Waterproof Aerated Bricks from Stone Powder Waste through Nano-TiO 2 Structured Hydrophobic Surface Modiﬁcation

: To eliminate the negative impacts of waste stone powder that arises from stone processing, the waste was recycled into aerated bricks with a porous structure that exhibited exceptional properties when applied in buildings. However, the pores easily absorb rainwater and dust, causing performance degradation and mold growth inside. In this paper, we have developed through hydrothermal reactions an environmentally friendly aqueous suspension, containing homemade highly dispersive TiO 2 nanoparticles modiﬁed with super-hydrophobic groups on the surface. The suspension was coated onto the aerated bricks, creating a super-hydrophobic surface with a highly textured hierarchical structure. A large contact angle of 146 ◦ tested on the surface and negligible water absorption for 24 h immersion demonstrate the excellent water prooﬁng performance, holding a great promise for large scale applications in construction and buildings.


Introduction
Owing to the increasing demand for stone construction and decoration materials world wide because of the continuing growth in the world's population, a tremendous amount of stone powder waste has been generated in the natural and artificial stone industry during the cutting and carving processing stages, which could account for up to 35% of the weight of the stone that was used. The waste powder ranging from nanometers to millimeters is disposed of in landfills or arbitrarily discharged to the surrounding environment, and is prone to drift in air, which has not only caused severe environmental and ecological damage, but also carried risks to public health [1]. Fortunately, a growing awareness of these issues has been raised by both local municipal managers and many researchers from different fields.
Several methods have been adopted and implemented to recycle the waste stone powder. At present, the reuse strategies concentrate upon incorporating the stone byproduct as additives into the traditional construction materials, such as cement and concrete, to regulate their mechanical properties [2][3][4][5]. Other applications for reusing the stone powder have been developed by researchers in the area of functional nanomaterials, such as a self-cleaning coating based on waste marble [6],

Fabrication of Aerated Bricks with Surface Modification
The recycling process from stone powder waste to aerated bricks was performed in Fengzhu Novel Construction Materials Co., LTD in Fujian Province, China. The stone waste was firstly milled by using a ball grinder, and then pumped to a slurry tank. Then lime, concrete, sand, and foaming agent were added at an appropriate ratio into the tank, followed by stirring with water. The slurry was then introduced into the injection molds and delivered to a still kettle after solidification. Under steam curing in the still kettle, porous aerated bricks were produced.
In this study, the aerated brick surface was coated with a novel homemade self-dispersed TiO 2 nanoparticle to form the hydrophobic hierarchical double structure. The TiO 2 particles were synthesized through a controllable hydrothermal reaction, as described in our previous studies [1,6,16]. In this research, firstly, 5 g of TiO 2 nanoparticles were dispersed into 20 ml of a mixture solution of water and ethylene glycol (purchased form Sigma-Aldrich) at a ratio of 1:1. Then, 0.8 wt.% of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTS, purchased from Nanjing Quanxi Chemical Co., LTD, China) and 2 wt.% of tetraethoxysliane (TEOS, purchased from Aladdin, China) were added into the mixture under a stirring condition at a speed of 500 r/min. After another 2 h of continuous stirring, a super-hydrophobic suspension could be obtained. Finally, the hydrophobic surface of the aerated bricks was achieved through dip coating or spray method.

Characterizations
The crystal phases of the samples were investigated by X-ray diffraction (XRD) using a Rigaku Smartlab 9 kW X-ray diffractometer, equipped with a Cu-K α1 radiation source (λ = 1.5406 Å). The microstructure of the materials was visualized using scanning electron microscopy (SEM) by employing a JEOL 6490 microscope at an accelerating voltage of 20 kV, while the elemental analysis of the samples was examined by using an energy dispersive spectrometer (EDS), which is attached to the JEOL 6490 microscope. Atomic force microscopy (AFM) was utilized to survey the surface morphology of the samples, which was performed on a Bruker NanoScope 8 in tapping mode using a silicon cantilever with a tip radius of less than 10 nm and resonance frequency of 278 kHz. The mean particle size and the width of the particle distribution of the prepared TiO 2 nanoparticles dispersed in deionized water were determined by using a Malvern Mastersizer 3000 laser particle size analyzer. Fourier transform infrared spectroscopy (FT-IR) measurements were carried out to examine the atomic bonding in the prepared coating by using a Bruker Vertex-70 spectrometer, and the absorption spectra were collected in the range of 400-1400 cm −1 applying a spectral resolution of 2 cm −1 . The surface wettability of the modified aerated brick was quantified by the contact angle, which was performed on a PowerEach JC2000D contact angle meter using the 5-point fitting method with a droplet volume of 2 µL.

