating surface. The 2D and 3D profile images of the coating surface show that the surface is rugged, just like the mountains and valleys on Earth. The average roughness (Ra) of the coating surface was approximately 4977 nm, belonging to the micron-scale roughness. Superhydrophobic surface usually contains groups such as ACH3, ACF3, and -Si-CH3 to ensure low surface energy of the coating [31-33]. Therefore, the surface chemical composition of the FC/PC composite based superhydrophobic coating was measured. The XPS survey spectrum indicates that the presence of oxygen, carbon, calcium, and silicon elements on the surface of the superhydrophobic coating. Elemental oxygen and silicon can be attributed to the cement hydration products, FD and PDMS, all of which contain elemental oxygen and silicon. Elemental calcium can be attributed to cement hydration products and FD. Three distinct peaks located at the binding energies of 284.8, 286.7, and 289.9 eV were observed in the C region, indicating three types of carbon sources on the coating surface, namely, carbonate (CaCO3) produced during the carbonation process of
HCP in air, carbon adsorption on HCP surface in air (often containing polar and non-polar carbon), and carbon introduced from PDMS (-Si-CH3). Obviously, the XPS peak at 289.9 eV was attributed to calcium carbonate. Obviously, the XPS peak at 286.7 eV is attributed to the polar carbon adsorbed on the HCP surface (C@ @O and/or COR) [31]. The XPS peak at 284.8 eV is mainly attributed to the silicon methyl group in PDMS, and a small part of it might come from the adsorption of non-polar carbon in the air (C–C). The ATP-FTIR spectrum of the superhydrophobic coating showed the presence of carbonate, methyl, and Si–O bonds on the surface of the coating, and this result is consistent with the XPS test results Fig. 11.
3.8. Surface mechanical robustness analysis The mechanical robustness of superhydrophobic coatings is one of the key factors to determine their applicability in practical engineering fields [16,24]. At present, there is no standard
Fig. 10. (a) and (b) FESEM images of the FD/PC composite based superhydrophobic coating; (c) 2-D surface profile measurement of the coating; (d) 3-D surface profile measurement of the coating.
Fig. 11. (a) XPS survey spectrum and (b) high resolution C 1 s spectrum of the FD/PC composite based superhydrophobic coating; (c) ATR-FTIR curve of the coating.
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equipment or method for measuring the mechanical robustness of superhydrophobic materials. The stability of the superhydrophobic materials is evaluated by measuring the changes in the surface CA and SA after various mechanical damage tests by artificial methods such as sandpaper abrading, double-sided adhesive adhesion and peeling, gravel impact, and knife cutting. Among these methods, sandpaper sanding is the most commonly used method for characterizing the mechanical robustness of superhydrophobic surfaces in the literature, because some important experimental parameters such as pressure, roughness of sandpaper, and abrasion distance (or cycles) of sanding are easily quantified. In the present
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work, the robustness of the superhydrophobic coating against mechanical damage was evaluated by sandpaper sanding and adhesion-peeling method, and the results are summarized in Fig. 12. Fig. 12(a) shows that the superhydrophobic coating of the single layer (brushing only once) still has superhydrophobicity after six cycles of polishing under a pressure of 3.1 kpa with a surface CA of 153.2° and SA of 6.9°. With the prolongation of polishing cycles, the surface CA of the coating decreases rapidly, while the SA cannot be measured (droplets can not roll). In other words, the surface of the coating loses its superhydrophobicity. When the superhydrophobic coatings are polished under low pressure
Fig. 12. Effect of abrading and peeling cycles on the surface wettability of the FD/PC composite based superhydrophobic coatings. (a) one layer coating, abrading test under a pressure of 3.1 KPa; (b) one layer coating, peeling test under a pressure of 2 MPa; (a1) one layer coating, abrading test under a pressure of 0.62 KPa; (b1) one layer coating, peeling test under a pressure of 31 KPa; (a2) two-layer coating, abrading test under a pressure of 3.1 KPa; (b2) two-layer coating, peeling test under a pressure of 2 MPa.
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(i.e., 0.62 MPa), the coatings remain superhydrophobic even after 50 polishing cycles (Fig. 12a1). The superhydrophobic coating loses its superhydrophobicity after six cycles of grinding under a pressure of 3.1 KPa, which can be attributed to the wear of the coating. The greater the pressure, the longer the distance, the more severe the wear of the superhydrophobic surface. In order to enhance the robustness of the superhydrophobic coating, a double layer coating was prepared. The double-layer coating remained superhydrophobicity after 10 cycles of abrasions under an abrasion pressure of 3.1 KPa (Fig. 12a2). The 3 M double-sided adhesive was adhered on the coating under a pressure of 2 MPa [10,16]. After the pressure was kept for 2 min, the double-sided adhesive was peeled off, and the CA and SA of the coating were measured (Fig. 13). The robustness of the coating against peeling is summarized in Fig. 12b, b1, and b2, indicating that the peeling stability of single layer superhydrophobic coating is only for five cycles under 2 MPa. However, the coating has a peeling stability larger than 100 cycles under the pressure of 31 KPa. In addition, the double-layer superhydrophobic coating has a peeling stability of more than 10 cycles under the adhesion pressure of 2 MPa, which is significantly higher than that of the single-layer superhydrophobic coating Therefore, mechanical robustness can be effectively improved by increasing the number of layers of superhydrophobic coating. Another important property for superhydrophobic surfaces is the repairability [34]. In nature, the lotus leaf maintains a stable superhydrophobic surface by secreting a wax layer of low surface energy. However, for artificially prepared superhydrophobic surfaces, simulating the function of lotus leaf to secrete low surface energy substances automatically is difficult. An alternative approach is to repair the degraded superhydrophobic surface in a simple manner. Our superhydrophobic coatings were prepared by a very simple brushing method and cured at room temperature. Therefore, the damaged coating can be easily repaired by repeating the preparation process. In order to demonstrate the repair process
Fig. 14. Repair test for the FD/PC composite based superhydrophobic coating. (a) picture of water droplets on the damaged coating surface; (b) picture of water droplets on the repaired coating surface.
