# Factors Affecting the Efficiency of Hydrophobic Coatings--Experience from Application on Sandstone
## 2 Materials and Methods
### Methods
\(WAC\) (g\(\cdot\)m\({}^{-2}\)) was calculated as the mass of water absorbed by the sample per unit area at time \(t\) as follows:  
\[WAC=\frac{m_{t}-m_{d}}{A} \tag{1}\]  
where  
\(m_{t}\) (g) is the mass of the sample determined at the time \(t\) (s) since the first contact with water, \(m_{d}\) (g) is the mass of the dry sample, and  
\(A\) (m\({}^{2}\)) is the area of the sample base in contact with water. These absorption tests were carried out on untreated samples (i.e., without any coating), as well as on samples coated according to the testing program shown in Table 2. There were three replicates for untreated samples and 2x/2d samples, but four replicates for 2x/2w and 3x/2w samples in each series (Table S1). After 48 h the water absorption rate dropped to \(<\)1% of the last measured sample mass, so the apparent \"steady state\" or \"constant mass\" was reached as defined in the technical standard. However, samples were allowed to saturate for 30 days to see what happened. After this time, the maximum water \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Number of Coating Layers**} & \multicolumn{2}{c}{**Curing Time**} \\ \cline{2-3}  & **2 Days** & **2 Weeks (14 Days)** \\ \hline
2 & 2x/2d & 2x/2w \\ \hline
3 & – & 3x/2w \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Designation of the sample series according to test conditions. absorption by capillarity (i.e., final specimen moisture due to capillary rise) \(N_{WAC}\) was calculated as follows:  
\[N_{WAC}=\frac{m_{\text{max}}-m_{d}}{m_{d}}\cdot 100(\%) \tag{2}\]  
where  
\(m_{\text{max}}\) (g) is the mass of the sample saturated with water only through capillary rise to a constant mass and  
\(m_{\text{d}}\) (g) is the mass of the dry sample. In the next step, the samples were completely immersed in water (the water reached 10 mm above the surface of the samples) for a period of 30 days, until the samples were saturated to a constant weight. Subsequently, the water absorption by complete immersion at atmospheric pressure (i.e., final moisture after complete immersion when full saturation is already expected) \(N_{WAI}\) was determined according to the technical standard EN 13755 [26] as follows:  
\[N_{WAI}=\frac{m_{sat}-m_{d}}{m_{d}}\cdot 100(\%) \tag{3}\]  
where  
\(m_{sat}\) (g) is the mass of the sample saturated with water to the constant mass and  
\(m_{\text{d}}\) (g) is the mass of the dry sample. From the water absorption values \(N_{WAC}\) or \(N_{WAI}\) calculated according to (2) or (3), the efficiency coefficient of the coating \(C_{\textit{ef}}\) can be expressed (or calculated) as follows:  
\[C_{\textit{ef}WAC}=\frac{N_{WACt}}{N_{WACu}} \tag{4}\]  
\[C_{\textit{ef}WAI}=\frac{N_{WAIt}}{N_{WAIu}} \tag{5}\]  
where  
\(N_{WACt}\) and \(N_{WAIt}\) (\(\%\)) is \(N_{WAC}\) or \(N_{WAI}\) of treated sample and  
\(N_{WACu}\) and \(N_{WAIu}\) (\(\%\)) is \(N_{WAC}\) or \(N_{WAI}\) of untreated sample. Values of \(C_{\textit{ef}}\) can vary between 0 and 1. The closer the \(C_{\textit{ef}}\) value is to 0, the higher the efficiency of the hydrophobic coating. After carrying out all absorption tests, the treated samples (from 2x/2w and 3x/2w series) were dried at a temperature of 70 \({}^{\circ}\)C. Next, the final dry masses of those samples were determined and compared with their dry masses before treatment and testing. The aim of this procedure was to determine the retention or loss of the coating on the samples by the leaching action of water.