# Factors Affecting the Efficiency of Hydrophobic Coatings--Experience from Application on Sandstone
## 2 Materials and Methods
### Methods  
To characterize the pore-size distribution of the sandstone, mercury intrusion porosimetry (MIP or Hg porosimetry) was used. Two factors were evaluated that can affect the efficiency of the coating: a different number of applied coating layers and different curing (or \"hardening\") time. The curing can be defined in general as a chemical and/or physical process that is generating the required properties or desired effect. Here, curing time is the minimum time span between applying the coating and reaching the intended effect. However, the curing time of the \begin{table}
\begin{tabular}{l c c} \hline \hline  & **Antipluviol S** & **Funcosil SNL** \\ \hline Consistency & liquid & liquid \\ \hline Colour & transparent & transparent \\ \hline Density (at 20 \({}^{\circ}\)C) & approx. 0.8 kg\(\cdot\)m\({}^{-3}\) & 0.78 kg\(\cdot\)m\({}^{-3}\) \\ \hline Application temperature range & from 5 \({}^{\circ}\)C to 30 \({}^{\circ}\)C & from 10 \({}^{\circ}\)C to 25 \({}^{\circ}\)C \\ \hline Consumption for natural stone & 0.1–0.8 kg\(\cdot\)m\({}^{-2}\) per coat & 0.6–1.2 L\(\cdot\)m\({}^{-2}\) \\ \hline Number of coat layers & 2 or 3 & min. 2 \\ \hline Penetration depth & 4 mm & no information \\ \hline Curing time & not given & not given \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Technical data of tested coatings and recommendations for their use. Figure 1: Two of the many buildings in Bratislava (Slovakia) with Horice sandstone facades. coating is rather questionable, i.e., when is the curing really completed. Therefore, the efficiency of 2-day- and 2-week-long curing of the coating was compared either with two or three layers of coating (Table 2). Coating layers were applied using a brush, at two-hour intervals according to the producer's recommendations. To determine the efficiency of applied coatings, the penetration depth was checked and water absorption tests under atmospheric conditions were used. To determine the penetration depth of a waterproofing coating, the authors of [23] tested four different indicators mixed with the coating before its application. However, to avoid any change of the coating's properties, a different procedure was preferred here. The penetration depth of impregnation liquids was determined on prismatic samples with dimensions of approximately 25 mm \(\times\) 25 mm \(\times\) 50 mm by a simple visual test according to [24], where a similar procedure was used on concrete surfaces. After the specified curing time, those coated prisms were saved perpendicularly to the coated surface into two cubic halves. The fresh cut surface was put in contact with blue-ink-colored water for 1 s to make the coating's penetration depth visible. Since the hydrophobic zone did not absorb as much colored water as the rest of the sample, it created a visible light rim around the dark blue center of the cut. The thickness of this rim was evaluated visually and measured by a caliper. The penetration depth of the coating was measured on sandstone with two and three layers of coating. The efficiency of the coatings was assessed by capillary absorption tests. Water absorption by capillarity (WAC) is one of the ways water penetrates from the surroundings into the building material. In this process, water is driven by capillary forces (i.e., capillary rise) in open pores of certain small sizes (capillaries) into the material, even against gravity. The test was carried out according to the procedure defined in the technical standard EN 1925 [25]. Samples dried to a constant mass (at 70 \({}^{\circ}\)C) were immersed 3 \(\pm\) 1 mm deep into water in a plastic tank on a non-oxidizing grid. The mass increase was due to absorbed water, measured in given time intervals (in minutes): 1, 3, 5, 10, 15, 30, 60, 120, 180, 240, 360, 480, 1440, and 2880 (i.e., 48 h). According to the standard, the test should end at this point, i.e., after 2 days. However, samples were left in the tank for whole 30 days to see what happens. Results were expressed as \(WAC=f(t)\) curves.