of the superhydrophobic coating, the surface of the coating was destroyed using a blade (Movie S6 in the SM,). The area immediately loses its superhydrophobicity after the coating is damaged (Fig. 14a). However, after a simple brushing process and curing at room temperature for 24 h, the damaged coating area restored its superhydrophobicity (Fig. 14b and the SM, Movie S7). 3.9. Water absorption analysis The water absorption measurement results of various FD/PC composites before and after superhydrophobic coating modification are listed in Fig. 15 [15,35]. Fig. 15a shows that the water absorption of FD/PC composites without superhydrophobic coating is closely related to the pore volume of the composites (also see Fig. 5a). The larger the total pore volume, the higher the water absorption rate and the faster the water absorption rate of the composites. The 96-hour water absorption of the FD/PC composite with an FD content of 10% was 3358 g/m2, which was the lowest in the composites. After the superhydrophobic coating treatment, the water absorption of the FD/PC composites decreased significantly, and their water absorption curves almost coincided [Fig. 15b]. The 96-hour water absorption of the FD/PC composites modified with superhydrophobic coatings fluctuated between 710 and 815 g/ m2, with a maximum value of 814 g/m2. When a water droplet was placed on the surface of the superhydrophobic coating (also see Fig. 8), the surface wettability could be described by the following Equation: [36]
coshw ¼ f 1 coshr f 2
Fig. 13. Mechanical robustness against peeling of the superhydrophobic coating.
ð2Þ
where hw is the apparent CA on a rough surface of a solid and hw = 155.3° (rough surface of PDMS), hr is the CA on a smooth surface of the same solid and hr = 110° (smooth surface of PDMS), and f1 and f2 are the area fractions of the water droplet in contact with solid and air of the superhydrophobic coating, respectively (f1 + f2 = 1). Both f1 and f2 can be determined by Equation (2) and were found as 0.1291 and 0.8609, respectively. The result indicates that 12.91% of the surface area of the superhydrophobic coating contacts with water. During the soaking period, water can gradually penetrate into the pores from the contacting areas with
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Fig. 15. Water absorption curves for different FD/PC composites before and after superhydrophobic coating modification.
increasing soaking time. As a result, the FD/PC composites with superhydrophobic coating absorb water, and their water absorption reduced by more than 76% after the superhydrophobic coating treatment. Therefore, superhydrophobic coating treatment is an effective method to reduce the water absorption of the cementbased composites.
Acknowledgements The authors acknowledge the financial support by the Science and Technology Support Program (Social Development) of Changzhou City (Grant No. CE20195038). Appendix A. Supplementary data
4. Conclusions In this study, FD/PC based composites and superhydrophobic coatings were prepared by partially replacing Portland cement with FD. The mechanical strength of the FD/PC composite decreased with increasing FD content. However, the FD/PC composite still exhibited a relatively high mechanical strength (the compressive strength is 31.70 kPa and the flexural strength is 8.32 kPa), which could ensure the application of FD/PC composite in the field of some construction materials. Superhydrophobic coatings based on the FD/PC composites were prepared by simply brushing the admixture of FD, PC, PDMS, and water on substrates. The coating exhibits superhydrophobicity after curing at room temperature for 24 h. The superhydrophobic coating possesses excellent mechanical robustness against abrasion and peeling tests. The mechanical robustness of the superhydrophobic coatings was further improved by double-layer coating method. In addition, the superhydrophobic coating can be easily repaired after getting damaged. Moreover, the water absorption of the FD/PC composites reduced by more than 76% after the superhydrophobic coating treatment. The FD/PC composite modified by the FD/PC composite based superhydrophobic coating exhibited high mechanical strength, low water absorption, and high utilization rate of FD and thus is expected to be widely used in the field of building materials. CRediT authorship contribution statement Fajun Wang: Conceptualization. Ting Xie: Data curation, Writing - original draft. Sheng Lei: Data curation, Writing - original draft. Junfei Ou: Writing - review & editing. Wen Li: Project administration. Mingshan Xue: Methodology. Daqi Huang: Methodology. 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.
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