Results and Discussion
The stone powder is mainly composed of silicon dioxide, calcium carbonate, and other aluminum silicate containing calcium [17], which is identified by XRD measurement shown in the lower red curve in Figure 1 and EDS element detection presented in Table S1 in the Supplementary Materials. The stone powder was mixed with concrete and sand, and then fabricated into aerated bricks. As displayed in Figure 1, an approximate coincidence of diffraction peaks between the two curves confirms that the composition of aerated brick is nearly the same as that of the waste stone powder. The unchanged composition from waste stone powder to aerated bricks was also verified by comparison of the chemical elements employing EDS, which is shown in Figure S1, Table S1 and Table S2  continuous stirring, a super-hydrophobic suspension could be obtained. Finally, the hydrophobic surface of the aerated bricks was achieved through dip coating or spray method.

Characterizations
The crystal phases of the samples were investigated by X-ray diffraction (XRD) using a Rigaku Smartlab 9 kW X-ray diffractometer, equipped with a Cu-K 1 radiation source ( = 1.5406 Å). The microstructure of the materials was visualized using scanning electron microscopy (SEM) by employing a JEOL 6490 microscope at an accelerating voltage of 20 kV, while the elemental analysis of the samples was examined by using an energy dispersive spectrometer (EDS), which is attached to the JEOL 6490 microscope. Atomic force microscopy (AFM) was utilized to survey the surface morphology of the samples, which was performed on a Bruker NanoScope 8 in tapping mode using a silicon cantilever with a tip radius of less than 10 nm and resonance frequency of 278 kHz. The mean particle size and the width of the particle distribution of the prepared TiO2 nanoparticles dispersed in deionized water were determined by using a Malvern Mastersizer 3000 laser particle size analyzer. Fourier transform infrared spectroscopy (FT-IR) measurements were carried out to examine the atomic bonding in the prepared coating by using a Bruker Vertex-70 spectrometer, and the absorption spectra were collected in the range of 400-1400 cm −1 applying a spectral resolution of 2 cm −1 . The surface wettability of the modified aerated brick was quantified by the contact angle, which was performed on a PowerEach JC2000D contact angle meter using the 5-point fitting method with a droplet volume of 2 μL.

Results and Discussion
The stone powder is mainly composed of silicon dioxide, calcium carbonate, and other aluminum silicate containing calcium [17], which is identified by XRD measurement shown in the lower red curve in Figure 1 and EDS element detection presented in Table S1 in the supplementary materials. The stone powder was mixed with concrete and sand, and then fabricated into aerated bricks. As displayed in Figure 1, an approximate coincidence of diffraction peaks between the two curves confirms that the composition of aerated brick is nearly the same as that of the waste stone powder. The unchanged composition from waste stone powder to aerated bricks was also verified by comparison of the chemical elements employing EDS, which is shown in Figure S1, Table S1 and Table S2 in the supplementary materials.    exhibiting a porous structure with the pore size on the micrometer scale. This porous structure enables the aerated brick to possess outstanding properties such as good thermal shielding, sound insulation, light weight, and large pressure tolerance. However, the pores, in turn, could also easily absorb water from rains, consequently facilitating mold growth, suffering from corrosion, and degrading the performances of the bricks.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 10 enables the aerated brick to possess outstanding properties such as good thermal shielding, sound insulation, light weight, and large pressure tolerance. However, the pores, in turn, could also easily absorb water from rains, consequently facilitating mold growth, suffering from corrosion, and degrading the performances of the bricks. To address the issue of water invasion of the aerated bricks, creating a water-repellent surface is an effective way, by modifying the surface with micro-and nanostructure and nanomaterials. Learning from the lotus effect, a hierarchical double structure, consisting of a characteristic epidermis equipped with micrometer-sized papilla arrays and the covering waxes [13,15], significantly reduces the contact area and the adhesion force between water droplets and the surface, thus enables the water repellency of the surface.
