STUDY ON MICROMECHANISM OF A SUPER-HYDROPHOBIC AND SELF-CLEANING COATING MADE BY RECYCLED CONCRETE WASTES

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1 STUDY ON MICROMECHANISM OF A SUPER-HYDROPHOBIC AND SELF-CLEANING COATING MADE BY RECYCLED CONCRETE WASTES L.P. Guo (1,2), W. Sun (1,2), T. Yu (1) and B. Chen (1,3) (1) School of Materials Science and Engineering, Southeast University, Nanjing, China (2) Jiangsu Key Laboratory of Construction Materials, Nanjing, China (3) Nanjing Hydraulic Research Institute, Nanjing, China Abstract An organic-inorganic hybrid coating was studied for the outside concrete wall by the recycled concrete wastes with sol-gel method. The waste concrete was grinded to pass the sieves with the size of 80 micrometers. The grinded waste concrete powders were used as the film-forming materials. Ethanol, methoxy-silane and ammonia were adopted as the solvent, modifier and solidifying agent, respectively. An optimized production method was also proposed through analyzing the different performance of coating with different mix design and stirring time. Four primary properties were evaluated by the smeared coating on dried concrete plates, such as surface contact angles for different ph droplets, self-cleaning property, micro-scale morphology and chemical group analysis. The environmental scanning electron microscope, atomic force microscope, infrared spectrometer and video based contact angle measuring device were employed for above-mentioned tests. The results show that the Contact Angle (CA) for water on the produced super-hydrophobic coating is greater than 150º. Furthermore, this hard coating has obvious advantages of self-cleaning and spatial rough surface. Finally, the alkali and acid resistance tests prove that the produced organic-inorganic hybrid coating can be used in any solutions of ph INTRODUCTION The 0.1 billion tons of annual output of construction waste is a great burden for social progress and economical development in China. Special attention should be paid on the tiny dusts with the maximum size of 0.15mm and 2% in mass of abandoned concrete wastes. These light weight of non-degradable dusts can not only pollute the atmosphere but leading to respiratory diseases for human. Recycling these tiny particles of concrete wastes to be a novel functional material is very meaningful for environment and health.

2 Besides, the super-hydrophobic surface technology has been developed in a high speed in recent years [1-3]. It has a good future in self-cleaning materials, micro-fluidic devices and biological materials [4-8]. In 1990s, Wilhelm Barthlott in Germany [1,2] firstly proposed a so-called Lotus Effect and reported a simple relation between hydrophobic and self-cleaning properties based on a series studies on the physical rough surface of the lotus leaf. Since then, the hydrophobic surface brings more and more concerns and interests in the field of nanomaterials and technology. If the water contact angle is greater than 150º, the prepared coating could be named as super-hydrophobic coating [1-3]. Besides, five different models were proposed and inducted for simulating this special surface state [4,5,8], i.e. Wenzel state, Cassie state, Lotus state, Wenzel-Cassie transition state and Gecko state. Each of above defined models has different physical meaning for simulating a particular super-hydrophobic mechanism. Therefore, the study herein focuses on inviting a novel coating with both superhydrophobic and self-cleaning properties through the physical Lotus-state morphology and a chemical hybrid method. The preparation method and some unique properties of this novel coating are detailed introduced in following sections. This hard coating can be used effectively for improving the durability of infrastructures exposed to any solutions with ph value from 5 to COATING PREPARATION The collected concrete waste was ground, sieved and oven-dried for preparation. And part of these powders with a particle size of less than 80 microns was used as the precursor. The analytically pure methoxy-silane, analytically pure ethanol and analytically pure ammonia were adopted as the modifier, the solvent and the solidifying agent, respectively. Wherein, the mass fraction of the precursor is 15%~20% of all raw materials. The optimized molar ratio of modifier, solvent and solidifying agent is 1: 35:13. A 3-mouth flask and an ultrasonic water-bath instrument like shown as follows were employed for casting the super-hydrophobic coating. All of raw materials and the original temperature of water in the ultrasonic wave instrument were kept in room temperature (20± 1 ) in advance. The collected concrete waste, the analytically pure ethanol and the analytically pure methoxy-silane were mixed together in a three mouth flask, and then were vibrated in an ultrasonic water-bath instrument for one hour. An additional 30 min of vibration was need after supplying the analytically pure ammonia. The ready paste was evenly brushed on a mortar specimen surface and was dried naturally in room temperature for 24 hours to produce a hard coating film. 3. PROPERTIES EVALUATION 3.1 Contact angle Three different solutions were selected for CA test in the room temperature (20±1 ), e.g. acid, neutral and alkali solutions. The ph value of them is 5, 7 and 12, respectively. A CA tester as shown in Figure 1 was utilized for qualitative and quantitative analysis in this research. The quantity of each droplet was carefully controlled as 10 microlitres through a micro-injector. The captured morphology and analyzed contact angles of droplets on the coating surface were shown in Figure 2 and 3 as examples, respectively. The CA of spherical

