3.1 Morphology
Figure 1 shows the structure of simple and hydrophobic Leca. As seen in Figure 1A, simple Leca has small and large holes that can absorb water, so it is considered a hydrophilic surface if, after hydrophobic sizing with stearic acid, the holes are almost (not completely) filled. The surface of Leca is physically and chemically covered with stearic acid (Figure 1B). Since stearic acid is used as a water-repellent material for various purposes and materials, the stone sample in question is well water-repellent.
3.2 TGA Analysis
Figure 2 A shows the TGA of a simple Leca. As can be seen, the change in mass in the temperature range of 0 to 300 °C is about 7%, which is due to the loss of moisture in its crystal structure. In the temperature range of 300 to 700 °C, the mass changes are not noticeable; it is about 2.5%, which can be due to the destruction of some of its internal structures. It can also be seen from the DTA diagram for simple Leca that the reaction in this analysis was heat sink. (Li et al. 2017)
Figure 2B shows the TGA analysis for the hydrophobic coating. In the temperature range of 0 to 124.5 °C, mass changes of approximately 1.5% were due to moisture loss in the structure and cavities. The biggest mass changes are in the temperature range of 124.5 to 300 °C, and it is about 9%. The reason for these mass changes was the loss of stearic acid from its surface and pores. The existing DTA diagram for the hydrophobic Leca is thermal because the concavity direction is downwards. (Mihajlović et al. 2009)
3.3 FT-IR Analysis
According to figure 3, for stearic acid, the wavelength cm-1 is 3415 for O-H, cm-1 2848 for C-H, 1702 cm-1 for C=O, 1464 cm-1, and 723 cm-1 for -CH2. For simple Leca, the wavelengths are 471, 791, and 1075 cm-1 for Si-O and Si-o-Si, respectively. The presence of a peak with a wavelength of 3453 cm-1 shows water in its structure. (Demirbas et al. in 2006, Wang et al. in 2021, Li et al. in 2013 and Peng et al. in 2017)
Hydrophobic Leca has more peaks than simple Leca and stearic acid, and the presence of the C=O peak indicates that the reaction between simple Leca and stearic acid was of a chemical type. (Peng et al. in 2017) The intensity of C-O and O-H decreased in the range of 1000 to 1500 while CH3 remained constant. These changes show a hydrogen bond between the Leca and stearic acid layers. On the other hand, with the constant intensity of CH3, it can be concluded that stearic acid has entered the perlite structure and is surrounded by it. (Zhang et al 2020, Myronyuk et al, 2022, and Hernández et al, 2019)
3.4 BET Analysis
The surface area of simple and hydrophobic Leca was measured by BET test under nitrogen gas at a temperature of 299 K, which can be seen in the table below. As expected, the specific surface area, the total volume, and the volume of the holes of the hydrophobic Leca have decreased compared to the simple Leca, which can be due to the presence of stearic acid in the cavities and its outer surface, which is covered with a white layer on the Leca surface.
Table 1: BET data for simple and hydrophobic Leca
sample
|
Specific surface area
aS, BET
(cm2/g)
|
Total volume
Vm
(cm3/g)
|
The volume of cavities (cm3/g)
|
Pure Leca
|
0.401
|
0.003337
|
0.0921
|
Hydrophobic Leca
|
0.0191
|
0.004388
|
0.00536
|
3.5 XRD analysis
Figure 4 compares the structure of simple and hydrophobic plica by XRD analysis. Hydrophobic Leca has additional peaks at 2θ=23.8, 21.5 degrees. This figure shows the presence of stearic acid in the structure of the hydrophobic sample is consistent and proves the presence of stearic acid in the structure of Leca. The peak corresponding to 2θ=6.5 degrees shows that stearic acid has entered the pores of Leca. (Li et al. in 2013 and Scavo et al. in 2019), but since the distance between the peaks in both structures has not changed much, it can be concluded that the amount of stearic acid entered into its structure was small. (Peng et al. 2017)
3.6 Contact angle
Figure 5 shows the contact angle of simple and hydrophobic coating. The contact angle of the simple Leca is almost zero, and the hydrophobic Leca is 143 degrees. In 2006, Gomari et al. made calcite hydrophobic using stearic acid and reached a contact angle of 100 degrees. And the findings of McHale et al. in 2007 also show that stones and soils reach a contact angle of 140 degrees after hydrophobicity in the best case.
From comparing the obtained contact angle with the researchers' findings, it can be concluded that stearic acid has been able to make the surface of Leca hydrophobic and cover the holes at an acceptable level.
3.7 Evaporation rate
Figure 6 shows the amount of water evaporation in 24 hours for simple Leca at a temperature of 20 °C. In this experiment, it can be seen that the simple Leca coating has increased the rate of evaporation compared to the uncoated water due to its tiny holes that act as capillary tubes for the movement of water molecules.
Figure 7 compares the rate of evaporation for hydrophobic Leca and hexadecanol at a temperature of 5°C. Hydrophobic Leca and hexadecanol efficiencies are 20% and 19% at this temperature, respectively. As you can see, the performance of these two samples was similar at a temperature of 5 degrees. In Figure 8, the hydrophobic coating was tested at 25°C for 24 hours. In this test, the hydrophobic Leca prevented evaporation by 28% and hexadecanol by 32%. The rate of evaporation at 40°C for hydrophobic Leca and hexadecanol in 24 hours can be seen in Figure 9. In this test, the efficiency of hexadecanol is equal to each other and about 17%. Figures 10 and 11 show the available data in the form of a diagram in the presence of radiation and wind. In these two tests, as you can see, the performance of hexadecanol is 6% better than hydrophobic Leca.
In general, the evaporation average efficiency (at 5, 25, and 40 °C and environmental conditions of wind and sunlight) for hydrophobic Leca was 26% and 30% for hexadecanol. According to Robert's findings in 1959, for monolayers, evaporation was prevented by 34% in the first year of the experiment and 22% in the second year. Also, Saggaei et al.'s data in 2018 showed that monolayer hexadecanol could prevent evaporation by 22%. Karim and colleagues have also achieved a yield of 24% for hexadecanol in 2018. In 2021 and 2022, Ghahramani et al. achieved the same efficiency as those obtained with titanium dioxide and bentonite hydrophobicity, 30% and 23%, respectively, in the environmental conditions. Results demonstrated that the hydrophobic bentonite efficiency under laboratory conditions was similar to that of hexadecanol and prevented water evaporation by 36%. However, under field conditions, hydrophobic bentonite and hexadecanol efficiencies were 40% and 23% in reducing evaporation for 30 days, respectively. Omer et al., 2022 also used low-cost polymer layers to minimize evaporation and yielded 26% after 45 days.
By comparing the results, it can be concluded that the results of the findings are consistent with the obtained results. Although the performance of hexadecanol was 4% better than that of hydrophobic coating, a hydrophobic coating is preferred due to its low stability and high cost.
3.8 Stability
Simple and hydrophobic Leca was placed in water for 200 days, and its stability was measured. On the first day, some of the simple Leca seeds settled down due to their weight after absorbing water, while the hydrophobic Leca did not settle. From the third day, the hydrophobic Leca starts shedding; on the fourteenth day, the amount of shedding increased, but from the fourteenth day to the 200th day, after that, the number of deposited particles remained constant for the hydrophobic Leca, the simple Leca still sheds. The reason for the shedding of hydrophobic Leca can be that the holes of the Leca are not filled, and some water is absorbed, and the filling of the holes and the surface of the Leca with stearic acid causes the heaviness of the Leca grains. (Figure 12)