As shown in the schematic diagram in Figure 3, homemade highly dispersive TiO2 nanoparticles were employed to coat on the aerated brick surface by dip coating method, creating a rough epidermis of the surface with TiO2 nanoparticle clusters. On the other hand, the TiO2 nanoparticles clad with TEOS undergo hydrolysis, forming -OH terminal groups on the surface. At the same time, the -Si-OH groups in the hydrolyzed PFOTS are dehydrated with the -OH group on the TiO2 surface by means of a self-assembling process and eventually build the hierarchical architecture. According to the Cassie-Baxter wetting model [18], this artificial highly textured surface modified with superhydrophobic groups is responsible for water repellency, holding great potential for self-cleaning and water-proofing applications [19][20][21]. To address the issue of water invasion of the aerated bricks, creating a water-repellent surface is an effective way, by modifying the surface with micro-and nanostructure and nanomaterials. Learning from the lotus effect, a hierarchical double structure, consisting of a characteristic epidermis equipped with micrometer-sized papilla arrays and the covering waxes [13,15], significantly reduces the contact area and the adhesion force between water droplets and the surface, thus enables the water repellency of the surface.
As shown in the schematic diagram in Figure 3, homemade highly dispersive TiO 2 nanoparticles were employed to coat on the aerated brick surface by dip coating method, creating a rough epidermis of the surface with TiO 2 nanoparticle clusters. On the other hand, the TiO 2 nanoparticles clad with TEOS undergo hydrolysis, forming -OH terminal groups on the surface. At the same time, the -Si-OH groups in the hydrolyzed PFOTS are dehydrated with the -OH group on the TiO 2 surface by means of a self-assembling process and eventually build the hierarchical architecture. According to the Cassie-Baxter wetting model [18], this artificial highly textured surface modified with super-hydrophobic groups is responsible for water repellency, holding great potential for self-cleaning and water-proofing applications [19][20][21]. Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 10 The highly dispersive TiO2 nanoparticles in water were prepared, enabling the preparation of environmentally friendly aqueous based suspension. Being a wide gap semiconductor [22,23] and nontoxic material, TiO2 has been extensively used in areas of solar cells [23], photocatalysis [1], whitening, and ultraviolet shielding. The crystal phase of the prepared TiO2 was revealed by XRD ( Figure S2 in the supplementary materials). The diffraction peaks located at 25.2°, 37.7°, 48.0°, 53.9°, 62.6° were well indexed to its (101), (004), (200), (105) and (204) reflections respectively (JCPDS Card Number 21-1272), indicating anatase crystal structure of TiO2 [1]. The dispersion characteristic of TiO2, which can be evaluated by the degree of nanoparticles aggregation in water, is of crucial importance to the aqueous based suspension fabrication. As shown in Figure 4, the median size of secondary particles is about 0.28 μm, and the narrow distribution ranging between 0.1 and 0.6 μm implies a high degree of dispersion for the prepared TiO2 in water. The surface morphology of the TiO2 nanoparticles with super-hydrophobic modification coated on a silicon wafer is visualized in Figure 5. From the AFM topographic image and the sectional view of the height variation, it is estimated that the diameter of a single TiO2 particle is about 50 nm on average, while the clusters range on the sub micrometer scale, in agreement with the secondary particle size distribution presented in Figure 4. The aerated brick was dipped into the TiO2 based aqueous super-hydrophobic suspension, introducing the micro-and nanostructure on the outmost The highly dispersive TiO 2 nanoparticles in water were prepared, enabling the preparation of environmentally friendly aqueous based suspension. Being a wide gap semiconductor [22,23] and nontoxic material, TiO 2 has been extensively used in areas of solar cells [23], photocatalysis [1], whitening, and ultraviolet shielding. The crystal phase of the prepared TiO 2 was revealed by XRD ( Figure S2 [1]. The dispersion characteristic of TiO 2 , which can be evaluated by the degree of nanoparticles aggregation in water, is of crucial importance to the aqueous based suspension fabrication. As shown in Figure 4, the median size of secondary particles is about 0.28 µm, and the narrow distribution ranging between 0.1 and 0.6 µm implies a high degree of dispersion for the prepared TiO 2 in water.  