3 droplets was 156º, 157ºand 155ºfor acid, neutral and alkali solutions in Figure 3. We adopted the average value of five CA data to be the final test result for each type of solutions. The test results show that all tested CA values are greater than 150º, meanwhile, the droplets of ph 5-12 have no negative effects on the super-hydrophobic property of this coating. If the ph values of droplets are less than 5 or greater than 12, they have obvious erosion on this coating and the super-hydrophobicity cannot be realized. Therefore, we recommend this unique coating to be suitably practically applied in conditions of ph Fig. 1 A contact angle tester Fig. 2 The morphology of droplets on the coating surface (a) Acid solution (ph 5) (b) Neutral water (ph 7) (d) Alkali solution (ph 12) Fig. 3 The test results of CA for droplets of ph Self-cleaning property Standard test: Since the self-cleaning is an environmental and economic performance for a novel coating to prevent from pollution by raindrop and dust in the air. In order to evaluate the selfcleaning property of the prepared coating, a Chinese standard Test method for dirt resistance of film of architectural coatings and paint with a series number of GB was referenced. According to this standard, the thin slurry including 100g of fly ash and 100ml of tape water was ready as the pollution source. A brightness meter was adopted to quantitatively evaluate the self-cleaning effect of this coating through the light reflection degree. Since the super-hydrophobic of this coating, any droplet of the brushed slurry could not stably adhere to the coating surface. Therefore, the light reflection degrees of the tested coating were nearly unchanged before and after contamination. The self-cleaning ratio R sc of the tested super-hydrophobic coating, which was defined in Equation (1), was close to 100%. A B Rsc (1 ) 100 (1) A C Wherein, R sc is the self-cleaning ratio, %. A, B is the light reflection degree of coating before and after this test, respectively, %. C is the light reflection degree of fly ash, %.

4 Non-standard test: In order to evaluate the self-cleaning property of the tested coating exposed to the dry dusts, a non-standard test was put forward. A mortar plate with a surface area of 160 mm x 80 mm was selected for coating covering. The cast and drying methods of this coating were same as those mentioned in Section 2. Firstly, this mortar plate was horizontally placed and the upper coated surface was evenly layover by 50g of oven-dried fly ash. Secondly, the dried fly ash particles roll down from the coating surface after putting this mortar plate perpendicularly within one second. The weights of the coated mortar specimen were recorded after finishing each of above stages. Finally, the self-cleaning ratio could also be calculated by the following equation. ma m0 Rsc (1 ) 100 (2) m0 Wherein, m 0 is the weight of the uncontaminated coated plate before the first test step, m a is that of the contaminated coated plate after the second test step. The test results in Table 1 show that the prepared novel coating made by the recycled concrete wastes has an outstanding self-cleaning performance for dry dust, because the average self-cleaning ratio great close to 100%. Table 1. The results on self-cleaning property of the coating by a non-standard method Mass of uncontaminated coated plate (g) Mass of contaminated coated plate after dust removing (g) Self-cleaning ratio of the coating (%) Analysis and discussions on micro mechanisms Because the physical modification and a chemical hybrid method efficiently contribute to the novel coating film with both super-hydrophobic and self-cleaning properties, the mechanisms of micro-scale physical and chemical modifications on this coating are critical for this research. Physical mechanisms: The effects of the physical modification on the coating surface could be enclosed by two approaches, i.e. the micro-scale physical morphology captured by a field emission environmental scanning electron microscope (ESTM) and the spatial roughness of micro-scale surface analyzed by an atomic force microscope (AFM). As present in Figure 4, the particles of the recycled concrete wastes are tightly interconnected and there are no obvious cracks for voids found on the tested coating surface. It means that optimized size distribution and uniform 2-dimensional dispersion of recycled concrete particles are prerequisites for the unique properties of the novel coating. Furthermore, the 3-dimensional morphology in a 10μm x 10μm area of a coating sample was quantitatively analyzed by a tapping-mode AFM system, shown in Figure 5. The maximum height of roughness of the coating surface is 1.3μm, meanwhile, the average space of particles is around 1μm. Moreover, these particles are stably glued to and embedded in each other through an organic superhydrophobic film. These results indicate that the tested coating has a rough morphology with both micro- and nano-scale structures, which is similar to that of the self-cleaning lotus leaf.

