Synthesis of Silica Particles from Sugarcane Bagasse Ash for Its Application in Hydrophobic Coatings †

: Wastes such as sugarcane bagasse ash (SCBA) can be used as raw material in ceramics by the elaboration of bricks and tiles and the glass industry, due the high amount of silica in its composition (>70%). Another application for SCBA is the synthesis of metallic silicates. In this work, we study the synthesis of sodium silicate with SCBA as the main raw material and the future application of sodium silicate for the preparation of silica particles in order to create hydrophobic surfaces for ceramic materials to prevent their erosion. The sodium silicate synthesis was carried out by the thermochemical method with batches of ash and sodium carbonate in a 1:1 sodium oxide– silicon oxide molar ratio. The thermal treatment was in an electric furnace at 800 °C for 8 h. Then, for the synthesis of the silica particles, the sodium silicate was dissolved in water, and then we added methanol in a 3:2 water methanol volume ratio. The solution was left to age for an hour to create the Si-OH bond. Finally, tetraethylorthosilicate (TEOS) was added and the solution was stirred for 2 h to create a hydrophobic and hydrolytically resistant siloxane by the displacement of H in the Si-OH bond. The application of the solution was by the spray-coating method over substrates of concrete and red clay with the application of 10, 15, and 20 layers. The hydrophobicity was evaluated with the water contact angle test, with the results of contact angles above the 110°, thus demonstrating the capacity of a waste for the generation of coatings to prolong the useful life of building materials.


Introduction
One of the most common problems in buildings and monument surfaces is erosion due to rain, which slowly dissolves materials, or by the propagation of water across the cracks, allowing the corrosion of the internal structure [1,2]. There are alternatives, such as covering the materials with hydrophobic materials to prevent the impact over the external structure. Treated silica particles are one of the main selected materials for this application, due their capacity to create siloxanes over the ceramic substrates [3]; siloxanes are known for their hydrolytically stability and hydrophobic behavior, making them a good alternative to protect buildings and outdoor sculptures [4]. In this work, to generate silica particles was used a solution of sodium silicate synthetized from sugarcane bagasse ash (SCBA), the main waste of the sugar industry, as the main raw material. The SCBA has been studied as additive for the elaboration of building materials; nevertheless, only 10% to 15% are allowed before the collapse of the materials due the high porosity [5].

Materials and Methods
For the sodium silicate synthesis, we used sugarcane bagasse ash from the Xicotencatl sugar mill located at the south of the state of Tamaulipas, Mexico. The SCBA was analyzed by an elemental analyzer (Perkin Elmer Inst. Series II 2400 CHNS-O) to quantify the volatile matter and then was calcinated at 900 °C for two hours [6]. The resulting ash was analyzed again by elemental analysis to make sure of the elimination of carbon and X-ray fluorescence (XRF) (Panalytical Epsilon 3) to quantify the inorganic matter. The ash was then milled and sieved through standard mesh #100 (150 μm) and finally leached with citric acid at 2% for 2 h and 60 °C to diminish the oxides [7] (CaO), which can interfere in the silicate synthesis. For the synthesis, we calculated the mass relations, considering a molar ratio of 1:1 and a batch of 1.5 g to make SCBA-sodium carbonate (Sigma Aldrich, reactant grade) pellets. The pellets were treated in a furnace (Carbolite, CWF 1300) at 800 °C and 8 h in air atmosphere to decompose the carbonate and let the sodium and silicon oxides react.
The synthetized material was milled and analyzed by X-ray diffraction (XRD) (Bruker D2 Phaser) to guarantee the presence of sodium silicate, then the powder was dissolved in water and methanol was added in a 2:3 water methanol ratio to generate the silanol group (Si-OH). The solution was aged for an hour to let the silicate and the alcohol react; after the hour, 3 mL of tetraethylorthosilicate (TEOS) was added and the solution was stirred for two hours to generate the siloxane group (R-Si-O-Si-R) by the displacement of the H of the silanol [8]. To apply the solution over the substrates, we used the spray coating method with 10, 15, and 20 wear cycles. Finally, the covered substrates were aged for a day before the water contact angle test.

Results and Discussion
The results from the chemical analysis of the ash are reported in Tables 1 and 2. From the elemental analysis (Table 1), is possible to demonstrate that the thermal process of calcination is able to eliminate almost 100% of the carbon in the ash, an important step considering the thermochemical synthesis method, in which it is important to keep the powders compacted to let the reaction occur. If there are carbon remains in the ash, the carbon dioxide generated can damage the pellet integrity and as well the synthesis reaction.  (Table 2), we obtain the inorganic composition of the ash, where silicon oxide is the main component of the SCBA, followed by calcium oxide. A high concentration of CaO (no leached SCBA) can interfere in the synthesis by the formation of a nonsoluble calcium-sodium silicate that cannot be used in the synthesis of siloxane. Nevertheless, the leached with the citric acid is sufficient to remove the necessary calcium, decreasing calcium-sodium silicategeneration. The high content of SiO2 makes it possible to use the SCBA in the synthesis of ceramic materials and silicates as well. In Figure 1, we report the XRD of the material after the synthesis of the sodium silicate, showing as the main phase the sodium silicate and a secondary phase of sodium-calcium silicate due the CaO concentration in the ash. The low intensities signal a polluting phase, with a low impact on the siloxane generation. The selection of the number of layers over the coated materials is due to the generation of a white crust over the substrate, an undesirable result in this kind of coating. To prove the coating, the surfaces selected were red clay and concrete, which were tested without coating as a control and after a day of being coated. In Figure 2, it is possible to observe the results of the water contact angle before and after the coatings. For the samples of red clay with 15 layers and concrete and red clay with 20 layers, it was impossible to quantify the water contact angle because the droplet bunces from the surface at the time it touches the material, a characteristic behavior of superhydrophobic surfaces [9]. The results of the water contact angles of Figure 2 are reported in Table 3. There is a great difference between the substrates without the coating, which are slightly hydrophilic, with water contact angles below 90°. Angle was increased to 140° after materials were coated, a nearly superhydrophobic behavior due the interactions between the organic structures of the siloxane and the water. Another important interaction occurs between the oxygen of the siloxane and the hydrogen of water, but the interaction is null compared with the covalent bond of water [3].

Conclusions
The SCBA can be used as a raw material for the synthesis of sodium silicate after being leached and calcinated to eliminate the metallic elements that can interfere in the synthesis. Although there is still a sodium-calcium silicate phase, its concentration is too low. This can be considered a polluting a) b) c) phase and does not negatively affect the coating. Sodium silicate can be used to generate coatings for ceramic materials. The samples coated showed a nearly superhydrophobic behavior, and when there were 15 layers over the clay and 20 layers over the materials both showed water-repellent behavior.