The highly dispersive TiO2 nanoparticles in water were prepared, enabling the preparation of environmentally friendly aqueous based suspension. Being a wide gap semiconductor [22,23] and nontoxic material, TiO2 has been extensively used in areas of solar cells [23], photocatalysis [1], whitening, and ultraviolet shielding. The crystal phase of the prepared TiO2 was revealed by XRD ( Figure S2 in the supplementary materials). The diffraction peaks located at 25.2°, 37.7°, 48.0°, 53.9°, 62.6° were well indexed to its (101), (004), (200), (105) and (204) reflections respectively (JCPDS Card Number 21-1272), indicating anatase crystal structure of TiO2 [1]. The dispersion characteristic of TiO2, which can be evaluated by the degree of nanoparticles aggregation in water, is of crucial importance to the aqueous based suspension fabrication. As shown in Figure 4, the median size of secondary particles is about 0.28 μm, and the narrow distribution ranging between 0.1 and 0.6 μm implies a high degree of dispersion for the prepared TiO2 in water. The surface morphology of the TiO2 nanoparticles with super-hydrophobic modification coated on a silicon wafer is visualized in Figure 5. From the AFM topographic image and the sectional view of the height variation, it is estimated that the diameter of a single TiO2 particle is about 50 nm on average, while the clusters range on the sub micrometer scale, in agreement with the secondary particle size distribution presented in Figure 4. The aerated brick was dipped into the TiO2 based aqueous super-hydrophobic suspension, introducing the micro-and nanostructure on the outmost The surface morphology of the TiO 2 nanoparticles with super-hydrophobic modification coated on a silicon wafer is visualized in Figure 5. From the AFM topographic image and the sectional view of the height variation, it is estimated that the diameter of a single TiO 2 particle is about 50 nm on average, while the clusters range on the sub micrometer scale, in agreement with the secondary particle size distribution presented in Figure 4. The aerated brick was dipped into the TiO 2 based aqueous super-hydrophobic suspension, introducing the micro-and nanostructure on the outmost surface of the brick. Along with the micrometer-sized pores in the brick, a certain rough surface modified with super-hydrophobic groups was built, as shown in Figure S3 in the Supplementary Materials.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 10 surface of the brick. Along with the micrometer-sized pores in the brick, a certain rough surface modified with super-hydrophobic groups was built, as shown in Figure S3 in the supplementary materials.
(a) (b) As displayed in Figure 6, FT-IR was employed to characterize the functional groups and the molecular bonding in the modified TiO2 nanoparticles with PFOTS. The upper blue curve in Figure  6 shows The middle red curve in Figure 6 shows the infrared absorption of the PFOTS modified nano-TiO2, while the bottom black line gives the spectrum of nano-TiO2 before treatment. It can clearly be seen that the middle spectrum is a superposition of the upper and bottom ones, which implies that the PFOTS is attached to the TiO2 nanoparticles.  As displayed in Figure 6, FT-IR was employed to characterize the functional groups and the molecular bonding in the modified TiO 2 nanoparticles with PFOTS. The upper blue curve in Figure 6 shows the absorption peaks of PFOTS (including TEOS), in which the existence of C-F bonds in the form of CF, CF 2  surface of the brick. Along with the micrometer-sized pores in the brick, a certain rough surface modified with super-hydrophobic groups was built, as shown in Figure S3 in the supplementary materials.
(a) (b) As displayed in Figure 6, FT-IR was employed to characterize the functional groups and the molecular bonding in the modified TiO2 nanoparticles with PFOTS. The upper blue curve in Figure  6 shows the absorption peaks of PFOTS (including TEOS), in which the existence of C-F bonds in the form of CF, CF2 and CF3 are situated at 520, 739, 954, 1120 and 1205 cm −1 [24]. The peaks at 1078 cm −1 and 800 cm −1 are associated with the asymmetric stretching vibration and bending mode of Si-O-Si bonds formed by dehydrate reaction of the silane coupling agent, respectively [25,26]. The peak located at 897 cm −1 is assigned to the C-H bonds [25,26]. The Si-O-C bond appears at 1142 cm −1 , which confirms that the functional groups from PFOTS are linked to the silica network [27].