5 Fig. 4 The 2-dimensional morphology of the micro-scale coating surface by ESEM Fig.5 The 3-dimensional roughness of the micro-scale coating surface analyzed AFM Chemical mechanisms: Except to the effect of the physical modification, replacing the active hydroxyl groups existing in the precursor particles through the active methyl groups supplying by methoxy-silane is another critical mechanism for the novel properties of the prepared coating. Analysis on the change of chemical elements and groups as a result of an inorganic-organic hybrid method could be conducted through energy dispersive system (EDS) coupled in ESEM and infrared spectrum (IR) system, respectively. The energy dispersive curve and element percentage analyzed by EDS were present in Figure 6. Note that the hydrogen element can not be detected by EDS. It can be found in Figure 6 that there are four main chemical elements in the prepared coating, e.g. calcium, silica, carbon and oxygen. Their total mass fraction or atomic percentage is greater than 90%. Wherein, the great fractions of oxygen and carbon elements shown in Figure 6 are the result of using the methoxy-silane in this coating. In addition, the atomic percentage of calcium and silica element in this coating is about 2.5, which is exactly the evidence of the presence of a dense calcium silicate hydrate gel (Type II C-S-H gel with network structure) in the employed recycled concrete wastes [9]. However, we can not reliably certify the chemical reactions existing between C-S-H gel and methoxy-silane agent, if relying solely on the element analysis herein. Therefore, an additional quantitative analysis is necessary. An infrared spectrum shown in Figure 7 is employed as an effective way for comprehensively evaluating the chemical group newborn on the surface of C-S-H gels. According to Figure 8, it could be analyzed that the major newborn active groups on the surface of C-S-H gels are -CH 3 and -CH 2 -, as well as more than 4 groups of -CH 2 - are generated on the surface of each cluster of C-S-H gel. By reducing the surface energy of the C-S-H gels, these newborn active groups are another inherent source for super-hydrophobic and self-cleaning properties of the novel coating. Therefore, the rough structures and the newborn super-hydrophobic active groups are believed as two inherent micro-mechanisms for super-hydrophobic and self-cleaning properties of the hard coating surface.

6 Fig. 6 Energy dispersive curve and element percentage analyzed by EDS Fig. 7 The employed IR system Fig. 8 IR curve of the tested coating 4. CONCLUSIONS An optimized mixture design and stirring time must be determined by trial-and-error as the first step for super-hydrophobic and self-cleaning coating preparation. The maximum particle size of the recycled concrete wastes should be less than 80 microns to be used as the precursor. Its optimized mass fraction is 15%~20% of all raw materials in the coating. Analytically pure methoxy-silane, ethanol and ammonia are satisfied to be adopted as the modifier, the solvent and the solidifying agent, respectively. And the optimized mixture of them should be 1: 35:13 in molar ratio. Since the tested CA values are greater than 150ºas well as the average self-cleaning ratio great close to 100%, the prepared coating has a outstanding super-hydrophobic and self-cleaning properties. Meanwhile, it is recommended to be practically applied in case of ph According to the quantitative evaluations on physical and chemical micro-mechanisms, the lotus-state rough structures and the newborn super-hydrophobic active groups are believed as two inherent resources for super-hydrophobic and self-cleaning properties of this novel coating surface. ACKNOWLEDGEMENTS This research was sponsored by the National Natural Science Foundation of China (Grant No ), the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No ), and the Teaching and Research Funding for Excellent Young Teacher in Southeast University of China (School Notification No. [2011] 154).

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