# Factors Affecting the Efficiency of Hydrophobic Coatings--Experience from Application on Sandstone
## 3 Results  
### Application and Penetration of Coatings  
Applied coatings did not create any crust on the stone surface. The Funcosil SNL coating did not even change the appearance of the stone and preserved its original color. The Antipluviol S slightly changed the color of the stone after applying two layers; the rusty smearing of the sandstone was highlighted (Figure 2). According to Munsell Color Charts [27], the color of the sandstone changed from very light brown (10 YR8/2) to yellow (10 YR8/6). The waterproofing effect of the coating could be seen already by dropping water onto the coated sandstone surface. A magnified photograph of the water droplet (Figure 3) revealed a water/sandstone contact angle of ca. 120\({}^{\circ}\), which proves the surface is hydrophobic [16]. The difference of the water-to-sandstone contact angles with and without coating is very clear from Figure 4. Water infiltrated immediately after contact with untreated sandstone; no contact angle could be measured/observed. Droplets on the coated samples show similar angles for both coatings. The pore-size distribution of the Horice sandstone is presented in Figure 5. A unimodal size distribution of pores is typical for this rock. The main pore-diameter range is between  
Figure 4: Waterproofing effect on the contact angle of a water droplet. H-29 with Antipluviol S, H-5 untreated/uncoated, H-36 with Funcosil SNL. Figure 3: Estimated contact angle between a water droplet and the sandstone surface coated with Funcosil SNL. Figure 2: Color changes of the Horice sandstone with coatings. (**a**) Untreated sample, (**b**) sample with Funcosil SNL coating, and (**c**) sample with Antipluviol S coating. 100 nm and 100,000 nm. Such pores can be classified as micropores or capillary pores [28]. Micropores hold maximum water by capillary forces and lead to high capillary rise. The impregnation depth of the tested coatings on the sandstone is visible in Figure 6. The blue indigo color intensity was used to visually distinguish three zones: 1--hydrophobic zone (original color of the stone), 2--partly hydrophobic zone (light blue color), 3--not impregnated (i.e., hydrophilic) zone (dark blue color). Because of the smooth transition of the color, border lines were drawn very subjectively and the penetration depth cannot be exactly quantified. The penetration depth is irregular even in one sample, probably due to local differences in the grain size and porosity. Thin sections could explain more, but they were not available. The samples from the series 2x/2d showed significantly weaker impregnation than the samples 2x/2w with both types of coatings. A very thin and irregular hydrophobic zone 1 could be recognized only in the sample coated with Funcosil SNL (1 to 2 mm). The color becomes darker towards the sample center (zones 2 and 3). The sample with Antipluviol S shows only colored zones, where the irregular zone 2 is ca. 3 mm; the rest is fully saturated with the ink water, i.e., hydrophilic. However, the hydrophobic zone 1 can be clearly recognized in all samples from the series 2x/2w. This means that the longer curing time enhanced the hydrophobic effect of the coatings. Some \"hardening\" of the coatings occurred with time, making the coatings  
Figure 5: Pore-size distribution of the Horice sandstone determined by Hg-porosimetry. Figure 6: Visual evaluation of the penetration depth of applied coatings, Horice sandstone. 1—hydrophobic zone, 2—partly hydrophobic zone, 3—hydrophilic zone. less soluble and more resistant in contact with water. The infiltration of the coatings into the samples is rather irregular, which is also visible in Figure 6. This can be caused by the heterogeneity of the rock material. The same may explain the fact that Funcosil-SNL-covered samples 2x/2w show thinner zones 1 and 2 than the 2x/2d ones. The best impregnation was achieved in the samples from the series 3x/2w. All three zones can be clearly recognized in samples with both types of coatings, whereby the hydrophobic zones are up to 3 mm thick.