The middle red curve in Figure 6 shows the infrared absorption of the PFOTS modified nano-TiO2, while the bottom black line gives the spectrum of nano-TiO2 before treatment. It can clearly be seen that the middle spectrum is a superposition of the upper and bottom ones, which implies that the PFOTS is attached to the TiO2 nanoparticles.  The middle red curve in Figure 6 shows the infrared absorption of the PFOTS modified nano-TiO 2 , while the bottom black line gives the spectrum of nano-TiO 2 before treatment. It can clearly be seen that the middle spectrum is a superposition of the upper and bottom ones, which implies that the PFOTS is attached to the TiO 2 nanoparticles.
The hydrophobicity of the surface can be determined by its contact angle. As displayed in Figure 7, a contact angle of 146 • on average demonstrates an excellent water-repellency performance of the aerated brick surface with hierarchical structure and super-hydrophobic material modification.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 10 The hydrophobicity of the surface can be determined by its contact angle. As displayed in Figure  7, a contact angle of 146˚ on average demonstrates an excellent water-repellency performance of the aerated brick surface with hierarchical structure and super-hydrophobic material modification. To further investigate the water-proofing performance of the bricks with a hydrophobic surface, a water immersion experiment was carried out, as shown in Figure 8. The sample with surface modification is shown in the left in Figure 8a,b), while the right in Figure 8a,b displays the bare brick. It is apparently observed that the brick with surface modification was floating on the water like a boat, while the control brick sank to the bottom of the water. The water-repellent surface dams the water into the pores of the brick, shielding the porous interior and forming relatively lower density than that of the water. By contrast, the water can easily permeate into the bare brick through the pores. To be more specific, the weight variations of the two brick samples were recorded before and after immersed into the water. A weight increment of 55.1% was detected for the bare brick after 24 h immersion, as shown in Figure 9. On the other hand, the weight of the brick with surface modification showed a small increase of about 4.3%, implying an excellent water resistance. This water immersion test preliminarily demonstrated the water proofing performance of the superhydrophobic coating on the aerated bricks. Further investigation on the aging test and weather fastness would be carried out to encourage the widespread application.  To further investigate the water-proofing performance of the bricks with a hydrophobic surface, a water immersion experiment was carried out, as shown in Figure 8. The sample with surface modification is shown in the left in Figure 8a,b), while the right in Figure 8a,b displays the bare brick. It is apparently observed that the brick with surface modification was floating on the water like a boat, while the control brick sank to the bottom of the water. The water-repellent surface dams the water into the pores of the brick, shielding the porous interior and forming relatively lower density than that of the water. By contrast, the water can easily permeate into the bare brick through the pores. To be more specific, the weight variations of the two brick samples were recorded before and after immersed into the water. A weight increment of 55.1% was detected for the bare brick after 24 h immersion, as shown in Figure 9. On the other hand, the weight of the brick with surface modification showed a small increase of about 4.3%, implying an excellent water resistance. This water immersion test preliminarily demonstrated the water proofing performance of the super-hydrophobic coating on the aerated bricks. Further investigation on the aging test and weather fastness would be carried out to encourage the widespread application.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 10 The hydrophobicity of the surface can be determined by its contact angle. As displayed in Figure  7, a contact angle of 146˚ on average demonstrates an excellent water-repellency performance of the aerated brick surface with hierarchical structure and super-hydrophobic material modification. To further investigate the water-proofing performance of the bricks with a hydrophobic surface, a water immersion experiment was carried out, as shown in Figure 8. The sample with surface modification is shown in the left in Figure 8a,b), while the right in Figure 8a,b displays the bare brick. It is apparently observed that the brick with surface modification was floating on the water like a boat, while the control brick sank to the bottom of the water. The water-repellent surface dams the water into the pores of the brick, shielding the porous interior and forming relatively lower density than that of the water. By contrast, the water can easily permeate into the bare brick through the pores. To be more specific, the weight variations of the two brick samples were recorded before and after immersed into the water. A weight increment of 55.1% was detected for the bare brick after 24 h immersion, as shown in Figure 9. On the other hand, the weight of the brick with surface modification showed a small increase of about 4.3%, implying an excellent water resistance. This water immersion test preliminarily demonstrated the water proofing performance of the superhydrophobic coating on the aerated bricks. Further investigation on the aging test and weather fastness would be carried out to encourage the widespread application.  The highly dispersive TiO2 nanoparticles modified with super-hydrophobic agent were prepared as aqueous based suspension, which is coated onto the aerated brick to create waterrepellent surface. This property could effectively prevent the aerated bricks made of stone powder waste from rainwater invasion and corrosion, maintaining the outstanding performance of the bricks and keeping fresh appearance of the buildings. Tailoring the aerated brick to the water-proofing needs by utilizing nanomaterials in an environmentally friendly way introduces a new strategy for waste recycling and enlarges the application scale of super-hydrophobic technology.

Conclusions
In this study, the stone powder waste from stone processing was recycled into aerated bricks, which possess a pore structure that makes them prone to rainwater erosion. In order to block the outside water and make sure the bricks keep their edge, a strategy of making the brick surface waterrepellent was introduced. Homemade highly dispersive TiO2 nanoparticles modified with hydrophobic groups deriving from PFOTS were prepared as an aqueous suspension. It is coated onto the aerated brick by means of a dip coating method, creating a super-hydrophobic surface. The contact angle for water droplet on the surface reached 146°, indicating exceptional water-proofing performance. Compared to an increase by 55.1% in weight for a bare brick immersed in water for 24 h, negligible water absorption for the brick with surface modification also practically verified the outstanding water repellency and the effectiveness. This approach opens up the possibility of applying nanomaterials and technologies to improve the performances of products from waste recycling, and expands the application scope of the prepared aqueous based hydrophobic coating.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: Images of EDS detected area of (a) the stone powder and (b) aerated brick, Figure S2: XRD pattern of TiO2 nanoparticles, Figure S3: SEM image of the aerated brick surface with modification, Table S1: Elemental composition of the sample in Figure S1 (a), Table S2: Elemental composition of the sample in Figure S1   The highly dispersive TiO 2 nanoparticles modified with super-hydrophobic agent were prepared as aqueous based suspension, which is coated onto the aerated brick to create water-repellent surface. This property could effectively prevent the aerated bricks made of stone powder waste from rainwater invasion and corrosion, maintaining the outstanding performance of the bricks and keeping fresh appearance of the buildings. Tailoring the aerated brick to the water-proofing needs by utilizing nanomaterials in an environmentally friendly way introduces a new strategy for waste recycling and enlarges the application scale of super-hydrophobic technology.

Conclusions
In this study, the stone powder waste from stone processing was recycled into aerated bricks, which possess a pore structure that makes them prone to rainwater erosion. In order to block the outside water and make sure the bricks keep their edge, a strategy of making the brick surface water-repellent was introduced. Homemade highly dispersive TiO 2 nanoparticles modified with hydrophobic groups deriving from PFOTS were prepared as an aqueous suspension. It is coated onto the aerated brick by means of a dip coating method, creating a super-hydrophobic surface. The contact angle for water droplet on the surface reached 146 • , indicating exceptional water-proofing performance. Compared to an increase by 55.1% in weight for a bare brick immersed in water for 24 h, negligible water absorption for the brick with surface modification also practically verified the outstanding water repellency and the effectiveness. This approach opens up the possibility of applying nanomaterials and technologies to improve the performances of products from waste recycling, and expands the application scope of the prepared aqueous based hydrophobic coating.
Supplementary Materials: The following are available online at http://www.mdpi.com/2076-3417/9/13/2619/s1, Figure S1: Images of EDS detected area of (a) the stone powder and (b) aerated brick, Figure S2: XRD pattern of TiO 2 nanoparticles, Figure S3: SEM image of the aerated brick surface with modification, Table S1: Elemental composition of the sample in Figure S1(a), Table S2: Elemental composition of the sample in Figure S1